CA1071768A - Light beam path for reading video discs - Google Patents
Light beam path for reading video discsInfo
- Publication number
- CA1071768A CA1071768A CA314,705A CA314705A CA1071768A CA 1071768 A CA1071768 A CA 1071768A CA 314705 A CA314705 A CA 314705A CA 1071768 A CA1071768 A CA 1071768A
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- path
- reading
- directing
- path portion
- reflected
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Abstract
ABSTRACT
The present invention relates to directing a reading light beam onto the surface of a video disc and receiving a modulated beam reflected therefrom. The prior art attempts to direct light beams to and from a video disc involved directing the incident and reflected beam portions along totally separate optical paths, resulting in bulky and complex playback appara-tus. The present invention provides an improved apparatus and method for directing the incident and reflected light beams along a common path portion. In accordance with the invention, a light source is provided for producing a reading beam of light radiation. The reading beam is directed from the light source along an optical path to the information track of the video disc. The reading beam is modulated and reflected by the information track to retrace at least a portion of the path.
A senor receives the reflected beam of radiation from the information track. The means for directing the reading beam includes a device portioned along the portion of the path common to the reading beam and the reflected beam for directing the reflected beam out of the path of the reading beam and toward the sensor. The beam separator may comprise a beam splitting prism, and a quarter wave plate disposed between the prism and the video disc.
The present invention relates to directing a reading light beam onto the surface of a video disc and receiving a modulated beam reflected therefrom. The prior art attempts to direct light beams to and from a video disc involved directing the incident and reflected beam portions along totally separate optical paths, resulting in bulky and complex playback appara-tus. The present invention provides an improved apparatus and method for directing the incident and reflected light beams along a common path portion. In accordance with the invention, a light source is provided for producing a reading beam of light radiation. The reading beam is directed from the light source along an optical path to the information track of the video disc. The reading beam is modulated and reflected by the information track to retrace at least a portion of the path.
A senor receives the reflected beam of radiation from the information track. The means for directing the reading beam includes a device portioned along the portion of the path common to the reading beam and the reflected beam for directing the reflected beam out of the path of the reading beam and toward the sensor. The beam separator may comprise a beam splitting prism, and a quarter wave plate disposed between the prism and the video disc.
Description
'' ' 'I ''' ' i :' ' ' -~ 1071768 LI~jHT BEAM PATH FOR READING
VIDEO DISCS
Systems have heretofore been,developed for repro-ducing signals at video frequencies from information recorded on discs, tapes, or other media. Such systems have utilized, among other things, optical recordings upon photosensitive discs, electron beam recording on thermo plastic surfaces and, in prior patents assigned to the assignee~of the present invention, systems u~ilizing a rotating disc which is responsive to impinging radiation to reflect or transmit radiation correspond-ing to and representative of the information stored on the surface of the disc.
For example, in U.S. Patent No. 3,530,258, issued to David Paul Gregg and Reith O. Johnson on September 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair of flexible, fibre optic elements. An air bearing supported an objective lens system. A light source of radiant energy was positioned below the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source which directed an energy beam to the surface of the disc and provided a transducer that was responsive to reflected energy.
One-of the major problems to be encountered in the recording and reproduction of video information, arises directly from a consideration of the energy levels involved in such a process and the restraints imposed by the considerations of size, weight and operating conditions.
, To be commercially desirable as a home instrument, the system should be able to store and reproduce a "program"
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of at least 15 to 30 minutes in length. The record disc should be of an easily handled size, comparable to the phonograph records currently in use. If the playback turntable was operated at 1800 rpm, some 54,000 revolutions would provide 30 minutes of playback. Assuming a 1 micron track width and 1 micron spacing between adjacent tracks, a circular band approximately 4.25 inches wide is required. Assuming that the smallest radius at which informatio~ can be stored is approxi-mately three inches, the resultant disc is about 15 inches in diameter. The duration of the program or the speed of the turn-table can change the dimensions of the recorded area, as can the width of the individual track and the spacing between adjacent tracks.
Assuming that the video information has been recorded in some digital fashion, the presence or absence of a signal can be detected at an appropriate information rate. If the width of the track is approximately one micron, and that the space between adjacent tracks is also one micron, the quantity of energy necessary to impart information from the disc can be determined. It is necessary to provide sufficient radiant energy to "illuminate" a "spot" of approximately one micron in diameter and, at the same time, provide sufficient radiant energy at the detector, so that the "presence" or "absence" of a signal can be distinguished.
It has been discovered, in attempting to utilize the transmitted radiation techniques of the prior art, that the provision of an inordinately large amount of radiation into the system is required in order to "transmit" a sufficiently useful ` increment of energy for detection through the record. It has has also been determined that a substantial magnification is
VIDEO DISCS
Systems have heretofore been,developed for repro-ducing signals at video frequencies from information recorded on discs, tapes, or other media. Such systems have utilized, among other things, optical recordings upon photosensitive discs, electron beam recording on thermo plastic surfaces and, in prior patents assigned to the assignee~of the present invention, systems u~ilizing a rotating disc which is responsive to impinging radiation to reflect or transmit radiation correspond-ing to and representative of the information stored on the surface of the disc.
For example, in U.S. Patent No. 3,530,258, issued to David Paul Gregg and Reith O. Johnson on September 22, 1970, there was shown and described a system in which a video signal transducer included a servo controlled pair of flexible, fibre optic elements. An air bearing supported an objective lens system. A light source of radiant energy was positioned below the disc and the transducer was responsive to transmitted light.
Other patents have shown the use of a radiant source which directed an energy beam to the surface of the disc and provided a transducer that was responsive to reflected energy.
One-of the major problems to be encountered in the recording and reproduction of video information, arises directly from a consideration of the energy levels involved in such a process and the restraints imposed by the considerations of size, weight and operating conditions.
, To be commercially desirable as a home instrument, the system should be able to store and reproduce a "program"
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of at least 15 to 30 minutes in length. The record disc should be of an easily handled size, comparable to the phonograph records currently in use. If the playback turntable was operated at 1800 rpm, some 54,000 revolutions would provide 30 minutes of playback. Assuming a 1 micron track width and 1 micron spacing between adjacent tracks, a circular band approximately 4.25 inches wide is required. Assuming that the smallest radius at which informatio~ can be stored is approxi-mately three inches, the resultant disc is about 15 inches in diameter. The duration of the program or the speed of the turn-table can change the dimensions of the recorded area, as can the width of the individual track and the spacing between adjacent tracks.
Assuming that the video information has been recorded in some digital fashion, the presence or absence of a signal can be detected at an appropriate information rate. If the width of the track is approximately one micron, and that the space between adjacent tracks is also one micron, the quantity of energy necessary to impart information from the disc can be determined. It is necessary to provide sufficient radiant energy to "illuminate" a "spot" of approximately one micron in diameter and, at the same time, provide sufficient radiant energy at the detector, so that the "presence" or "absence" of a signal can be distinguished.
