CA1124402A - Apparatus for scanning variable area optical sound tracks - Google Patents
Apparatus for scanning variable area optical sound tracksInfo
- Publication number
- CA1124402A CA1124402A CA304,524A CA304524A CA1124402A CA 1124402 A CA1124402 A CA 1124402A CA 304524 A CA304524 A CA 304524A CA 1124402 A CA1124402 A CA 1124402A
- Authority
- CA
- Canada
- Prior art keywords
- sound track
- signal
- noise
- optical sound
- optical
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
- 230000003287 optical effect Effects 0.000 title claims abstract description 30
- 238000012545 processing Methods 0.000 claims abstract description 13
- 230000005236 sound signal Effects 0.000 claims description 10
- 230000007704 transition Effects 0.000 claims description 8
- 230000000670 limiting effect Effects 0.000 claims description 6
- 108091008695 photoreceptors Proteins 0.000 claims description 6
- 238000005286 illumination Methods 0.000 claims description 4
- 230000004044 response Effects 0.000 claims description 2
- 238000006467 substitution reaction Methods 0.000 claims description 2
- 230000002146 bilateral effect Effects 0.000 description 13
- 238000000034 method Methods 0.000 description 9
- 230000009977 dual effect Effects 0.000 description 7
- 230000004048 modification Effects 0.000 description 4
- 238000012986 modification Methods 0.000 description 4
- 230000008901 benefit Effects 0.000 description 3
- 238000010586 diagram Methods 0.000 description 3
- 238000004377 microelectronic Methods 0.000 description 3
- 230000001629 suppression Effects 0.000 description 3
- 230000009471 action Effects 0.000 description 2
- 238000001514 detection method Methods 0.000 description 2
- 230000000694 effects Effects 0.000 description 2
- 230000009467 reduction Effects 0.000 description 2
- 241000448280 Elates Species 0.000 description 1
- 241000364027 Sinoe Species 0.000 description 1
- 241000519995 Stachys sylvatica Species 0.000 description 1
- 230000002547 anomalous effect Effects 0.000 description 1
- 238000013459 approach Methods 0.000 description 1
- 230000008878 coupling Effects 0.000 description 1
- 238000010168 coupling process Methods 0.000 description 1
- 238000005859 coupling reaction Methods 0.000 description 1
- 230000002950 deficient Effects 0.000 description 1
- 230000001934 delay Effects 0.000 description 1
- 238000005530 etching Methods 0.000 description 1
- 230000036039 immunity Effects 0.000 description 1
- 238000010348 incorporation Methods 0.000 description 1
- 238000007689 inspection Methods 0.000 description 1
- 238000009434 installation Methods 0.000 description 1
- 230000007774 longterm Effects 0.000 description 1
- 238000004519 manufacturing process Methods 0.000 description 1
- 230000000873 masking effect Effects 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 238000012856 packing Methods 0.000 description 1
- 239000002245 particle Substances 0.000 description 1
- 238000005070 sampling Methods 0.000 description 1
- 239000007787 solid Substances 0.000 description 1
Classifications
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B7/00—Recording or reproducing by optical means, e.g. recording using a thermal beam of optical radiation by modifying optical properties or the physical structure, reproducing using an optical beam at lower power by sensing optical properties; Record carriers therefor
- G11B7/002—Recording, reproducing or erasing systems characterised by the shape or form of the carrier
- G11B7/003—Recording, reproducing or erasing systems characterised by the shape or form of the carrier with webs, filaments or wires, e.g. belts, spooled tapes or films of quasi-infinite extent
- G11B7/0032—Recording, reproducing or erasing systems characterised by the shape or form of the carrier with webs, filaments or wires, e.g. belts, spooled tapes or films of quasi-infinite extent for moving-picture soundtracks, i.e. cinema
-
- G—PHYSICS
- G01—MEASURING; TESTING
- G01B—MEASURING LENGTH, THICKNESS OR SIMILAR LINEAR DIMENSIONS; MEASURING ANGLES; MEASURING AREAS; MEASURING IRREGULARITIES OF SURFACES OR CONTOURS
- G01B11/00—Measuring arrangements characterised by the use of optical techniques
- G01B11/02—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness
- G01B11/024—Measuring arrangements characterised by the use of optical techniques for measuring length, width or thickness by means of diode-array scanning
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B20/00—Signal processing not specific to the method of recording or reproducing; Circuits therefor
- G11B20/24—Signal processing not specific to the method of recording or reproducing; Circuits therefor for reducing noise
-
- G—PHYSICS
- G11—INFORMATION STORAGE
- G11B—INFORMATION STORAGE BASED ON RELATIVE MOVEMENT BETWEEN RECORD CARRIER AND TRANSDUCER
- G11B23/00—Record carriers not specific to the method of recording or reproducing; Accessories, e.g. containers, specially adapted for co-operation with the recording or reproducing apparatus ; Intermediate mediums; Apparatus or processes specially adapted for their manufacture
- G11B23/0007—Circuits or methods for reducing noise, for correction of distortion, or for changing density of recorded information
Abstract
ABSTRACT
Apparatus for scanning variable area optical sound tracks wherein a plurality of discrete photo sensors examine small fractional portions of the sound track width. Selection of the number of sensors in relation to photographic grain dimensions results in converting annoying quantizing noise to more tolerable random noise while using a practical number of sensors. Electronic processing of the sensors' outputs reduces impulse noise from dirt and scratches.
