Abstract:
The system eliminates the noise, rumble and hiss from any standard 35 mm analog optical sound track. By simply feeding the film through the projector sound head in a normal manner the system automatically converts the analog optical sound tracks to digital quality. No special storing of digital data on film is required and no special digital decoder equipment is needed. The system produces noise-free sound, increased frequency response, expanded dynamic range and clarity of the dialogue. Film studios will no longer need to carry a double inventory of films having digital and analog sound tracks or to process the sound tracks for noise reduction.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to motion pictures and more specifically to an analog sound track digitizer for motion pictures. 
     The best prior art analog sound track digitizer is described and illustrated in the Carlsen U.S. Pat. No. 5,526,075. This earlier version of the inventors scanner was found difficult to adjust for the variations in errors in film sound tracks. Some of these included variations in the location of the sound track or weave, the minimum width of the tracks when there was no modulation, changes in film density, scratches, blotches and the inability to deal with very large noise transients caused by breaks in the film and splices. 
     The present standard for sound-tracks imaged on motion picture film dates back to 1967 wherein the location on, and the area covered by stereo sound tracks on film was specified. This standard describes the dimensions of the track and the related standards used today. This method places two tracks along one edge of the film which are of two types, variable density and variable area, the last of which is used almost exclusively. Inherent to this method of reproducing sound is the undesirable effect of background noise and rumble due to the nature of the plastic medium and residue of the film emulsion passing through the light beam of the photo sound detector. Dolby Corporation with its Dolby B,C,H, etc has pretty much dominated the industry both with its noise reduction systems for movie film and those for magnetic tape. However even with the best system Dolby can offer, the sound suffers distortion and amplitude variations caused by the extreme processing needed to remove random noise from between the normal sound peaks. 
     The introduction of the compact disk or CD with its almost perfect sound reproduction ability has stimulated interest by several companies to try and incorporate digital-quality sound on film. 
     This has proven to be a formidable problem because of the immense amount of digital information required to produce the multiple channel digital formats while keeping the old stereo sound tracks intact for general use by most movie theaters. Also the cost of the digital reader-heads to decode the new digital sound tracks is very high. Dolby Laboratories has recently developed its Theater Digital System that is currently being tested in a number of theaters. Sony Corporation also has introduced their digital system. It should be noted that these systems are expensive and complicated. Also the economics of necessary maintenance and the actual working-life of a digitally-encoded film sound-track is yet to be established. 
     None of the new digital sound systems that are being developed by these major corporations at great expense, does anything to improve old analog film sound-track that remain on the film. 
     The background of the present invention relates to the reproduction of sound from motion picture film. Two of the biggest hurdles to overcome in the effort to improve sound from the movie film are the increase of high frequency response and the reduction of background noise. Filters to reduce background noise also reduce high frequency response which forces the user to use compression and dynamic filter techniques. These lead to unwanted distortions and complexity. 
     Present analog sound track readers used in moving picture theater projectors read the variable width sound tracks on motion picture film by back lighting the sound track portion of the film with a focused slit of light which is arranged perpendicular to  1 l the direction of travel of the film and just wide enough to span both tracks of the normal stereo print. A dual photo detector is placed on the opposite sides of the film to intercept the light which passes through the sound track portion of the film. The two tracks, which are transparent to light, vary in width as the film moves past the slit light source. The thickness of the slit light source and the instantaneous width of the sound track as it passes the photo detector determines the total amount of light falling on the photo detector and in turn the amount of electrical output from the detector. As the film continues to move past the detector the sound information which modulates the width of the tracks is converted to an electrical audio signal. Present photodiode detectors are linear devices and any change in the amount of light falling on the detector causes a corresponding electrical output. Electrical output changes due to different changes in the width of the sound tracks cause useful output. However changes due to residual emulsion, scratches, dirt and light aberration through the film plastic medium cause unwanted light modulation and are perceived as background noises in the electrical output. 
     The frequency response of present systems, irrespective of the noise limitation, is ultimately limited by the recording camera response. In reality, the limitation is based on the thickness of the slit light source used during playback. The best of the slit lenses produce about a 0.5 mil slit thickness which produces a high frequency limit of about 5 KHZ. These wide band slit lenses are usually used in special playback systems such as Dolby stereo or Surround Sound and are followed by dynamic noise filters high frequency boost amplifiers and expanders. These require that the film be specially recorded with compression techniques to improve signal and noise ratios. Generally slit sizes are used which produce high frequency limits of about 9 to 16 KHZ. 
     It is an object of the invention to provide an improved analog sound track digitizer having the highest accuracy and having a minimum amount of noise and distortion. 
     It is also an object of the invention to provide an improved analog sound track digitizer having a novel weave tracking circuit. 
     It is another object of the invention to provide an improved analog sound track digitizer having a circuit which allows the scanner to track minimum width tracks that are as little as  {fraction (1/4+L )}the minimum standard set by the industry.    