It has been discovered, in attempting to utilize the transmitted radiation techniques of the prior art, that the provision of an inordinately large amount of radiation into the system is required in order to "transmit" a sufficiently useful ` increment of energy for detection through the record. It has has also been determined that a substantial magnification is
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1071~68 required to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a light source illuminates the entire field which can be scanned by the detector under control of the servo system, it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity to register upon the photosensitive device.
In a preferred embodiment of the present invention, an articulated mirror is utilized in conjunction with a second ` mirror to provide multiple reflecting paths. With a plurality of reflections, assuming the use of a highly collimated source, small amounts of mirror motion are necessary to move the point of impingement of the radiant spot upon the disc. Moreover, a plurality of reflections provides a longer optical path which - enables the use of longer focal-length lenses, for directing a radiant spot to the disc and for focusing the image of the re-flected spot upon the photosensitive transducer.
An important aspect of the present invention is the ability to direct the illuminating radiation to a particular spot and to return the information from the spot thus illuminated I to a detector system. The prior art has suggested the use of a pair of transducers in conjunction with a summing amplifier to provide signal information and a differential amplifier to provide feedback servo information for error correction. However, ~ given the limitations of the extremely low radiation levels, ; the diffraction limited characteristics of the image and the extreme sensitivity of the system to noise and vibration, such an approach is not entirely satisfactory. A difference "curve
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1071~68 required to enable a state-of-the-art transducer to respond to a one micron diameter radiant spot.
If a light source illuminates the entire field which can be scanned by the detector under control of the servo system, it will be seen that an extraordinary light intensity must be provided before the light transmitted through or reflected from the disc will be of sufficient intensity to register upon the photosensitive device.
In a preferred embodiment of the present invention, an articulated mirror is utilized in conjunction with a second ` mirror to provide multiple reflecting paths. With a plurality of reflections, assuming the use of a highly collimated source, small amounts of mirror motion are necessary to move the point of impingement of the radiant spot upon the disc. Moreover, a plurality of reflections provides a longer optical path which - enables the use of longer focal-length lenses, for directing a radiant spot to the disc and for focusing the image of the re-flected spot upon the photosensitive transducer.
An important aspect of the present invention is the ability to direct the illuminating radiation to a particular spot and to return the information from the spot thus illuminated I to a detector system. The prior art has suggested the use of a pair of transducers in conjunction with a summing amplifier to provide signal information and a differential amplifier to provide feedback servo information for error correction. However, ~ given the limitations of the extremely low radiation levels, ; the diffraction limited characteristics of the image and the extreme sensitivity of the system to noise and vibration, such an approach is not entirely satisfactory. A difference "curve
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following" technique described in the patent to W.D. ~unro, U.S. Patent No. 2,838,683, issued June 10, 1958, has suggested an alternative solution.
According to the present invention there is provided an apparatus arranged to read information recorded in an information track on a reflective surface of a disc, the apparatus having source means for producing a reading beam of light radiation, beam directing means for directing the reading beam from the source means along the beam path to the information track, the reading beam being modulated and reflected by the information track to retrace at least a portion of the path, and sensing means for receiving the reflected beam of radiation from the information track. The beam directing means includes beam separating means positioned along the portion of the path common to the reading beam and the reflected beam for directing the reflected beam out of the path of the reading beam and toward the sensing means.
In a specific embodiment of the invention, the beam separating means may include a beam splitting prism and a quarter wave plate disposed between the prism and the disc.
The beam directing means may include optical means for directing the reading beam from the source means along a folded U-shaped optical path and then onto the disc, the common path portion including at least a part of one leg of the U-shaped path. The source may include a laser beam generator for producing the reading beam of radiation.
According to another aspect of the invention, there is provided a method for reading information recorded in an information track on a reflective surface of a disc, the method !
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' including producing a reading beam from a source of light radiation, directing the reading beam from the source along the beam path to the information track, the reading beam being modulated and re1ected by the information track to retrace at least a portion of the path and sensing the reflected beam of radiation from the information track. The beam directing step includes directing the reflected beam out of the portion of the path common to the reading beam and the reflected beam, to be sensed in the sensing step.
In a specific embodiment of the invention the step of directing the reflected beam out of the path portion includes optically separating the reflected beam from the reading beam. The beam separating step may include passing the reading beam in the non-reflected direction through a beam splitting prism and passing the emerging beam through a quarter wave plate prior to impinging on the beam of the disc, passing the reflected beam through the ~uarter wave plate in opposing direction to the reading beam, and passing the reflected beam emerging from the quarter wave plate to a reflective surface of the beam splitting prism. The beam directing step may include optically directing the reading beam from the source of radiation l along a folded U-shaped optical path and then onto the disc, the `! common path portion defining at least a part of one leg of the U-shaped path.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which ~. .
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'; ' ; ! ' ' ' ; :' ' 10717~8 several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an idealized side view of a playback assembly according to the present invention, FIG. 2 is a more detailed block diagram of the elements in the optical playback system;
FIG. 3 is an idealized view of an alternative articulated mirror assembly;
FIG. 4 is a block diagram of a suitable detector and tracking circuit;
FIG. 5 is a block diagram of an optical detector of the prior art suitable for use in the present invention;
FIG. 6 is an enlarged side view of the optical head and air bearing assembly.
FIG. 7 is a top idealized view of a cam and follower assembly for controlling the bias on the air bearing assembly; and .,~ .
FIG. 8 is a side view of another alternative articulated mirror arrangement useful in the system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, there is shown, in side view, a playback assembly 10 suitable for use in the present invention. The playback assembly 10 includes a laser element 12 which moves with the playback assembly 10. It is, however, within the state-of-the-art to provide a stationary ~ !
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laser which is coupled optically to the movable assembly 10.
Preferably, the laser 12 provides coherent, polarized light~
A read head 14 is mounted in arm 16 of the playback assembly 10 .
A video disc 20, which has video information recorded upon it is mounted on a turntable 22, which is adapted to rotate the disc 20 at a relatively high speed. In the preferred embodiment, the turntable ~ ' .
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-' speed is set at 1800 rpm.
- Suitable video discs have been described and claimed in the patents to Gregg, Johnson, supra.
The playback assembly 10 is mounted on a rotatable element 24 which, in the view of FIG. 1, translates the reading head in the radial direction relative to the disc 20 and in an arc that is generally orthogonal to the plane of the drawing.
The laser 12 generates a reading beam 26 which - generally passes f~om the laser 12 through an optical system to the playback head 14. The beam is then directed to the surface of the disc 20 and returns through the pla~-back head 14 along the same optical path until a read assembly 28 is encountered.