Apparatus for scanning variable area optical sound tracks wherein a plurality of discrete photo sensors examine small fractional portions of the sound track width. Selection of the number of sensors in relation to photographic grain dimensions results in converting annoying quantizing noise to more tolerable random noise while using a practical number of sensors. Electronic processing of the sensors' outputs reduces impulse noise from dirt and scratches.
Description
~z4~V2 B~CXGROUND OF T~IE Ir~ENTION
The invention r.elates generally to playback equipment for variable area optical soundtracks and more particularly to an improved apparatus for scanning such sound tracks.
. Variable area optical sound tracks on motion picture film have been used in substantially their present form since the earliest days of sound-on-film in the cinema. In their earliest form, a single monophonic optical souhd track ~as use.d, the width of the clear area being proportional to the recorded .
modulation amplitude. Later modifications intended to reduce distortion provided for bilateral and dual bilateral trac};s adjacent to each other, carrying the same modulation information ., !
and being identical in their pattern. A further modification provides for separately m~dulating the dual bilateral tracks to pr~vide stereo-phonic reproductlon.
~ dern o~ rereial film projectors ~ntinue to use essentially the same illumination and light sensing arrangements for reading variable area optical sound tracks as those that were used in the earliest sound pr~jection equiprrent- a light source and narrow mechanical slit to pro-vide a line source illumination, with a single photocell for detection or with dual photocells in the case of stereophonie dual bilateral traeks.
A oontinuing problem in optieal sound traek rep~duction is the suppression of noise while seeking to provide wide frequency range. Op-tieal sound tracks are partielllarly susoeptible to impulse noise caused by dirt and scratehes, whieh increase with the nubber of times a print is projected. Other types of noise include grain noise in the white Iclear~
area of the track and photoeleetrie oell noise, both of which are çssen-tially proportional to the width of the traek being replayed and eause a noise modulation effect when the signal is repn~dueed.
Various techniques have been introduoed to improve the quality of optical sound traeks and indications are that the medium is not in-herently as deficient as had been supposed in -the 1950's and 1960's, when attempts were made to popularize magnetie sound tracks. A useful dis-eussion of the history and potential of optical sound tracks ineludes "The Pro~uction of Wide-P~nge, L~w-Distortion Optical sound Tracks Utilizing the D~lby ~oise Peduction System," by Ioan Allen in the Journal of the SMPTE! Septenber, 1975, Volum~e 84, pages 720-729. Tkle paper includes a bibliography in the subject area.
An early attempt at a different approach in replaying the op-tical sound track is set forth in U.S. Patent 2,347,084 in which the trac3c is repeatedly scanned across its width by a very small scanning spot and detected by a single photo responsive.
rb/ - 2 -4632`
- 11 to produce an essentially two-level pulse width modulated signal. The signal ls limited to suppress noise and is integrated to provide an amplitude varying audio signal. This system is useful in eliminating a substantial amount of noise which is caused by dark spots on the clear area of the track, the black areas being substantially noise-free, except where the negative may have had imperfections, causing white spots~ However, in spite of the scanning technique some dark spots on the clear area will still produce noise.
In an improved scanning system the knowledge that the noise arises primarily in the clear area is taken advantage of by squelching the output signal after initial detection of the black to white transition on the film track during each scan.
The reset is effected after each scan. Further refinement of the technique employs a two-way scan so that the white to black transition on a dual bilateral black can be utilized. Signal delays and logic circuits are also employed for this purpose.
In both prior scanning systems described, a problem is in the complexity of the optical scanning mechanisms whic~
would require exten~ive modification to existing film projectors.
Also, long term reliability and relative immunity from frequent adjustments are essential in commercial projection equipment.
The prior art scanning systems are susceptible to such problems because of their use of cathode ray tube, mechanical, laser, or other electro-optical scanning techniques.
~A - 3 -csm/~
~24410Z
SUMMPRY OF THE INVENTION
. . .
In aco~rdan oe with the teachings of the present invention an improved optical sound track playback system is provided which retains the advantages of optical scanning and yet is essentially electLonic in character. The method permits uniquely tailored processing tech-niques to be employed in order to reduce the particular types of noise afflicting optical sound tracks.
The developm~nt of the invention is as follows. First, the need for scanning of the track with a light beam would-apparently be eliminated if a photosensor array could be arranged to sense the changing width of the optical track. Optical enlargem~nt of the track oould be employed but to make the system practical for use with existing projectors, the sensor array should preferably be situated immediately behind the film plane. However, a deterrent in employing a sensor array technique is the knowledge that quantizing noise would be a problem. In the audio art it is conventionally acoepted that to quanti æ an audio signal with high quality requires something in the order of 10,000 or more quantizing steps. Sinoe the total track width of a single stereo optical track on 35 mm film is 0.033 inches, or about 0.016 inches for each of the bilateral recordings, a photosensor array of 10,000 elements spaced at 1.6 microinch intervals is indicated.
