Multi-bandwidth optical playback apparatus having minimum baseband distortion

An optical playback system is used to recover data recorded in an elongated information track, comprising undulations representative of the data, on a surface of a record medium. The data is recorded over two substantially different frequency bands, for example, a video signal may be recorded over a high frequency deviation range and a sound signal may be recorded over a low frequency deviation range. The optical playback system is provided with a dual response characteristic. The response of the system to the low frequency signals is improved by utilizing a properly oriented linearly polarized readout light beam for recovering the low frequency signals, by using a light source that provides a relatively short wavelength light beam, and by providing an input stop for varying the effective numerical aperture of the objective lens and an output stop which is matched to the shape of the input stop.

This invention relates to an apparatus for optically reading information 
recorded on the surface of a record medium and, more particularly, to 
apparatus for reading information which has been recorded over a 
relatively wide frequency band, for example, for reading video information 
recorded over a high frequency deviation range and audio information 
recorded over a relatively low frequency deviation range. 
BACKGROUND OF THE INVENTION 
The playback principles of the present invention are applicable to the 
recovery of data recorded in an information track as a succession of 
undulations of varying length along the length of the track. 
In certain high density information playback systems, video information is 
recorded as relatively short wavelength (e.g., 0.4 .mu.m relief variations 
along the length of an information track. Illustratively, the method of 
recording may be of the type shown in U.S. Pat. No. 4,044,379, issued to 
J. B. Halter. Pursuant to the Halter method, an electromechanically driven 
stylus (e.g., of diamond) responsive to a combined video and audio signal, 
records relatively short geometric variations representative of the time 
variations of the signal in a metal substrate. After the electromechanical 
recording operation, the metal substrate has a relief pattern 
corresponding to that which is desired in the final record. Stampers which 
are used to produce production line records are made from the substrate 
and a vinyl record is formed, having the desired relief pattern, from the 
stamper. 
In one illustrative format for electromechanical cutting disclosed in the 
Halter patent, an encoded video signal is additively combined with the 
accompanying encoded audio signal. In accordance with this method, the 
accompanying encoded audio signal is obtained by causing the audio signal 
to frequency modulate a low frequency sound carrier over a low frequency 
deviation range (illustratively, 716 .+-.50 KHz). The encoded video signal 
is obtained from a picture modulator, wherein the composite color video 
signal (including luminance signals occupying a given band of frequencies 
and chrominance signals appearing as sideband components of a modulated 
chrominance subcarrier interleaved with the luminance signal components in 
an intermediate region of the given band) is caused to frequency modulate 
a high frequency picture carrier over a high frequency deviation range 
illustratively, 4.3-6.3 MHz). The peak to peak amplitude of the sound 
modulator output is held at a level which is small relative to the 
peak-to-peak amplitude level of the picture modulator output, with an 
illustrative level ratio being 1:10. The respective modulated carriers are 
combined in a linear adder and applied to a recorder which may be a Halter 
electromechanical recorder controlled in response to the signal developed 
by the adder. The recorder is used to record the composite signal as 
geometric variations (i.e., undulations) on the metal substrate. 
The specification of the sound carrier recorded on a video disc is 
generally critical to the performance of the video disc system. The 
peak-to-peak amplitude of the sound carrier recorded on a high density 
information record, such as a video disc described in U.S. Pat. No. 
3,842,194 to J. K. Clemens is very small--illustratively, the sound 
carrier amplitude may be 85 .ANG. peak-to-peak. Deviation of the amplitude 
of the sound carrier from that which is specified may adversely affect the 
quality of the video and audio reproduction. For example, if the sound 
carrier is not cut deep enough the signal-to-noise ratio may be degraded 
or, on the other hand, if it is cut too deep then sound beats may be 
visible during the video reproduction. 
To assure high quality video and audio reproduction during disc playback, 
it is generally agreed that certain measurements regarding the quality of 
the information recorded on the metal substrate should be made prior to 
producing production line records. The present invention provides an 
optical playback apparatus that may be used for reproducing, and thus 
veryifying, the information recorded according to the Halter 
electromechanical recording method. 