~ The read assembly 28 is mounted on the arm 16.
; In operation, the laser directs a reading light beam 26 to the surface of the dlsc 20 through the optical system.
The information recorded upon the disc interacts with the impinging beam and a reflected beam is produced which contains the recorded information. The reflected light beam is returned to the optical system which "analyzes" the returned beam to determine whether the beam is properly tracking the signal channel.
^ If the electronics determine that the laser spot is not being directed to a predetermined area of the information : channel, appropriate servo signals are derived which, when applied to the read head 14, cause the point of impingement of the laser beam to shift in the radial direction to retain alignment with the track that is being read.
In an alternative embodiment, the driver for the rotatable element 24 for the playback asse~bly 10 can also be .
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- ~071768 controlled by the servo signals whlch changes the position of the laser spot. In yet other embodiments, a motor can be coupled to the turntable driver to provide a predetermined increment of radlal motion for each revolution of the turntable 22. In any case, the playback head 10 can be made to track the information channel recorded on the disc 20 with a "coarse"
adjustment being applied to the driver of the rotatable element 24 and a "fine" adjustment being applied to an articulated mirror, described in greater detail below.
~urning next to FIG. 2, there is shown a diagram of the elements of the reading system. The reading laser beam 26 is applied to a beam splitting prism 30. The prism 30 is rotated slightly with respect to the optical path. ~ lens 32 ` is provided to better form the beam 26 at the surface 20 and to;~ optimize the resolving power of the system. The transmitted 'r~ portion of the beam 26 is applied through a quarter wave plate - 36 and is then directed through the reading head 14 to the disc ;~ .
20.
A returning beam 38 containing the information from 2Q the disc 20 follows substantially the identical path. At the - quarter wave plate 36, the returning beam is now given an additional quarter wave shift for a total polarization of one-half wavelength. The returning beam 38 reaches the beam splitter 30 and is reflected therefrom to a suitable optical system 40. Light from the laser 12 that is initially reflected in the prism 30 and re-reflected from the base of the prism will, due to the slight rotation of the prism 30, be aimed at a point that wholly misses the detector 40. Moreover, the cumulative effect of the ~uarter wave plate which polarizes the _g_ ~ bm.
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` 1071768 returning beam by ~/2 substantially attenuates any transmitted component. What is transmitted is cross polarized with respect to the laser 12.
. The read head 14 includes a fluid-bearing member 50 which is adjacent to and supportive of a microscope objective lens 52. A limited amount of vertical adjustment is available in the objective lens 52. Directing the illumination to the objective lens 52 is an articulated mirror 54 which is mounted adjacent to and cooperates with a second or fixed mirror 56 that is substantially parallel with the articulated mirror 54.
The fixed mirror receives the reading beam 26 and directs it to the articulated mirror 54. 5 The reading beam 26 undergoes at least one reflection from the artlculated mirror 54 before the beam is applied to the-objective lens 52. Two such reflections.are illustrated in the embodiment of FIG. 2. Similarly, the beam path is such that a reflected beam 38 returning from the surface of the disc ~:
20 would also undergo two reflections from the articulated mirror 54 and two reflections from the fixed mirror 56 before :,,........ . . .~
proceeding into the optical path including an additional fixed ~ .
mirror 57 which ultimately leads to the read assembly 28. ~
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` In the embodiment illustrated, the articulated mirror 54 is mounted on a point pivot 58 that is centrally located with respect to the mirxor 54. The mi.rror 54 may have an oblong shape with the long axis in the plane of the drawing and the short axis orthogonal to the plane of the drawings. As shown, a mirror driver 60 is connected to one end of the mirror 54 and is operable to impart motion about the central pivot 58.
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~ ` 1071768 If the driver 60 rotates the mirror 54 in the clock- ;~
wise direction, as viewed in FIG. 2, the point of impingement of the read beam 26 will be shifted to the left. This would represent a deflection of the beam in a first radial direction.
If the driver 58 rotates the mirror 54 in the counter-clockwise - direction, then the point of impingement of the transmitted beam 26 will be shifted to the right, as seen in FIG. 2, or in a ` second, opposite radial direction.
It will be obvious that the reflected beam 38 and the reading beam 26 trace identical paths between the surface of ., the disc 20 and the beam splitter 30. The articulated mirror :. 54 serves to "steer" the reading spot to a desired location `~ and then "reads" only the illuminated area, transmitting that 1 information back to the read assembly 28.
:~ In alternative embodiments, the articulated mirror 54 and the stationary mirror 56 can be adjusted and repositioned . to provide a greater plurality of reflections between the two . , , mirrors before the beam continues either to or from the disc surface 20. In such an arrangement, the magnitude of mirror .. 20 deflection required to steer the reading spot appropriately can be greatly reduced. The driver 60 therefore, need only impart small, incremental motions to the articulated mirror 54.
In an alternative embodiment, as shown in FIG. 3, a first articulated mirror 54' is provided which is mounted on a central piVQt member 58', and is driven about an axis orthogonal to the plane of the FIGURE and in the clockwise and counter-clockwise direction by a first driver 60' that is coupled .to the mirror 54' at the end of a long axis.
A second driver 60" is coupled to one end of a third bm.
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- 107~768 mirror 54" for imparting rotational motion to the third mirror 54" about the long axis that is in the plane of the FIGURE.
In operation, the ~irst driver 60' permits translation of the beams in the "radial" direction to permit "fine" tracking of the information channel. The second driver 60" is uxed to translate the beam in the circumferential direction, to provide time synchronization, if desired, and to compensate for eccentricity.
In other embodiments, the problem of time synchroni- ' zation can be handled mathematically, as a step in the process , of electronically compensating for eccentricity of the disc 20 and in such embodiments, only the single articulated mirror is used.
Turning next to FIG. 4, there is shown a preferred embodiment of the optical detector assembly 40 which utilizes sQme of the electronics of the Munro patent, supra. As shown in FIG. 4, the returned optical image 38 is directed to impinge upon a photocell 70 when a channel is being tracked properly, with the spot on the outer half of the track, a predetermined output signal is generated. The output of the photocell 70 is applied to a comparator 72. An adjustable bias 74 is applied to the other input of the comparator 72 and is adjusted to provide a null when the predetermined output signal is being applied. The error signals resulting from drift can be inte-grated, and the output of the integrator can be applied to an appropriate circ~it to urge the movable playback assembly 10 relative to the center of the disc 20. The error signal is also used to apply a signal directly to the mirror driver 60 of FIG. 2 to urge the beam to follow the track.
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If, however, the track is not being followed properly, depending, of course, upon the characteristics of the disc surface, a condition will be presented in which the energy impinging upon the photocell 70 will be di~erent than the bias provided by bias circuit 74, and accordingly, the error signal of appropriate polarity will be provided to correct the position of the light spot relative to the information channel.