This is out of the question in the current state of microelectronic art; even with the use of an optical magnification system, which would greatly increase installation difficulties with existing pro]ectors, an array of 40,000 sensors for the whole track width would be a formidible undertaking.
~b/
r ~ . , ~L~.Z4~L~3Z
Further study of the matter leads to a more hopeful result. In quantizing audio signals it is known to add a wide-band dither signal to the input signal in order to reduce cor-relation between the signal and the quantizing noise. This reduces the annoyance of the quantizing noise significantly. If the dither signal is large enough then a smaller number of digit-izing steps suffices. Microscopic inspection of variable area optical sound tracksshows that the clear to black boundary of the track is not an abrupt one but rather has a ragged edge due to the uncertainty introduced by film grain. This uncertainty extends over a distance of 0.0001" - 0.0002". If this uncertainty is regarded as a dither signal, added to the input signal, then the sound track could be quantizea in steps of, say, 0.0001".
Thus each half of each bilateral recording could be quantized into only 160 steps, a very low number compared with the 10,000 conventionally required in quantizing an audio signal.
A photo-sensor array with a pitch of 0.0001" represents a packing density about ten times closer than in current micro electronic practice. While new masking and etching techniques may be required, this increase in density does not seem entirely out of the question. In any case, optical magnification can be used until such time that the necessary microelectronic production techniques are available. Thus an important element of the inven-tion is the discovery and recognition that optical sound tracks I have an inherent property which makes a quantized reproduction ! system feasible.
In a possible embodiment of the invention the photo-sensor elements are scanned sequentially at a high rate by means of clock circuits and switches. The pulse width modulated output 3Q signal is then limited and otherwise processed as in the scanning proposals of the prior art. While the above scanning embodiment jk/~ _ ~ ~Z44~2 ght well be practical, it is desirable to simplify the apparatus even further is possible. Thus another element of the invention is the recognition that with a digital readout system lateral scanning of the sound track is unnecessary, thereby avoiding the considerable complica-tion of clock circuits, switching, and pulse width demodulation. In accordance with this aspect of the invention the output of each photosensor element is processed in parallel. Each sensor is directly fed to its own amplifier and limiter circuit; most of the spurious outputs due to dirt and scratGhes are eliminated by the limiting action. Logic can then be applied to the circuit outputs in order to achieve further noise reduction. The outputs of all the circuits are then combined and smoothed to form-the audio output signal. The circuitry required in these steps is all very simple, permitting the incorporation on a single integrated circuit chip of all photosensor elements, amplifier/limiters, noise suppression logic, combining circuitry, and audio signal output amplifiers. In accordance with the figures mentioned previously, 640 sensing and processing channels would be involved; in most commercial motion picture applications the final output of the integrated circuit would, however, comprise only the two audio channels.
The invention is applicable to all types of variable width optical sound tracks including monophonic bilateral, monophonic dual bilateral and stereophonic dual bilateral by selecting the portion of photodiodes to be processed as a separate group. Two, three, four or more tracks and processing chains can be used to provide signals for left, center, right, rear, surround effects, control signals for special purposes, and the like.
~A
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~24~32 Since the inventive system uses the conventional illuminating source in a projector, the only modification to the projector mechanical structure requirecl is to substitute the photodiode and processing IC for the photoelectric cell.
Optional optics may be possible or desirable in some instances to effectively place the photodiode strip at the film track image plane.
In accordance with the present invention, therefore, there is provided a scanning system for reproducing a variable area optical sound track comprising means for illuminating a substantial width of the sound track, a linear array of photo-receptor means disposed relative to the sound track from the illuminating means for generating analog electrical signals in response to the level of illumination received at each of the photoreceptor means from a respective small fractional portion of the sound track, means for processing in parallel the electrical signals, and means for combining the processed electrical signals to generate an audio signal.
These and other advantages and features of the present invention will be appreciated as the following description of the preferred embodiments is read in conjunction with the drawings.
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I - ~3 ~24~Z
.;
1¦ 13RIEF l~l:S~ Il'TION OF Tlll. Dl'l~1iIN(.;S
4 I Figure l is a plot of density versus distance of an 5~ exemplary variable area optical sound track scan useul in 6 understanding the present invention.
8 ¦ Figure 2 is a magnified plot of the black to white 9 ¦ transition portion of Figure 1.
10 l 11 ¦ Figure 3 is a partially block schematic perspective 12 view of an optical playback system embodying the present 13 ¦invention.
15 ¦ Figure 4 is a functional block diagram of the sensor 16 ¦array and processing circuitry of the preferred embodiment of the 17 invention.
18 l 19 1 Figure S is a schematic diagram of a portion of the 20 ¦logic circuit usable in Figure 4.
22 ¦ Figure 6 is a functional block diagram of the sensor 23 ¦array and processing circuitry of a further embodiment of the 24 invention.
2a ~
31 l ~ -B-.p'~ 2 DESCRIPTION OF IHE PPEFEP~ED EMBODI~ENTS
Referring now to ~igure 1, the photographic density versus distance of an exemplary scan across a single bilateral variable area sound track is shown. In scanning systems the waveform information is in the pulse width w. Ihe initial black level is essentially a oonstant level and is therefore noise free. Hcwever, the white level varies substantially in amplitude due to dark particles typically encountered in the white area. Without limiting to rem~ve such variations the resulting reproduced audio signal would oontain substantial noise.