In its simplest form, the surface pattern of a metal substrate can be 
considered as a set of adjacent and parallel one-dimensional gratings with 
no guard space between the adjacent gratings. The gratings correspond to 
the signal tracks. The video signal wavelengths, especially on the inner 
radius signal tracks, are much smaller than the track-to-track spacings. 
Illustratively, the track spacing is about 2.5 .mu.m. 
An objective lens of an optical playback system of numerical aperture 
(N.A.) whose aperture is fully illuminated with a plane wave of light 
having wavelength .lambda. produces at its focal plane a focused spot such 
that about one-half of the optical power is within a circle of diameter D 
where 
EQU D=.lambda./2NA (1) 
The illuminating optics of the optical playback apparatus should be chosen 
such that the focused spot diameter is small enough to both resolve the 
shortest wavelength of interest and to maintain adjacent track crosstalk 
at an acceptably low level. Therefore, in practice extremely high 
numerical apertures (e.g., NA&gt;0.8) must be used to resolve the smallest 
signal wavelengths on the metal substrates. 
One optical playback system for reading a metal substrate having signals 
cut according to a Halter method is described in U.S. Pat. No. 4,065,786 
issued on Dec. 27, 1977 to W. C. Stewart. According to the Stewart system, 
the differential phase representative of the recorded information of a 
light beam reflected from the metal substrate surface is detected by a 
split photodetector. Thus, the output signal from the split photodetector 
is representative of the signal recorded on the metal substrate surface. 
The frequency response of a differential phase optical playback system to 
sine wave signals may be approximated by a triangular response 
characteristic having a peak response in the middle of the frequency band 
with a linear roll off to an upper and lower cutoff frequency. 
Ideally, lenses should be selected such that frequencies of the recorded 
information occur in the vicinity of the peak response of the optical 
system. However, it is very difficult to provide an optical system having 
a uniform response to wideband signals. For example, in a recording system 
where slot shaped signal elements are recorded on a flat surface, if the 
differential phase optical readout system is operated with a uniformly 
illuminated diffraction limited objective lens having a rectangular 
aperture and the lens is chosen so that the optical readout system is 
optimized for a video signal of 5 MHz at a particular radius, the response 
of the system to the 716 KHz audio signal will be about 17 dB lower. 
In accordance with U.S. patent application Ser. No. 242,250 entitled 
"Multi-Bandwidth Optical Playback Apparatus" filed on Mar. 10, 1981 for 
Istvan Gorog et al., now U.S. Pat. No. 4,375,096 (hereinafter, the Gorog 
apparatus) an optical playback apparatus is described for reproducing the 
information recorded on the metal substrate to verify the quality thereof. 
The Gorog apparatus includes a high numerical aperture lens for use in 
reading the information recorded. If the high numerical aperture objective 
lens performed in a truly ideal manner, i.e., as an ideal diffraction 
limited focusing device, then the reproduction of the information recorded 
would contain no signal distortions because signal distortions produced in 
one half of a split detector, as described in the aforementioned Stewart 
patent, would exactly cancel the signal distortions produced in the other 
half of the split detector. However, a real objective lens is not ideal 
and signal distortions that are the result of the imperfections of the 
lens system and are produced in the square-law detector are present in the 
reproduced output. The particular type of distortion of interest here is 
known as baseband distortion. This distortion, that can be produced by an 
imperfect real optical system when reading video signals encoded according 
to the Clemens patent, may significantly interfere with the recovery of 
the audio signals encoded according to the Clemens patent. To reduce these 
distortions, the Gorog apparatus provides a dual bandwidth apparatus for 
reading the information. In accordance with the Gorog apparatus the video 
information, recorded in accordance with the aforementioned Halter patent, 
is read out using an objective lens having a high numerical aperture while 
the audio information is readout using an objective lens having a low 
numerical aperture. The same objective lens is used to readout the video 
and audio but a stop is interposed in the readout beam path of the audio 
readout beam. The stop modifies the readout system by effectively reducing 
the numerical aperture of the objective lens. In other words, the stop 
acts as a filter for eliminating the video information that, as a result 
of baseband distortions produced by an imperfect real optical system, may 
produce interference with the recovery of the audio information. 