; The integrator output then is applied to the movable playback assembly 10, and if the bias signal is greater, a forcing function is generated tending to send the spot toward the periphery of the disc. If the received signal is greater, the ; spot is directed to the center of the disc. As the spot follows the spiral track properly, the differential output tends toward the null.
In FIG. 5, there is illustrated the prior art optical detector electronics utilized and shown as FIG. 10 in the previously issued Gregg, et al., U.S. Patent No. 3,530,258, assigned to the assi~nee of the present invention. For con- -venience, the same reference numbers are used in Gregg, et al and herein. A pair of photo detectors 96, 98 are employed which, in combination, provide an additive information signal and, when differenced, an error signal which controls servo elements that redirect the reading elements. As applied to the present invention, the radial error signal could be applied to either of the drivers 60, 60' of the articulated mirror assemblies of FIGS. 2 and 3, respectively.
As shown in FIG. 5, a dual photo detector has two sections 96, 98 whose outputs are applied to respective amplifiers 100, 101. The outputs of the amplifiers 100, 101 are bm.
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summed in a summing network 106. The output from the summing network represents the sum signal from the two photo detector sections 96, 98 and constitutes the modulated signal output of the transducer.
-~ The signal amplitude from the first photo detector section is applied to a detector 102, and this detector produces a negative unidirectional signal representative there-.,'!' of. ~he signal amplitude from the second photo detector ~ection lS applied to a detector 103, and the latter detector produces a negative unidirectional signal in response thereto. The two signals are added algebraically in a summing network 105 which produces an error signal.
In the present example, the resulting error signal is amplified in an amplifier 104, and it is applied to the circuits of FIG. 3 and driver 60'. The error signal applied to the -driver 60' causes the mirror 54' to shift the beams in a radial direction with respect to the disc 20, as explained above. The ~-direction and amount of the shift depends on the polarity and amplitude of the error signal, so as to maintain the spot in perfect registry with the recording track on the record 20.
The output signal from the summing network 106 is applied to appropriate video detection and reproducing circuitry such as illustrated in FIGS. 17 and 18 of Gregg et al, supra, and described therein.
The DC component of the output of the amplifier 104 when properly processed, may be used in several ways to move the pick-up arm of FIG. 1 across the disc 20 at very nearly the rate which makes the signal approach zero. One method is to integrate this component over short intervals until it reaches bm.
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a predetermined value, at which it triggers a solenoid. This solenoid, in turn, actuates a light-duty friction ratchet which then turns the pick-up arm through a very small angle as is ` taught in Gregg et.al, supra.
. Another method also suggested in G_egg et al, supra, :~ is to use an inexpensive electric clock movement with a reduction gear to drive the arm continuously across the disc ; at a rate just slightly above 2 microns for each 1/30 second or revolution of the disc. In this case, the integrated signal of . 10 the first method is used to interrupt the motor voltage occasionally. To assist the process, the arm 16 of FIG. 1 may . i .
be biased slightly towards the center of the disc 20.
In FIG. 6, there is shown an enlarged side view of the lens and air bearing assembly of the playback head 14. The movable arm 16 connects to the playback head 14 through a pair of parallel leaf springs 120, 122. The spring force of the leaf springs 120, 122 is generally insufficient to maintain the springs in the horizontal position with the playback head 14 unsupported by the fluid bearing that is generated by the rotating disc 20. Within the read head 14 is the fluid bearing member 50 and the microscope type objective lens 52. Also contained in the read head 14 are the fixed and articulated ; mirrors 54, 56, 57 necessary to direct the beam of light from the source to the lens 52 and back from the surface o~.the disc 20.
A support post 124 extends outward of the read head 14 toward the inner end of the arm 16, Mounted to this support post 124 is a bias spring 126, the other end of which is fastened to a lever 128. The lever 128 is coupled to the arm .
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16 and, through a flexible cable 130, connects to a cam and ` follower assembly 132, to be described in connection with FIG.
7, below.
Also included, but not described in detail, are appropriate interlocking solenoid assemblies operating in conjunction with the cam and follower assembly to maintain the .. i. .
read-head 14 out of contact with the disc 20 as the arm 16 swings out of engagement with the disc 20, and which act to prevent damage if, for any reason, the disc 20 should slow appreciably while being tracked by the read head 14.
. r~he bias spring 126, when compressed, acts like a solid rod, enabling the lever 128 to directly cam the read head :~ 14 upward and away from the disc 20, if this configuration is desired. Alternatively, when the read head 14 is in positian over thé disc, the le~er 128 rotates in the opposite direction, relieving the compression on the spring 126. Under normal circumstances, the weight of the read head 14 is supported by - fluid bearing member 50 on the disc, thereby enabling the leaf springs 120, 122 to be substantially parallel and horizontal.
. According to the present invention, an additional bias is provided through the use of the bias spring 126 to maintain a substantially constant separation between the read head 14 and the fluid bearing member 50 and the surface of the disc 20.
The relative surface velocity changes as.the moving arm 16 progresses toward the center of the disc and the fluid bearing is less able to support the read head. Therefore, at the outset, the lever 128 is rotated in the downward direction, applying a stretch to the spring 126 which, in turn, imparts a downward force to the support arm 124, thereby increasing the .. - : .
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107~7~;8 . ` ', bias on the ~luid bearing 50 while the fluid pressure is at .~ its greatest.
`: As the arm 16 moves inwardly of the disc 20 and the surface velocity is reduced, a cam follower arrangement .
:~ gradually rotates the lever 128 in the upward direction, reducing the tension of the spring 126, thereby lessening the bias on the read head 14. By selecting an appropriate cam contour, the bias on the fluid bearing S0 can be maintained at an optimum value for constant separation from the disc 20 for ~ 10 the surface velocity of the disc at any radial loca~ion~
. Turning now to FIG. 7, there is shown one form of cam and follower assembly 132 that can drive the lever 128 through the flexible cable 130 (also shown in FIG. 1). A cam 140 is cut so that at the outermost position of the arm 16, a follower 142 rests on a high lobe which maintains the head 14 in an "up"
position r safely out of contact with the edge of the rotating disc 20.
; As the arm 16 tracks inwardly, the follower 142 immediately proceeds to the innermost point on the cam 140 surface, applying maximum bias to the read head 14. As the arm then continues inwardly in the radial direction, the follower :~ 142 gradually rides outwardly from the center of the cam 140, thereby reducing the bias forces on the read head 14.
It is clear that techniques are readily available for transmitting simple mechanical motion from the cam follower assembly 132 to the arm 16, and the specific details are unnecessary in the present application.
In FIG. 8, there is shown an alternative configuration for the articulated mirror assembly that is mounted on the read bm.
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head 14. In this alternative embodiment, a fixed mirror lS0 and an articulated mirror 152 are arranged on converging planes.