However, limiting doe s not rem~ve all amplitude variations, nor dbes it affect the noise generated by the un oe rtainty in the black/white ~(and white~black) transitions as depicted in Figure 2. Figure 2 has a magnified scale in order to show the manner in which the signal builds up in the transition area. The dotted lines show another possible a~plitude build up. Thus, depending on the level of limiting, the apparent position of the black to white transition will vary, which creates hiss in the reproduced signal.
In Figure 3 the general arrangement of the present system is shown, including a oonventional la~p 10 and mechanical slit 12 for horizontally illuminating a na~row strip of the variable area sound track 14 of a m~tion picture film 16. Although a bilateral sound track is shown, the invention is also applicable to multiple track films.
m e light modulated by sound track 14 is sensed by photore oe ptor means oomprising an array of solid state sensors 18 that sense, respectively, small fractional portions of the sound track width. As necessary, various lenses may be provided amDng lamp 10, slit 12, film 16 and sensors 18 in order to optically place the slit image and sensors at the film plane. Processing circuitry 20 re oe ives the sensors 18 ~b/
::
Z~32 outputs and generates the reproduced audio therefrom. ~he invention has as one of its purposes to yield a system in which both the sensors and the pro oessing circuitry may be contained on a single integrated circuit chip.
~ he number of sensors is chosen to provide about one sample per 0.0001 inches across the sound track, or about 640 sensors for the active areas of a 0.076 inch standard track. This nu~ber of sensors is sufficiently large to avoid quantizing noise yet not so large as to sample exoessively over the areas of unoertainty. About two or three samples are taken over each area of uncertainty thus providing a random sampling and substituting ran~om noise, which is psychoacoustically acoeptable, for annoying quantizing noise.
In the preferred embodi~ent shown in Fig. 4 the sensors 24 of array 18 are each connected to their own amplifiers 34a, 34b, etc.
and limiters 36a, 36b, etc. and the limiters' outputs are applied to a combiner 48. In its simplest form, co~biner 48 sums the applied signals and applies the result to the integrator or audio band pass filter 40 to provide the reproduoe d audio signal.
If sensors 24 are arranged in other than an evenly spaoe d manner, as by a greater density in the oenter of the track area, the resulting signals can be appropriately weighted in the combiner to oompensate.
In the case of multiple sound tracks, such as stereo bilateral tracks, the sensors 24 of Figure 4 may be divqded into gr~ups depending on which reads eadh particular track and the grDups are applied to separate pro oessing chains to provide a separate reproduced audio signal for each track. ~he gLoups would then be applied to separate combiners.
~ile most of the dirt and scratch noise reduction action is ~b/
~2~4~32 provided by the action of limiters 36, it is possible to employ logic circuits in each of the limiter outputs to achieve further noise re-duction, shown by way of example as logic 37b receiving the limiter 36b in Figure 4. Similar logic circuits (not shown) can be pn~vided in th~
outputs of the other limiters 37a, 37c, etc. Scratch noise manifests itself by non-uniformity of the outputs of the limiters. For exa~ple, in the clear area of the track, the limiter outputs might all be positive in polarity. A aeep scratch might cause one (or a small nu~berof) ~ limiter outputs to beo~me negative in polarity. By comparing the polarity of the output of a given limiter (say 36b-) with that of referen oe limiters on either side (say 36a and 36c, or perhaps removed by two or m~re elements), then the presen oe of an anom~lous signal can be detected.
If a polarity differen oe is detected between a particular limiter and its referenoe limiters then that limiter output is can oelled and the polarity of the reference limiters is substituted.
One possible logic circuit to determine if the limiter output is spurious is represented by the oombination of an OR-gate 44, AN~}
gate 46, ANr~gate 50 and OR~gate 52 as in Figure 5 in which the output m is compared with the outputs say one sensor removed on either side.
If it is the same, the signal is considered valid; if there is a dif-feren oe, then the polarity of the referen oe sensors is pr~vided at the output. The logic circuit o~uld be simplified by omitting elements 50 and 52, in ~hich case an anomalous white signal in the black area is not changed to a black signal.
As a practical matter, the amplifiers 34, limiters 36, and logic 37 can be oombined in a single electr~nic circuit, with suitable CrDSS couplings between the circuits to provide the logic inputs.
~/
z Figure 6 shows a further em~odiment of the invention in which the processing circuitry 20 includes means for electron-ically scanning the array of photosensors 18. The scanning means is shown functionally as a scan control 30 controlling a switch 32 that is selectively connected to each of the sensors 24. A
suitable scanning rate is in the arder of 100 kHz to accommodate audio frequencies up to about 20 kHz.
The switch 32 output is applied to an amplifier 34 which is preferably a low noise, wideband amplifier. The amplifier will also preferably convert the photosensor current output to a voltage. The resulting signal is limited or clipped by limiter 36 in order to remove most of the fluctuations in the white and black area signals to provide a substantially bi-level signal to logic 38.