The Gorog apparatus is quite effective in reducing some of the video 
baseband distortions which appear in the audio channel, nevertheless, some 
distortions are still evident and may distract a viewer during 
verification of a metal substrate. For example, when certain structured 
scenes, i.e., a picket fence, recorded according to the Halter and Clemens 
patents on the inner disc radii are being readout with an optical system, 
using a circularly polarized 633 nm wavelength light beam so much baseband 
distortion may be produced in the audio channel that the audio information 
becomes unintelligible. 
Baseband distortions in optical readout systems may be thought of as two 
types. During optical playback of a metal substrate formed in accordance 
with the aforementioned Clemens format baseband distortion may be produced 
in an imperfect real optical system by the combination of the aperture 
response of the detection system and the non-linearity inherent in 
square-law optical detectors as indicated above. The distortion produced 
by the aperture and detector responses may be reduced or eliminated by the 
aforementioned Gorog apparatus. The second source of baseband distortion 
is the spatial frequency dependent phase shift that is exhibited by light 
diffracted from a grating wherein the ratio of the wavelength of the 
readout light beam to the period of the grating approaches, and possibly 
exceeds, unity. As the grating pitch varies with the content of the video 
information, phase shifts in the readout light beam used for recovery of 
the audio information effects signal distortions in the audio channel. 
This second source of baseband distortion has not been adequately 
compensated for in prior art devices. 
SUMMARY OF THE INVENTION 
In accordance with the principles of the present invention, an optical 
playback apparatus is provided wherein baseband distortions are minimized. 
In accordance with one aspect of the present invention, an optical playback 
system for recovering data from a disc-shaped storage medium is provided. 
In the system, the data is recorded on a surface of the storage medium 
along the length of a spiral information track. The data is recorded 
within a first given band of low frequencies and a second given band of 
high frequencies. The information track has a succession of undulations 
representative of the data along the length thereof. The undulations have 
a first given range of spatial wavelengths corresponding to the first 
given band of frequencies and a second given range of spatial wavelengths 
corresponding to the second given band of frequencies. The system includes 
a first light source which emits a beam of radiation of a first given 
wavelength. The beam of radiation is linearly polarized. A first light 
path couples the first light source and the surface of the disc. An 
objective lens, positioned in the first light path, focuses the beam of 
radiation to a spot on the information track, the spot has a dimension 
along the length of the information track. The shape of the light beam is 
altered by first means for modifying the shape of the light beam such that 
the dimension of the light spot along the length of the information track 
is greater than the diameter of a diffraction limited spot formed from the 
beam of radiation by the objective lens. In the system, relative motion is 
established between the light spot and the information track. Further, the 
system includes means, responsive to the interaction of the focused light 
and the undulations during the occurrence of the relative motion, for 
developing signals representative of the data recorded in the information 
track. A second light path couples the surface of the disc and the 
developing means. Positioned in the second light path is second means for 
modifying the shape of the light beam that is reflected off of the disc 
surface. The beam shape is modified such that light responsive to the 
interaction of the focused light and the undulations of the second given 
range of spatial wavelengths is blocked from reaching the developing 
means. 
In accordance with another aspect of the present invention, an optical 
playback system is provided which includes first and second light sources. 
The first light source emits a first beam of radiation of a first given 
wavelength which is linearly polarized. The second light source emits a 
second beam of radiation of a second wavelength. The first and second 
beams of radiation are combined by means for combining to form a single 
beam path from the combining means to the surface of the disc. An 
objective lens, positioned in the single beam path, focuses the beams of 
radiation to respective spots on the information track of the record 
medium. The light spot formed from the first beam of radiation has a 
dimension along the length of the information track. The shape of the 
light beam from the first light source is altered by first means for 
modifying, interposed between the first light source and the combining 
means. The shape of the beam is altered such that the dimension of the 
light spot formed from the first beam is greater than the diameter of a 
diffraction limited spot formed from the first beam by the objective lens. 