An incoming beam in the horizontal direction impinges upon the articulated mirror 152, and through multiple reflection between the fixed mirror 150 and the articulated mirror 152, the beam is ultimately rotated through 90 and is directed downward into the reading assembly. Similarly, the returning beam retraces the same path. The mirror 152 is articulated to ::
rotate about an axis that is in the plane of the drawing to deflect the transmitted beam in a direction that is perpendicular - to the plane of the drawing.
The angle of incidence of the mirror 150 and the angle of convergence between the mirrors 150 and 152 are con-tolled so that the incoming beam makes a plurality of reflections off of the two mirrors before being directed into the disc.
Moreover, since the pair of mirrors, in addition to providing a "folded" light path, also rotates the beam through 90, a separate 45 mirror can be omitted, thereby increasing the intensity of available light to the disc. Of course, this would permit at least one extra reflection between the mirror pair without in any way degrading the quality of the light beam.
The same number of internal reflections as in the embodiment of FIG. 2 could be employed with less light loss in the mirror system.
Thus, there has been shown an improved video disc reading assembly which steers the illuminating radiation to the information track on the surface of the disc and steers the return signal from the track to an optical detector. An articulated mirror enables the steering of both the transmitted and the retuxned light beam.
, :: ~ . . .. . . . .
1071~768 :
An improved optical detector is utilized in combination with a fixed bias source so that a single detector provides both the information signal and the servo signals necessary to track the information channel.
A novel air bearing assembly has also been disclosed which enables a microscope lens to travel at a fixed distance . above the disc supported on a fluid bearing, and means are provided to impart a variable bias to the fluid bearing as a function of relative velocity between the disc and the bearing member.
~ . ~19-- .
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following" technique described in the patent to W.D. ~unro, U.S. Patent No. 2,838,683, issued June 10, 1958, has suggested an alternative solution.
According to the present invention there is provided an apparatus arranged to read information recorded in an information track on a reflective surface of a disc, the apparatus having source means for producing a reading beam of light radiation, beam directing means for directing the reading beam from the source means along the beam path to the information track, the reading beam being modulated and reflected by the information track to retrace at least a portion of the path, and sensing means for receiving the reflected beam of radiation from the information track. The beam directing means includes beam separating means positioned along the portion of the path common to the reading beam and the reflected beam for directing the reflected beam out of the path of the reading beam and toward the sensing means.
In a specific embodiment of the invention, the beam separating means may include a beam splitting prism and a quarter wave plate disposed between the prism and the disc.
The beam directing means may include optical means for directing the reading beam from the source means along a folded U-shaped optical path and then onto the disc, the common path portion including at least a part of one leg of the U-shaped path. The source may include a laser beam generator for producing the reading beam of radiation.
According to another aspect of the invention, there is provided a method for reading information recorded in an information track on a reflective surface of a disc, the method !
, ~ :.. . .. .
:, ;:: :
' including producing a reading beam from a source of light radiation, directing the reading beam from the source along the beam path to the information track, the reading beam being modulated and re1ected by the information track to retrace at least a portion of the path and sensing the reflected beam of radiation from the information track. The beam directing step includes directing the reflected beam out of the portion of the path common to the reading beam and the reflected beam, to be sensed in the sensing step.
In a specific embodiment of the invention the step of directing the reflected beam out of the path portion includes optically separating the reflected beam from the reading beam. The beam separating step may include passing the reading beam in the non-reflected direction through a beam splitting prism and passing the emerging beam through a quarter wave plate prior to impinging on the beam of the disc, passing the reflected beam through the ~uarter wave plate in opposing direction to the reading beam, and passing the reflected beam emerging from the quarter wave plate to a reflective surface of the beam splitting prism. The beam directing step may include optically directing the reading beam from the source of radiation l along a folded U-shaped optical path and then onto the disc, the `! common path portion defining at least a part of one leg of the U-shaped path.
The novel features which are believed to be characteristic of the invention, both as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings in which ~. .
., . - . ,.
'; ' ; ! ' ' ' ; :' ' 10717~8 several preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for the purpose of illustration only and are not intended as a definition of the limits of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is an idealized side view of a playback assembly according to the present invention, FIG. 2 is a more detailed block diagram of the elements in the optical playback system;
FIG. 3 is an idealized view of an alternative articulated mirror assembly;
FIG. 4 is a block diagram of a suitable detector and tracking circuit;
FIG. 5 is a block diagram of an optical detector of the prior art suitable for use in the present invention;
FIG. 6 is an enlarged side view of the optical head and air bearing assembly.
FIG. 7 is a top idealized view of a cam and follower assembly for controlling the bias on the air bearing assembly; and .,~ .
FIG. 8 is a side view of another alternative articulated mirror arrangement useful in the system of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
Turning first to FIG. 1, there is shown, in side view, a playback assembly 10 suitable for use in the present invention. The playback assembly 10 includes a laser element 12 which moves with the playback assembly 10. It is, however, within the state-of-the-art to provide a stationary ~ !
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laser which is coupled optically to the movable assembly 10.
Preferably, the laser 12 provides coherent, polarized light~
A read head 14 is mounted in arm 16 of the playback assembly 10 .
A video disc 20, which has video information recorded upon it is mounted on a turntable 22, which is adapted to rotate the disc 20 at a relatively high speed. In the preferred embodiment, the turntable ~ ' .
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, ; ' , : ' : : - . ~ .. .
-' speed is set at 1800 rpm.
- Suitable video discs have been described and claimed in the patents to Gregg, Johnson, supra.
The playback assembly 10 is mounted on a rotatable element 24 which, in the view of FIG. 1, translates the reading head in the radial direction relative to the disc 20 and in an arc that is generally orthogonal to the plane of the drawing.
The laser 12 generates a reading beam 26 which - generally passes f~om the laser 12 through an optical system to the playback head 14. The beam is then directed to the surface of the disc 20 and returns through the pla~-back head 14 along the same optical path until a read assembly 28 is encountered.
~ The read assembly 28 is mounted on the arm 16.
; In operation, the laser directs a reading light beam 26 to the surface of the dlsc 20 through the optical system.
The information recorded upon the disc interacts with the impinging beam and a reflected beam is produced which contains the recorded information. The reflected light beam is returned to the optical system which "analyzes" the returned beam to determine whether the beam is properly tracking the signal channel.
^ If the electronics determine that the laser spot is not being directed to a predetermined area of the information : channel, appropriate servo signals are derived which, when applied to the read head 14, cause the point of impingement of the laser beam to shift in the radial direction to retain alignment with the track that is being read.
In an alternative embodiment, the driver for the rotatable element 24 for the playback asse~bly 10 can also be .