Logic 38 also receives information from scan control 30 as to the scan cycle timing. Alternatively, the logic circuit 38 may include storage means and compare other samples in a scan or multiple scans to determine if a sample is valid rather than noise induced. One possible logic circuit to determine if a black or white signal is valid and for making an interpolated substitution in the case of invalidity is the circuit of Figure 5.
The output of logic 38 is a series of pulses having varying widths related to the width of the scanned optical track, width noise suppression resulting from the signal limiting and the logic processing. The pulses are integrated over a suitable time constant in integrator or audio band pass filter 40 and the resulting reproduced aud~o signal is applied to the theatre sound system.
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The e~kodiment of Figure 4 may also be useful in applications other than types of scanning systems where an analog output signal is required. ~br example, in systems scanning a workpie oe to measure thickness, width, hole size or the like.
~b/ , : '
The invention r.elates generally to playback equipment for variable area optical soundtracks and more particularly to an improved apparatus for scanning such sound tracks.
. Variable area optical sound tracks on motion picture film have been used in substantially their present form since the earliest days of sound-on-film in the cinema. In their earliest form, a single monophonic optical souhd track ~as use.d, the width of the clear area being proportional to the recorded .
modulation amplitude. Later modifications intended to reduce distortion provided for bilateral and dual bilateral trac};s adjacent to each other, carrying the same modulation information ., !
and being identical in their pattern. A further modification provides for separately m~dulating the dual bilateral tracks to pr~vide stereo-phonic reproductlon.
~ dern o~ rereial film projectors ~ntinue to use essentially the same illumination and light sensing arrangements for reading variable area optical sound tracks as those that were used in the earliest sound pr~jection equiprrent- a light source and narrow mechanical slit to pro-vide a line source illumination, with a single photocell for detection or with dual photocells in the case of stereophonie dual bilateral traeks.
A oontinuing problem in optieal sound traek rep~duction is the suppression of noise while seeking to provide wide frequency range. Op-tieal sound tracks are partielllarly susoeptible to impulse noise caused by dirt and scratehes, whieh increase with the nubber of times a print is projected. Other types of noise include grain noise in the white Iclear~
area of the track and photoeleetrie oell noise, both of which are çssen-tially proportional to the width of the traek being replayed and eause a noise modulation effect when the signal is repn~dueed.
Various techniques have been introduoed to improve the quality of optical sound traeks and indications are that the medium is not in-herently as deficient as had been supposed in -the 1950's and 1960's, when attempts were made to popularize magnetie sound tracks. A useful dis-eussion of the history and potential of optical sound tracks ineludes "The Pro~uction of Wide-P~nge, L~w-Distortion Optical sound Tracks Utilizing the D~lby ~oise Peduction System," by Ioan Allen in the Journal of the SMPTE! Septenber, 1975, Volum~e 84, pages 720-729. Tkle paper includes a bibliography in the subject area.
An early attempt at a different approach in replaying the op-tical sound track is set forth in U.S. Patent 2,347,084 in which the trac3c is repeatedly scanned across its width by a very small scanning spot and detected by a single photo responsive.
rb/ - 2 -4632`
- 11 to produce an essentially two-level pulse width modulated signal. The signal ls limited to suppress noise and is integrated to provide an amplitude varying audio signal. This system is useful in eliminating a substantial amount of noise which is caused by dark spots on the clear area of the track, the black areas being substantially noise-free, except where the negative may have had imperfections, causing white spots~ However, in spite of the scanning technique some dark spots on the clear area will still produce noise.
In an improved scanning system the knowledge that the noise arises primarily in the clear area is taken advantage of by squelching the output signal after initial detection of the black to white transition on the film track during each scan.
The reset is effected after each scan. Further refinement of the technique employs a two-way scan so that the white to black transition on a dual bilateral black can be utilized. Signal delays and logic circuits are also employed for this purpose.
In both prior scanning systems described, a problem is in the complexity of the optical scanning mechanisms whic~
would require exten~ive modification to existing film projectors.
Also, long term reliability and relative immunity from frequent adjustments are essential in commercial projection equipment.
The prior art scanning systems are susceptible to such problems because of their use of cathode ray tube, mechanical, laser, or other electro-optical scanning techniques.
~A - 3 -csm/~
~24410Z
SUMMPRY OF THE INVENTION
. . .
In aco~rdan oe with the teachings of the present invention an improved optical sound track playback system is provided which retains the advantages of optical scanning and yet is essentially electLonic in character. The method permits uniquely tailored processing tech-niques to be employed in order to reduce the particular types of noise afflicting optical sound tracks.
The developm~nt of the invention is as follows. First, the need for scanning of the track with a light beam would-apparently be eliminated if a photosensor array could be arranged to sense the changing width of the optical track. Optical enlargem~nt of the track oould be employed but to make the system practical for use with existing projectors, the sensor array should preferably be situated immediately behind the film plane. However, a deterrent in employing a sensor array technique is the knowledge that quantizing noise would be a problem. In the audio art it is conventionally acoepted that to quanti æ an audio signal with high quality requires something in the order of 10,000 or more quantizing steps. Sinoe the total track width of a single stereo optical track on 35 mm film is 0.033 inches, or about 0.016 inches for each of the bilateral recordings, a photosensor array of 10,000 elements spaced at 1.6 microinch intervals is indicated.