The system further includes first and second means for developing signals 
representative of the data recorded. Means are provided for directing the 
first beam such that the first beam is incident on the first means for 
developing and for directing the second beam such that it is incident on 
the second means for developing. Interposed between the means for 
directing and first means for developing is a second means for modifying 
the shape of the first beam of radiation. The second means for modifying 
blocks light responsive to the interaction of the light spot formed from 
the first beam and the undulations of the second given range of spatial 
wavelengths from reaching the first means for developing.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
Referring to FIG. 1, in the apparatus shown, a disc-shaped record carrier 
(substrate) 1 is rotated by a motor 3. Illustratively, record carrier 1 
may be of a type described in the aforementioned Halter patent, i.e., a 
metal substrate which has been recorded in accordance with the method 
described in the aforementioned Clemens patent. After the 
electromechanical recording operation, the recorded surface of the metal 
substrate 1 has a relief pattern corresponding to that which is desired in 
a final production record, i.e., the plastic replica disc. 
The apparatus of FIG. 1 provides a two-beam system, one beam for verifying 
the video information cut on the metal substrate and the other beam for 
verifying the low frequency audio information. A source of radiation 5 
emits a beam of light 7. Illustratively, radiation source 5 may be a 
helium-cadmium laser emitting a beam of linearly polarized coherent light 
of generally circular contour at a wavelength of 442 nm. The beam of light 
7 enters beam expander 9 and emerges generally with no change in shape but 
enlarged in cross section. This enlarged light beam enters attenuator 11. 
Attenuator 11 which may be a neutral density filter reduces the amount of 
light reflected from carrier 1 that can reach laser 5 to effect unwanted 
feedback. The beam 7 passes through the aperture 13 of stop 15. A side 
view of stop 15 is shown in auxiliary view 12. The operation of stop 15 
will be explained in detail herein. The enlarged light beam enters the 
entrance face of beam combiner 17 and emerges therefrom. Beam combiner 17 
may be a dichroic element that passes light of one wavelength and reflects 
light of another wavelength. The light beam is incident on tracking mirror 
19 which reflects the beam to pass through objective lens 21. Lens 21, 
interposed between tracking mirror 19 and substrate 1, receives the light 
beam and focuses the beam to form a light spot on an information track of 
substrate 1. Illustratively, lens 21 may be maintained in a focus position 
by means as described in copending U.S. patent application Ser. No. 
218,073, filed on Dec. 19, 1980 for M. J. Lurie, now U.S. Pat. No. 
4,376,303. Further, light beams 7 may be directed to track the center of 
an information track on the surface of substrate 1 by use of a quadrant 
detector described in the aforementioned Stewart patent. 
A second source of radiation 23 emits a beam of light 25. Illustratively, 
radiation source 23 may be a helium-neon laser emitting a beam of coherent 
light of generally circular contour at a wavelength of 633 nm. Beam 25 
passes through beam expander 27 and attenuator 29. Beam expander 27 and 
attenuator 29 perform the same functions as beam expander 9 and attenuator 
11 perform with respect to beam 7. Expanded beam 25 is reflected off of 
mirror 31 to be incident on beam combiner 17. Beam combiner 17, in this 
case, reflects beam 25 to follow the path of beam 7 to the surface of 
substrate 1. 
The focused light beams 7 and 25 are diffracted by the recorded data which 
appears in an information track as a succession of undulations. The 
reflected light is collected by lens 21 and is transmitted through 
tracking mirror 19 on the second pass. The reflected light arrives at beam 
splitter 33. Beam splitter 33, which may be a dichroic element which 
reflects one wavelength of light and transmits a second wavelength of 
light, separates the combined light beams. Light beam 7 is reflected in a 
direction orthogonal to the incident path and light beam 25 is transmitted 
through beam splitter 33. From beam splitter 33 light beam 7 passes 
through the aperture in stop 35, the operation of stop 35 will be 
explained in detail herein, to impinge onto a light accepting region of 
light detector 37. Light beam 25 impinges on a light accepting region of 
light detector 39. Illustratively, detectors 37 and 39 may be split 
photodetectors of a type illustrated in the aforementioned Stewart patent. 
The output signals from detectors 37 and 39 are delivered to suitable 
circuitry (not shown) for processing and subsequent display on a 
television receiver (for example). 