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- ~071768 controlled by the servo signals whlch changes the position of the laser spot. In yet other embodiments, a motor can be coupled to the turntable driver to provide a predetermined increment of radlal motion for each revolution of the turntable 22. In any case, the playback head 10 can be made to track the information channel recorded on the disc 20 with a "coarse"
adjustment being applied to the driver of the rotatable element 24 and a "fine" adjustment being applied to an articulated mirror, described in greater detail below.
~urning next to FIG. 2, there is shown a diagram of the elements of the reading system. The reading laser beam 26 is applied to a beam splitting prism 30. The prism 30 is rotated slightly with respect to the optical path. ~ lens 32 ` is provided to better form the beam 26 at the surface 20 and to;~ optimize the resolving power of the system. The transmitted 'r~ portion of the beam 26 is applied through a quarter wave plate - 36 and is then directed through the reading head 14 to the disc ;~ .
20.
A returning beam 38 containing the information from 2Q the disc 20 follows substantially the identical path. At the - quarter wave plate 36, the returning beam is now given an additional quarter wave shift for a total polarization of one-half wavelength. The returning beam 38 reaches the beam splitter 30 and is reflected therefrom to a suitable optical system 40. Light from the laser 12 that is initially reflected in the prism 30 and re-reflected from the base of the prism will, due to the slight rotation of the prism 30, be aimed at a point that wholly misses the detector 40. Moreover, the cumulative effect of the ~uarter wave plate which polarizes the _g_ ~ bm.
.: . , -, . ,, -:. . : . :
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` 1071768 returning beam by ~/2 substantially attenuates any transmitted component. What is transmitted is cross polarized with respect to the laser 12.
. The read head 14 includes a fluid-bearing member 50 which is adjacent to and supportive of a microscope objective lens 52. A limited amount of vertical adjustment is available in the objective lens 52. Directing the illumination to the objective lens 52 is an articulated mirror 54 which is mounted adjacent to and cooperates with a second or fixed mirror 56 that is substantially parallel with the articulated mirror 54.
The fixed mirror receives the reading beam 26 and directs it to the articulated mirror 54. 5 The reading beam 26 undergoes at least one reflection from the artlculated mirror 54 before the beam is applied to the-objective lens 52. Two such reflections.are illustrated in the embodiment of FIG. 2. Similarly, the beam path is such that a reflected beam 38 returning from the surface of the disc ~:
20 would also undergo two reflections from the articulated mirror 54 and two reflections from the fixed mirror 56 before :,,........ . . .~
proceeding into the optical path including an additional fixed ~ .
mirror 57 which ultimately leads to the read assembly 28. ~
:.~
` In the embodiment illustrated, the articulated mirror 54 is mounted on a point pivot 58 that is centrally located with respect to the mirxor 54. The mi.rror 54 may have an oblong shape with the long axis in the plane of the drawing and the short axis orthogonal to the plane of the drawings. As shown, a mirror driver 60 is connected to one end of the mirror 54 and is operable to impart motion about the central pivot 58.
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~ ` 1071768 If the driver 60 rotates the mirror 54 in the clock- ;~
wise direction, as viewed in FIG. 2, the point of impingement of the read beam 26 will be shifted to the left. This would represent a deflection of the beam in a first radial direction.
If the driver 58 rotates the mirror 54 in the counter-clockwise - direction, then the point of impingement of the transmitted beam 26 will be shifted to the right, as seen in FIG. 2, or in a ` second, opposite radial direction.
It will be obvious that the reflected beam 38 and the reading beam 26 trace identical paths between the surface of ., the disc 20 and the beam splitter 30. The articulated mirror :. 54 serves to "steer" the reading spot to a desired location `~ and then "reads" only the illuminated area, transmitting that 1 information back to the read assembly 28.
:~ In alternative embodiments, the articulated mirror 54 and the stationary mirror 56 can be adjusted and repositioned . to provide a greater plurality of reflections between the two . , , mirrors before the beam continues either to or from the disc surface 20. In such an arrangement, the magnitude of mirror .. 20 deflection required to steer the reading spot appropriately can be greatly reduced. The driver 60 therefore, need only impart small, incremental motions to the articulated mirror 54.
In an alternative embodiment, as shown in FIG. 3, a first articulated mirror 54' is provided which is mounted on a central piVQt member 58', and is driven about an axis orthogonal to the plane of the FIGURE and in the clockwise and counter-clockwise direction by a first driver 60' that is coupled .to the mirror 54' at the end of a long axis.
A second driver 60" is coupled to one end of a third bm.
- . :, . : . ..
- ~. ~ :, ., .
. .
- 107~768 mirror 54" for imparting rotational motion to the third mirror 54" about the long axis that is in the plane of the FIGURE.
In operation, the ~irst driver 60' permits translation of the beams in the "radial" direction to permit "fine" tracking of the information channel. The second driver 60" is uxed to translate the beam in the circumferential direction, to provide time synchronization, if desired, and to compensate for eccentricity.
In other embodiments, the problem of time synchroni- ' zation can be handled mathematically, as a step in the process , of electronically compensating for eccentricity of the disc 20 and in such embodiments, only the single articulated mirror is used.
Turning next to FIG. 4, there is shown a preferred embodiment of the optical detector assembly 40 which utilizes sQme of the electronics of the Munro patent, supra. As shown in FIG. 4, the returned optical image 38 is directed to impinge upon a photocell 70 when a channel is being tracked properly, with the spot on the outer half of the track, a predetermined output signal is generated. The output of the photocell 70 is applied to a comparator 72. An adjustable bias 74 is applied to the other input of the comparator 72 and is adjusted to provide a null when the predetermined output signal is being applied. The error signals resulting from drift can be inte-grated, and the output of the integrator can be applied to an appropriate circ~it to urge the movable playback assembly 10 relative to the center of the disc 20. The error signal is also used to apply a signal directly to the mirror driver 60 of FIG. 2 to urge the beam to follow the track.
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If, however, the track is not being followed properly, depending, of course, upon the characteristics of the disc surface, a condition will be presented in which the energy impinging upon the photocell 70 will be di~erent than the bias provided by bias circuit 74, and accordingly, the error signal of appropriate polarity will be provided to correct the position of the light spot relative to the information channel.
; The integrator output then is applied to the movable playback assembly 10, and if the bias signal is greater, a forcing function is generated tending to send the spot toward the periphery of the disc. If the received signal is greater, the ; spot is directed to the center of the disc. As the spot follows the spiral track properly, the differential output tends toward the null.
In FIG. 5, there is illustrated the prior art optical detector electronics utilized and shown as FIG. 10 in the previously issued Gregg, et al., U.S. Patent No. 3,530,258, assigned to the assi~nee of the present invention. For con- -venience, the same reference numbers are used in Gregg, et al and herein. A pair of photo detectors 96, 98 are employed which, in combination, provide an additive information signal and, when differenced, an error signal which controls servo elements that redirect the reading elements. As applied to the present invention, the radial error signal could be applied to either of the drivers 60, 60' of the articulated mirror assemblies of FIGS. 2 and 3, respectively.