This is out of the question in the current state of microelectronic art; even with the use of an optical magnification system, which would greatly increase installation difficulties with existing pro]ectors, an array of 40,000 sensors for the whole track width would be a formidible undertaking.
~b/
r ~ . , ~L~.Z4~L~3Z
Further study of the matter leads to a more hopeful result. In quantizing audio signals it is known to add a wide-band dither signal to the input signal in order to reduce cor-relation between the signal and the quantizing noise. This reduces the annoyance of the quantizing noise significantly. If the dither signal is large enough then a smaller number of digit-izing steps suffices. Microscopic inspection of variable area optical sound tracksshows that the clear to black boundary of the track is not an abrupt one but rather has a ragged edge due to the uncertainty introduced by film grain. This uncertainty extends over a distance of 0.0001" - 0.0002". If this uncertainty is regarded as a dither signal, added to the input signal, then the sound track could be quantizea in steps of, say, 0.0001".
Thus each half of each bilateral recording could be quantized into only 160 steps, a very low number compared with the 10,000 conventionally required in quantizing an audio signal.
A photo-sensor array with a pitch of 0.0001" represents a packing density about ten times closer than in current micro electronic practice. While new masking and etching techniques may be required, this increase in density does not seem entirely out of the question. In any case, optical magnification can be used until such time that the necessary microelectronic production techniques are available. Thus an important element of the inven-tion is the discovery and recognition that optical sound tracks I have an inherent property which makes a quantized reproduction ! system feasible.
In a possible embodiment of the invention the photo-sensor elements are scanned sequentially at a high rate by means of clock circuits and switches. The pulse width modulated output 3Q signal is then limited and otherwise processed as in the scanning proposals of the prior art. While the above scanning embodiment jk/~ _ ~ ~Z44~2 ght well be practical, it is desirable to simplify the apparatus even further is possible. Thus another element of the invention is the recognition that with a digital readout system lateral scanning of the sound track is unnecessary, thereby avoiding the considerable complica-tion of clock circuits, switching, and pulse width demodulation. In accordance with this aspect of the invention the output of each photosensor element is processed in parallel. Each sensor is directly fed to its own amplifier and limiter circuit; most of the spurious outputs due to dirt and scratGhes are eliminated by the limiting action. Logic can then be applied to the circuit outputs in order to achieve further noise reduction. The outputs of all the circuits are then combined and smoothed to form-the audio output signal. The circuitry required in these steps is all very simple, permitting the incorporation on a single integrated circuit chip of all photosensor elements, amplifier/limiters, noise suppression logic, combining circuitry, and audio signal output amplifiers. In accordance with the figures mentioned previously, 640 sensing and processing channels would be involved; in most commercial motion picture applications the final output of the integrated circuit would, however, comprise only the two audio channels.
The invention is applicable to all types of variable width optical sound tracks including monophonic bilateral, monophonic dual bilateral and stereophonic dual bilateral by selecting the portion of photodiodes to be processed as a separate group. Two, three, four or more tracks and processing chains can be used to provide signals for left, center, right, rear, surround effects, control signals for special purposes, and the like.
~A
csm/~
~24~32 Since the inventive system uses the conventional illuminating source in a projector, the only modification to the projector mechanical structure requirecl is to substitute the photodiode and processing IC for the photoelectric cell.
Optional optics may be possible or desirable in some instances to effectively place the photodiode strip at the film track image plane.
In accordance with the present invention, therefore, there is provided a scanning system for reproducing a variable area optical sound track comprising means for illuminating a substantial width of the sound track, a linear array of photo-receptor means disposed relative to the sound track from the illuminating means for generating analog electrical signals in response to the level of illumination received at each of the photoreceptor means from a respective small fractional portion of the sound track, means for processing in parallel the electrical signals, and means for combining the processed electrical signals to generate an audio signal.
These and other advantages and features of the present invention will be appreciated as the following description of the preferred embodiments is read in conjunction with the drawings.
csm/ ~
I - ~3 ~24~Z
.;
1¦ 13RIEF l~l:S~ Il'TION OF Tlll. Dl'l~1iIN(.;S
4 I Figure l is a plot of density versus distance of an 5~ exemplary variable area optical sound track scan useul in 6 understanding the present invention.
8 ¦ Figure 2 is a magnified plot of the black to white 9 ¦ transition portion of Figure 1.
10 l 11 ¦ Figure 3 is a partially block schematic perspective 12 view of an optical playback system embodying the present 13 ¦invention.
15 ¦ Figure 4 is a functional block diagram of the sensor 16 ¦array and processing circuitry of the preferred embodiment of the 17 invention.
18 l 19 1 Figure S is a schematic diagram of a portion of the 20 ¦logic circuit usable in Figure 4.
22 ¦ Figure 6 is a functional block diagram of the sensor 23 ¦array and processing circuitry of a further embodiment of the 24 invention.
2a ~
31 l ~ -B-.p'~ 2 DESCRIPTION OF IHE PPEFEP~ED EMBODI~ENTS
Referring now to ~igure 1, the photographic density versus distance of an exemplary scan across a single bilateral variable area sound track is shown. In scanning systems the waveform information is in the pulse width w. Ihe initial black level is essentially a oonstant level and is therefore noise free. Hcwever, the white level varies substantially in amplitude due to dark particles typically encountered in the white area. Without limiting to rem~ve such variations the resulting reproduced audio signal would oontain substantial noise.