An explanation of the operation of the optical playback system will now be 
made. FIG. 2A represents a V-shaped signal track 30 having undulations 32 
recorded therein at a spatial wavelength F.sub.H. For ease of 
illustration, the undulations of FIGS. 2A and 2B are shown as having 
steep, square wave-like transitions. The following discussion applies 
equally to sinusoidal undulations such as in the preferred Halter method. 
As a matter of fact, wherever reference is made to spatial wavelength 
(F.sub.H, F.sub.L), it is to be understood that the reference is being 
made to a sinusoidal Fourier component having spatial wavelength F.sub.H, 
F.sub.L. Illustratively, the spatial wavelength F.sub.H may be in the 
middle of the video bandwidth described in Halter (e.g., 5 MHz of a 
Clemens type video disc recorded at the inside radius of the substrate). 
The read beam of this system is shown as a light spot 34 impining in the 
center of signal track 30. Preferably light spot 34 is focused according 
to equation 1 as a diffraction limited spot on information track 30. To 
readout the video information lens 21 of FIG. 1 must have a high numerical 
aperture, typically, greater than 0.8, and light beam 25 must completely 
fill the aperture of lens 21. 
Referring to FIG. 3, curve 50 indicates a triangular frequency response 
which is an approximation of the complex response of the optical system of 
FIG. 1 when the aperture of lens 21 is fully illuminated. The aperture is 
chosen such that the peak response 1/F.sub.H occurs near the middle of the 
high frequency band (illustratively, at 5 MHz for the Clemens system). The 
problem with such an arrangement is that when the aperture is chosen to 
optimize the high frequency response, the low frequency response suffers. 
For example, with respect to the Clemens disc, when the system is chosen 
for peak response in the middle of the video band (e.g., 5 MHz) the 
response in the audio band is down almost 20 dB, thus adversely affecting 
the measurement of test bands on a Halter substrate or providing an audio 
response which is substantially degraded. 
In accordance with the Gorog apparatus, the optical playback system is 
provided with an improved response at low frequencies. According to Gorog, 
a system is provided that has two differently dimensioned illumination 
systems and, therefore, a substantial improvement in signal response at 
the low frequency end is realized. 
Referring to FIG. 2B, a signal track 30' is shown having undulations 32' 
recorded therein at a spatial wavelength F.sub.L. Illustratively, the pit 
spacing at low frequencies may be seven times the pit spacing at high 
frequencies. According to the Halter system, the spatial wavelength 
F.sub.L may be at the audio frequency (e.g., 716 KHz of a Clemens type 
video disc). 
The Gorog apparatus discloses that the signal-to-noise ratio of the optical 
system response at low frequencies may be improved if the numerical 
aperture of lens 21 is reduced for reading the low frequency signals. For 
example, if a slit is used to modify the spot dimensions the S/N is 
improved. The slit has the effect of reducing the amount of non-signal 
bearing light and relocating the signal bearing light to the center of the 
aperture. Both of these consequences effected an improvement of the system 
performance. The Gorog apparatus includes a stop having a slit-shaped 
opening interposed between laser 5 and substrate 1, i.e., stop 15. With 
stop 15 positioned as shown in FIG. 1, light beam 7 passes through slit 
13, thus shaping the light beam which passes through to objective lens 21. 
By interposing slit 13 in the light beam path the effective numerical 
aperture of lens 21 is reduced in the signal direction and thus the light 
spot 34' on the surface of the substrate is extended along the length of 
the information track as shown in FIG. 2B. 
The second source of baseband distortion discussed supra has not been 
eliminated by the aforementioned Gorog apparatus. It can, however, be 
substantially reduced by the following: 
(1) Using a linearly polarized light beam whose electric field vector, E, 
is oriented such that the electric field vector is perpendicular to the 
information track at the surface of the substrate; 
(2) Using a laser having a short wavelength for the low resolution audio 
detection beam, e.g., using a helium-cadmium laser operating at 442 nm 
wavelength for the audio beam and a helium-neon laser operating at 633 nm 
wavelength for the video beam; and 
(3) Using an output stop between the substrate and light detector whose 
opening is matched to that of the input stop. 
The modification of the optical playback system to provide improved low 
frequency response will now be discussed with reference to FIGS. 1 and 4. 