As shown in FIG. 5, a dual photo detector has two sections 96, 98 whose outputs are applied to respective amplifiers 100, 101. The outputs of the amplifiers 100, 101 are bm.
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summed in a summing network 106. The output from the summing network represents the sum signal from the two photo detector sections 96, 98 and constitutes the modulated signal output of the transducer.
-~ The signal amplitude from the first photo detector section is applied to a detector 102, and this detector produces a negative unidirectional signal representative there-.,'!' of. ~he signal amplitude from the second photo detector ~ection lS applied to a detector 103, and the latter detector produces a negative unidirectional signal in response thereto. The two signals are added algebraically in a summing network 105 which produces an error signal.
In the present example, the resulting error signal is amplified in an amplifier 104, and it is applied to the circuits of FIG. 3 and driver 60'. The error signal applied to the -driver 60' causes the mirror 54' to shift the beams in a radial direction with respect to the disc 20, as explained above. The ~-direction and amount of the shift depends on the polarity and amplitude of the error signal, so as to maintain the spot in perfect registry with the recording track on the record 20.
The output signal from the summing network 106 is applied to appropriate video detection and reproducing circuitry such as illustrated in FIGS. 17 and 18 of Gregg et al, supra, and described therein.
The DC component of the output of the amplifier 104 when properly processed, may be used in several ways to move the pick-up arm of FIG. 1 across the disc 20 at very nearly the rate which makes the signal approach zero. One method is to integrate this component over short intervals until it reaches bm.
;' ,` ' ~ ' ` ' ~ ' `' ' ' ` `:
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a predetermined value, at which it triggers a solenoid. This solenoid, in turn, actuates a light-duty friction ratchet which then turns the pick-up arm through a very small angle as is ` taught in Gregg et.al, supra.
. Another method also suggested in G_egg et al, supra, :~ is to use an inexpensive electric clock movement with a reduction gear to drive the arm continuously across the disc ; at a rate just slightly above 2 microns for each 1/30 second or revolution of the disc. In this case, the integrated signal of . 10 the first method is used to interrupt the motor voltage occasionally. To assist the process, the arm 16 of FIG. 1 may . i .
be biased slightly towards the center of the disc 20.
In FIG. 6, there is shown an enlarged side view of the lens and air bearing assembly of the playback head 14. The movable arm 16 connects to the playback head 14 through a pair of parallel leaf springs 120, 122. The spring force of the leaf springs 120, 122 is generally insufficient to maintain the springs in the horizontal position with the playback head 14 unsupported by the fluid bearing that is generated by the rotating disc 20. Within the read head 14 is the fluid bearing member 50 and the microscope type objective lens 52. Also contained in the read head 14 are the fixed and articulated ; mirrors 54, 56, 57 necessary to direct the beam of light from the source to the lens 52 and back from the surface o~.the disc 20.
A support post 124 extends outward of the read head 14 toward the inner end of the arm 16, Mounted to this support post 124 is a bias spring 126, the other end of which is fastened to a lever 128. The lever 128 is coupled to the arm .
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-~ ~071768 ~ .
16 and, through a flexible cable 130, connects to a cam and ` follower assembly 132, to be described in connection with FIG.
7, below.
Also included, but not described in detail, are appropriate interlocking solenoid assemblies operating in conjunction with the cam and follower assembly to maintain the .. i. .
read-head 14 out of contact with the disc 20 as the arm 16 swings out of engagement with the disc 20, and which act to prevent damage if, for any reason, the disc 20 should slow appreciably while being tracked by the read head 14.
. r~he bias spring 126, when compressed, acts like a solid rod, enabling the lever 128 to directly cam the read head :~ 14 upward and away from the disc 20, if this configuration is desired. Alternatively, when the read head 14 is in positian over thé disc, the le~er 128 rotates in the opposite direction, relieving the compression on the spring 126. Under normal circumstances, the weight of the read head 14 is supported by - fluid bearing member 50 on the disc, thereby enabling the leaf springs 120, 122 to be substantially parallel and horizontal.
. According to the present invention, an additional bias is provided through the use of the bias spring 126 to maintain a substantially constant separation between the read head 14 and the fluid bearing member 50 and the surface of the disc 20.
The relative surface velocity changes as.the moving arm 16 progresses toward the center of the disc and the fluid bearing is less able to support the read head. Therefore, at the outset, the lever 128 is rotated in the downward direction, applying a stretch to the spring 126 which, in turn, imparts a downward force to the support arm 124, thereby increasing the .. - : .
bm.
107~7~;8 . ` ', bias on the ~luid bearing 50 while the fluid pressure is at .~ its greatest.
`: As the arm 16 moves inwardly of the disc 20 and the surface velocity is reduced, a cam follower arrangement .
:~ gradually rotates the lever 128 in the upward direction, reducing the tension of the spring 126, thereby lessening the bias on the read head 14. By selecting an appropriate cam contour, the bias on the fluid bearing S0 can be maintained at an optimum value for constant separation from the disc 20 for ~ 10 the surface velocity of the disc at any radial loca~ion~
. Turning now to FIG. 7, there is shown one form of cam and follower assembly 132 that can drive the lever 128 through the flexible cable 130 (also shown in FIG. 1). A cam 140 is cut so that at the outermost position of the arm 16, a follower 142 rests on a high lobe which maintains the head 14 in an "up"
position r safely out of contact with the edge of the rotating disc 20.
; As the arm 16 tracks inwardly, the follower 142 immediately proceeds to the innermost point on the cam 140 surface, applying maximum bias to the read head 14. As the arm then continues inwardly in the radial direction, the follower :~ 142 gradually rides outwardly from the center of the cam 140, thereby reducing the bias forces on the read head 14.
It is clear that techniques are readily available for transmitting simple mechanical motion from the cam follower assembly 132 to the arm 16, and the specific details are unnecessary in the present application.
In FIG. 8, there is shown an alternative configuration for the articulated mirror assembly that is mounted on the read bm.
,., . ~ . ' .
head 14. In this alternative embodiment, a fixed mirror lS0 and an articulated mirror 152 are arranged on converging planes.
An incoming beam in the horizontal direction impinges upon the articulated mirror 152, and through multiple reflection between the fixed mirror 150 and the articulated mirror 152, the beam is ultimately rotated through 90 and is directed downward into the reading assembly. Similarly, the returning beam retraces the same path. The mirror 152 is articulated to ::
rotate about an axis that is in the plane of the drawing to deflect the transmitted beam in a direction that is perpendicular - to the plane of the drawing.