However, limiting doe s not rem~ve all amplitude variations, nor dbes it affect the noise generated by the un oe rtainty in the black/white ~(and white~black) transitions as depicted in Figure 2. Figure 2 has a magnified scale in order to show the manner in which the signal builds up in the transition area. The dotted lines show another possible a~plitude build up. Thus, depending on the level of limiting, the apparent position of the black to white transition will vary, which creates hiss in the reproduced signal.
In Figure 3 the general arrangement of the present system is shown, including a oonventional la~p 10 and mechanical slit 12 for horizontally illuminating a na~row strip of the variable area sound track 14 of a m~tion picture film 16. Although a bilateral sound track is shown, the invention is also applicable to multiple track films.
m e light modulated by sound track 14 is sensed by photore oe ptor means oomprising an array of solid state sensors 18 that sense, respectively, small fractional portions of the sound track width. As necessary, various lenses may be provided amDng lamp 10, slit 12, film 16 and sensors 18 in order to optically place the slit image and sensors at the film plane. Processing circuitry 20 re oe ives the sensors 18 ~b/
::
Z~32 outputs and generates the reproduced audio therefrom. ~he invention has as one of its purposes to yield a system in which both the sensors and the pro oessing circuitry may be contained on a single integrated circuit chip.
~ he number of sensors is chosen to provide about one sample per 0.0001 inches across the sound track, or about 640 sensors for the active areas of a 0.076 inch standard track. This nu~ber of sensors is sufficiently large to avoid quantizing noise yet not so large as to sample exoessively over the areas of unoertainty. About two or three samples are taken over each area of uncertainty thus providing a random sampling and substituting ran~om noise, which is psychoacoustically acoeptable, for annoying quantizing noise.
In the preferred embodi~ent shown in Fig. 4 the sensors 24 of array 18 are each connected to their own amplifiers 34a, 34b, etc.
and limiters 36a, 36b, etc. and the limiters' outputs are applied to a combiner 48. In its simplest form, co~biner 48 sums the applied signals and applies the result to the integrator or audio band pass filter 40 to provide the reproduoe d audio signal.
If sensors 24 are arranged in other than an evenly spaoe d manner, as by a greater density in the oenter of the track area, the resulting signals can be appropriately weighted in the combiner to oompensate.
In the case of multiple sound tracks, such as stereo bilateral tracks, the sensors 24 of Figure 4 may be divqded into gr~ups depending on which reads eadh particular track and the grDups are applied to separate pro oessing chains to provide a separate reproduced audio signal for each track. ~he gLoups would then be applied to separate combiners.
~ile most of the dirt and scratch noise reduction action is ~b/
~2~4~32 provided by the action of limiters 36, it is possible to employ logic circuits in each of the limiter outputs to achieve further noise re-duction, shown by way of example as logic 37b receiving the limiter 36b in Figure 4. Similar logic circuits (not shown) can be pn~vided in th~
outputs of the other limiters 37a, 37c, etc. Scratch noise manifests itself by non-uniformity of the outputs of the limiters. For exa~ple, in the clear area of the track, the limiter outputs might all be positive in polarity. A aeep scratch might cause one (or a small nu~berof) ~ limiter outputs to beo~me negative in polarity. By comparing the polarity of the output of a given limiter (say 36b-) with that of referen oe limiters on either side (say 36a and 36c, or perhaps removed by two or m~re elements), then the presen oe of an anom~lous signal can be detected.
If a polarity differen oe is detected between a particular limiter and its referenoe limiters then that limiter output is can oelled and the polarity of the reference limiters is substituted.
One possible logic circuit to determine if the limiter output is spurious is represented by the oombination of an OR-gate 44, AN~}
gate 46, ANr~gate 50 and OR~gate 52 as in Figure 5 in which the output m is compared with the outputs say one sensor removed on either side.
If it is the same, the signal is considered valid; if there is a dif-feren oe, then the polarity of the referen oe sensors is pr~vided at the output. The logic circuit o~uld be simplified by omitting elements 50 and 52, in ~hich case an anomalous white signal in the black area is not changed to a black signal.
As a practical matter, the amplifiers 34, limiters 36, and logic 37 can be oombined in a single electr~nic circuit, with suitable CrDSS couplings between the circuits to provide the logic inputs.
~/
z Figure 6 shows a further em~odiment of the invention in which the processing circuitry 20 includes means for electron-ically scanning the array of photosensors 18. The scanning means is shown functionally as a scan control 30 controlling a switch 32 that is selectively connected to each of the sensors 24. A
suitable scanning rate is in the arder of 100 kHz to accommodate audio frequencies up to about 20 kHz.
The switch 32 output is applied to an amplifier 34 which is preferably a low noise, wideband amplifier. The amplifier will also preferably convert the photosensor current output to a voltage. The resulting signal is limited or clipped by limiter 36 in order to remove most of the fluctuations in the white and black area signals to provide a substantially bi-level signal to logic 38.