The apparatus of FIG. 1 is provided with a source of radiation 5 which 
emits a linearly polarized light beam for recovering the low frequency 
signals. Source 5 is oriented such that the electric field vector E of 
light beam 7 impinges on substrate 1 with the vector pointed orthogonal to 
the length of the information track. It is believed that vector wave 
diffraction effects are responsible for the improvement in the 
signal-to-noise ratio which occurs when the polarization of the readout 
beam is oriented as discussed above. Although the scalar theory commonly 
used to analyze optical diffraction phenomena, does not predict a change 
if the polarization of the incident beam is varied it has been shown 
experimentally that there is a measurable improvement when the light beam 
is properly oriented. 
As the spatial wavelength of the high frequency information, e.g., the 
video information at the inside radius of the disc, approaches the 
wavelength of the readout light beam in a dual bandwidth system, 
variations in the frequency of the high frequency information distort the 
signal recovered in the low frequency range, i.e., baseband distortion. In 
the dual beam system, the baseband distortion, interfering with the 
faithful recovery of the low frequency information, is believed to be the 
result of spatial frequency dependent phase shifts experienced by the low 
resolution beam upon reflection from the surface having high frequency 
undulations thereon. These distortion effects become especially strong 
when the range of the spatial wavelengths of the high frequency recorded 
information is such that the wavelength of the light beam used to recover 
the low frequency information falls within this range. One way to reduce 
these distortion effects is to choose the wavelength of the low frequency 
readout light beam to be of a value shorter than the shortest recorded 
wavelength of high frequency information. Contrary to prior art thinking, 
associating short wavelength readout beams with the recovery of hiqh 
frequency information and long wavelength readout beams with the recovery 
of low frequency information, in accordance with the teachings of the 
present invention, the short wavelength light beam is preferably used to 
recover the low frequency information. Recovery of the high frequency 
information with the longer wavelength light beam is satisfactory. 
The third modification involves stop 35 shown in FIG. 1. For an explanation 
of stop 35 reference may be made to FIG. 4. Referring to FIG. 4 light beam 
100 is shown bounded by light rays 101 and 102. Stop 15' is positioned in 
the path of beam 100 such that rays outside of the region between rays 101 
and 102, illustrated by rays 103 and 104, are blocked. Lens 21' is used to 
focus beam 100 onto substrate surface 106. Light incident on surface 106 
is diffracted by the undulations on the substrate surface. The light 
diffracted by the substrate surface undulations is collected by lens 21". 
(For the convenience of illustration, the reflective optical system 
commonly used to readout the signals recorded as surface undulations on 
reflective substrates, is shown in FIG. 4 in the form of its equivalent 
transmission type optical system. Thus, lenses 21' and 21" are the same 
lens in the actual reflection case, as shown in FIG. 1.) With respect to 
the audio channel the interference of the various light components is 
detected by a differential detector whose output contains signal 
components of the original recorded signal. Light diffracted by the high 
frequency video signal elements serves no useful purpose in audio 
detection, and, in fact, may produce unwanted distortion in the audio 
channel. To eliminate or reduce the effect of the video signal elements in 
the audio channel a stop 35' having an opening complementary to the 
opening in stop 15' is interposed between the collecting objective 21" and 
the detector (not shown). The light with the audio information between 
rays 101' and 102' is permitted to pass through the aperture of stop 35' 
while unwanted light illustrated by rays 110, 111, 112, 113 is blocked 
from reaching the detector. The unwanted light generally is that which 
interacts with the video signal elements. 
The frequency response of the system of FIG. 1 may be represented by FIG. 3 
which is shown as an approximation. Curve 50 represents the response 
characteristic at high frequencies obtained with light beam 25 and curve 
60 represents the frequency response at low frequencies obtained with 
light beam 7. 
While the principles of the present invention have been demonstrated with 
particular regard to the illustrated structure of the FIGURES, it will be 
recognized that various departures from such illustrated structure may be 
undertaken in practice of the invention. For example, stop 15 in FIG. 1 
having a slit-shaped opening may be replaced with other devices such as an 
anamorphic beam expander that fills the aperture in one direction without 
the use of a slit. Also, the system is not limited to providing for only 
two bandwidths, of course, as many bandwidths may be provided for as 
physical considerations permit.