The angle of incidence of the mirror 150 and the angle of convergence between the mirrors 150 and 152 are con-tolled so that the incoming beam makes a plurality of reflections off of the two mirrors before being directed into the disc.
Moreover, since the pair of mirrors, in addition to providing a "folded" light path, also rotates the beam through 90, a separate 45 mirror can be omitted, thereby increasing the intensity of available light to the disc. Of course, this would permit at least one extra reflection between the mirror pair without in any way degrading the quality of the light beam.
The same number of internal reflections as in the embodiment of FIG. 2 could be employed with less light loss in the mirror system.
Thus, there has been shown an improved video disc reading assembly which steers the illuminating radiation to the information track on the surface of the disc and steers the return signal from the track to an optical detector. An articulated mirror enables the steering of both the transmitted and the retuxned light beam.
, :: ~ . . .. . . . .
1071~768 :
An improved optical detector is utilized in combination with a fixed bias source so that a single detector provides both the information signal and the servo signals necessary to track the information channel.
A novel air bearing assembly has also been disclosed which enables a microscope lens to travel at a fixed distance . above the disc supported on a fluid bearing, and means are provided to impart a variable bias to the fluid bearing as a function of relative velocity between the disc and the bearing member.
~ . ~19-- .
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Claims (21)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus arranged to read information recorded in an information track on a reflective surface of a disc, com-prising: source means for producing a reading beam of light radiation; beam directing means for directing said reading beam from said source means along a beam path to said information track, said reading beam being modulated and reflected by said information track to retrace at least a portion of said path;
and sensing means for receiving said reflected beam of radiation from said information track; said beam directing means in-cluding beam separating means positioned along said portion of said path common to said reading beam and said reflected beam for directing said reflected beam out of the path of said reading beam and toward said sensing means.
and sensing means for receiving said reflected beam of radiation from said information track; said beam directing means in-cluding beam separating means positioned along said portion of said path common to said reading beam and said reflected beam for directing said reflected beam out of the path of said reading beam and toward said sensing means.
2. Apparatus as claimed in Claim l, wherein said beam separating means comprises: a beam splitting prism; and a quarter wave plate disposed between said prism and said disc.
3. Apparatus as claimed in Claim l, wherein said beam directing means includes optical means for directing said reading beam from said source means along a folded U-shaped optical path and then onto said disc, said common path portion including at least a part of one leg of said U-shaped path.
4. Apparatus as claimed in Claim l, wherein said source means comprises a laser beam generator for producing said reading beam of radiation.
5. Apparatus as claimed in Claim 3, wherein said optical means is arranged for directing said reading beam along first and third path portions parallel to the disc surface, a second path portion joining one end of said first path portion with one end of said third path portion, and a fourth path portion joining the end of said third path portion remote from said second path portion with said informa-tion track, the end of said first path portion remote from said second path portion joining with said source means.
6. Apparatus as claimed in Claim 5, wherein said first and third path portions of said folded path are parallel to each other.
7. Apparatus as claimed in Claim 5, wherein said fourth path portion is perpendicular to said first and third path portions and perpendicular to said disc surface.
8. Apparatus as claimed in Claim 7, including path adjusting means in said fourth path portion for altering the path of said reading beam in relation to said disc surface to cause said reading beam to shift laterally across said information track.
9. Apparatus as claimed in Claim 8, wherein said path adjusting means includes means for adjusting the path of said reading beam in relation to said disc surface to cause said reading beam to move along said information track.
10. Apparatus as claimed in Claim 8, including a microscope-type objective lens in said fourth path portion between said path adjusting means and said disc, said objective lens being adjustable along said fourth path portion to focus said beam on said infor-mation track and to pass said reflected beam along said common path portion to said sensing means.
11. Apparatus as claimed in Claim 5, wherein said sensing means is positioned in said third path portion,
12. A method for reading information recorded in an information track on a reflective surface of a disc, comprising: producing a reading beam from a source of light radiation; directing the reading beam from the source along a beam path to the information track, the reading beam being modulated and reflected by the information track to retrace at least a portion of said path; and sensing the reflected beam of radia-tion from the information track, said beam directing step including directing the reflected beam out of said portion of said path common to the reading beam and the reflected beam, to be sensed in said sensing step.
13. The method as claimed in Claim 12, wherein said step of directing the reflected beam out of sald common path portion includes optically separ-ating the reflected beam from the reading beam.
14. The method as clalmed in Claim 13, wherein said beam separating step incIudes: passing the reading beam in the non-reflected direction through a beam splitting prism and passing the emerg-ing beam through a quarter wave plate prior to im-plngement of the beam on the disc; passing the reflected beam through said quarter wave plate in opposing direction to the reading beam; and passing the re-flected beam emerging from the quarter wave plate to a reflective surface of the beam splitting prism.
15. The method as claimed in Claim 12, wherein said beam directing step included optically directing the reading beam from the source of radiation along a folded U-shaped optical path and then onto the disc, said common path portion defining at least a part of one leg of said U-shaped path.
16. The method as claimed in Claim 15, wherein said beam directing step included optically directing the reading beam along the path to produce first and third path portions parallel to the disc surface, a second path portion Joining one end of said first path portion with one end of said third path portion, and a fourth path portion joining the end of said third path portion remote from said second path portion with said information track, the end of said first path portion remote from said second path portion Joining with said source means.
17. The method as claimed in Claim 16, wherein said first and third path portions of said folded path are parallel to each other.
18. The method as claimed in Claim 16, wherein said fourth path portion is arranged perpendicular to said first and third path portions and perpendicular to said disc surface.
19. The method as claimed in Claim 18, in-cluding the step of altering the path of the reading beam in said fourth path portion relative to the disc surface to cause the reading beam to shift laterally across the information track.
20. The method as claimed in Claim 19, wherein said path altering step includes adjusting the path of the reading beam in relation to the disc surface to cause the reading beam to move along said information track.
21. The method as claimed in Claim 19, in-cluding the steps of: positioning a microscope-type objective lens in said fourth path portion adjacent said disc; and adjusting the objective lens along said fourth path portion to focus the beam on the information track and to pass the reflected beam along said common path portion to be sensed in said sensing step.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA314,705A CA1071768A (en) | 1972-10-24 | 1978-10-30 | Light beam path for reading video discs |
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US00299893A US3829622A (en) | 1972-10-24 | 1972-10-24 | Video disc player with variably biased pneumatic head |
| CA314,705A CA1071768A (en) | 1972-10-24 | 1978-10-30 | Light beam path for reading video discs |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1071768A true CA1071768A (en) | 1980-02-12 |
Family
ID=25668809
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA314,705A Expired CA1071768A (en) | 1972-10-24 | 1978-10-30 | Light beam path for reading video discs |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1071768A (en) |
-
1978
- 1978-10-30 CA CA314,705A patent/CA1071768A/en not_active Expired
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