Logic 38 also receives information from scan control 30 as to the scan cycle timing. Alternatively, the logic circuit 38 may include storage means and compare other samples in a scan or multiple scans to determine if a sample is valid rather than noise induced. One possible logic circuit to determine if a black or white signal is valid and for making an interpolated substitution in the case of invalidity is the circuit of Figure 5.
The output of logic 38 is a series of pulses having varying widths related to the width of the scanned optical track, width noise suppression resulting from the signal limiting and the logic processing. The pulses are integrated over a suitable time constant in integrator or audio band pass filter 40 and the resulting reproduced aud~o signal is applied to the theatre sound system.
A csm/~
4~Z
The e~kodiment of Figure 4 may also be useful in applications other than types of scanning systems where an analog output signal is required. ~br example, in systems scanning a workpie oe to measure thickness, width, hole size or the like.
~b/ , : '
Claims (5)
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A scanning system for reproducing a variable area optical sound track comprising means for illuminating a substantial width of the sound track, a linear array of photoreceptor means disposed relative to the sound track from said illuminating means for generating analog electrical signals in response to the level of illumination received at each of said photoreceptor means from a respective small fractional portion of the sound track, means for processing in parallel said electrical signals, and means for combining said processed electrical signals to generate an audio signal.
2. The combination of claim 1 wherein the number of photoreceptor means is in the order of one for every 0.0001 inches of optical sound track width, whereby the uncertainty of the black boundary transitions in the optical sound track provides a wideband dither signal to reduce the correlation between the signal and the quantizing noise.
3. The combination of claim 1 wherein said means for separately processing includes means for limiting each of said electrical signals.
4. The combination of claim 3 further comprising logic means responsive to said limited signals for determining the validity of each signal sample and for making an interpolated substitution in the case of invalidity.
5. The combination of claim 3 wherein the number of photoreceptor means is in the order of one for every 0.0001 inches of optical sound track width, whereby the uncertainty of the black boundary transitions in the optical sound track provides a wideband dither signal to reduce the correlation between the signal and the quantizing noise.
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US80304277A | 1977-06-03 | 1977-06-03 | |
| US803,042 | 1985-11-29 |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1124402A true CA1124402A (en) | 1982-05-25 |
Family
ID=25185428
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA304,524A Expired CA1124402A (en) | 1977-06-03 | 1978-05-31 | Apparatus for scanning variable area optical sound tracks |
Country Status (7)
| Country | Link |
|---|---|
| JP (1) | JPS542703A (en) |
| CA (1) | CA1124402A (en) |
| DE (1) | DE2823853A1 (en) |
| FR (1) | FR2393342A1 (en) |
| GB (1) | GB1598113A (en) |
| IT (1) | IT1192315B (en) |
| NL (1) | NL189322C (en) |
Families Citing this family (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4577305A (en) * | 1983-03-14 | 1986-03-18 | Dolby Laboratories Licensing Corporation | Stereophonic motion picture photographic sound-tracks compatible with different sound projection formats and record and playback apparatus therefore |
| US7400821B2 (en) * | 2000-09-11 | 2008-07-15 | Thomson Licensing | Device and method for reproducing photographic sound recordings |
Family Cites Families (5)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| FR885556A (en) * | 1941-08-30 | 1943-09-20 | Philips Nv | Method for the electro-optical exploration of oscillation recordings |
| BE789499A (en) * | 1971-09-30 | 1973-01-15 | Siemens Ag | PROCESS FOR SEQUENTIALLY RECORDING ANALOGUE DATA ON A MEMORIZATION TAPE |
| US3848095A (en) * | 1972-12-20 | 1974-11-12 | I O Metrics Corp | Three dimensional electro-optical retrieval system |
| US3959784A (en) * | 1974-12-23 | 1976-05-25 | Actron | High speed optical read-out of data stored in an array |
| DE2543276A1 (en) * | 1975-09-27 | 1977-03-31 | Licentia Gmbh | Reproduction system for data carried on tapes - has monochromatic light source and pick-up units for optical signals to be converted to audio output |
-
1978
- 1978-05-22 GB GB21023/78A patent/GB1598113A/en not_active Expired
- 1978-05-25 NL NLAANVRAGE7805719,A patent/NL189322C/en not_active IP Right Cessation
- 1978-05-31 JP JP6563278A patent/JPS542703A/en active Granted
- 1978-05-31 DE DE19782823853 patent/DE2823853A1/en active Granted
- 1978-05-31 CA CA304,524A patent/CA1124402A/en not_active Expired
- 1978-06-02 IT IT49669/78A patent/IT1192315B/en active
- 1978-06-02 FR FR7816650A patent/FR2393342A1/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| DE2823853C2 (en) | 1988-05-19 |
| IT1192315B (en) | 1988-03-31 |
| JPS542703A (en) | 1979-01-10 |
| DE2823853A1 (en) | 1978-12-14 |
| FR2393342B1 (en) | 1984-11-16 |
| NL7805719A (en) | 1978-12-05 |
| JPS6250893B2 (en) | 1987-10-27 |
| NL189322B (en) | 1992-10-01 |
| GB1598113A (en) | 1981-09-16 |
| IT7849669A0 (en) | 1978-06-02 |
| FR2393342A1 (en) | 1978-12-29 |
| NL189322C (en) | 1993-03-01 |
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