     It is an additional object of the invention to provide an improved analog sound track digitizer having an automatic gain control (AGC) that was added to the video amplifier circuit which allows for changes of 10 to 1 in light density and/or film density. 
     It is a further object of the invention to provide an improved analog sound track digitizer having a new method for comparing the video signal with a reference signal. 
     It is another object of the invention to provide an improved analog sound track digitizer having novel noise canceling circuits which work independently to remove the effect of breaks or scratches as large as 1/10 of an inch on the film and blotches of any size. 
     SUMMARY OF THE INVENTION 
     The novel analog sound track digitizer for motion picture projectors has been designed to be retrofit to existing sound head housings but the system can also be incorporated in newly manufactured sound head housings. 
     The present invention includes structure for linearly scanning a light beam of appropriate dimensions and intensity perpendicularly across the movie film sound tracks. A photo detector is placed on the opposite side of the film to intercept the light beam such that when the light beam is directed on the dark or emulsion portion of the film there is no output from the photo detector. When the light beam is directed on the transparent portion of the film the photo detector is saturated. The resulting output of the photo detector is a group of electrical pulses each having a width proportional to the width of the related transparent portion of the film sound tracks and all having a fixed amplitude. Continuously scanning the light beam at a fixed rate and frequency across the films sound tracks produces a continuous stream of pulses each changing in width at a rate related to the instantaneous changes in the width of the transparent portions of the sound track. The scanning frequency of the beam is chosen to produce the desired high frequency response of the system, usually twice the desired frequency. 
     The resulting width modulated pulse streams are passed through level comparators then integrators which convert the pulse streams to audio signals. The benefits of this method are first that since the detected pulses are either zero amplitude at dark emulsion or saturated at transparent, all noise due to residual emulsion or aberration in the film medium are eliminated and other defects such as scratches and dirt have to exceed an adjustable noise threshold before they are detected. Second, the high frequency response is set by the scan rate and beam size and not limited by the characteristics of a light slit and a photo linear detector. 
     Different methods of scanning are possible: 
     1. Scanning beam of light and a fixed photo detector 
     a. Cathode ray tube and a photo detector. 
     b. Mirrored galvanometer and a photo detector. 
     c. Scanning LED array and a photo detector. 
     2. Fixed slit of light and a scanning photo detector 
     a. Fixed slit of light and a Videcon camera tube. 
     b. Fixed spot of light and a charge coupled device (CCD). 
     The above methods are possible alternatives but the preferred method is (b) of group 2. It consists of a light source, not a slit, and a CCD linear scanner. In this configuration a spot of light just slightly larger than the width of the sound track is directed toward the sound track of one side of the film and the reverse side of the illuminated area is focused with a lens on the active area of the CCD linear scanner. The effective slit width of the scanner is reduced by the magnification value of the lens. For example, the CCD scanner has an aperture of 13 um and the lens has a magnification value of 2.25 which effectively reduces the slit to 6 um giving a maximum frequency response of 20.0 KHZ, more than double the present value. The scan frequency of the CCD device is set by the electronics used to drive it and is chosen to produce the desired output band width. 
     In preferred embodiment, the system includes a small halogen lamp with integrated condensing lens or light emitting diode or laser diode as a light source to produce a uniform light spot. The lens on the other side of the film picks up the back lighted image of the sound track and magnifies it 2.25 times then projects the resulting larger image of the sound track onto the image sensor in the scanning photo detector (CCD). The scanning photo detector effectively scans across the track image and converts the image of the sound track into one or two pulses depending on whether the film is mono or stereo. As the film moves past the photo sensor the sound tracks vary in width and the output pulses from the scanning photo detector vary in width accordingly. The scanning photo detector electronically scans the magnified image 40,000 times per second which sets the rate of the output pulse or pulses. The scanning photo sensor consists of an integrated row of photo sensors in a silicon chip numbering  512  which are sequentially read by the on chip scan circuitry. As each of the 512 photo sensors is read an output voltage is produced from each which is proportional to the amount of light falling on it and when all are combined a video signal is produced which is a profile of the light intensity across the scanned image. The effective width of the scan line made by the photo sensor array is 6 um or 0.00024 inches. This line thickness determines the resolution or frequency response of the system. The very best present systems have a maximum frequency response of about 10 KHZ, and are followed by a high frequency boost amplifier to achieve a 16 to 18 KHZ response. The novel analog sound track digitizer has a response of 20 KHZ requiring no boost circuits. 
     The scanning photo detector output is connected to a pulse width modulation (PWM) electronics circuit where the pulse stream is converted into one or two channels of audio. It is then transmitted to pre amps and amplifiers of the existing theater sound system. The sound thus produced is noise-free, has an increased frequency response, has an expanded dynamic range and produces an increased clarity of dialogue. 
     Normally there are only two sound tracks on  35 mm film for stereo sound. Some older films have only one sound track. The number of pulses from the scanner for each scan is equal to the number of sound tracks on the particular film being used. For stereo sound two pulses are generated. Each of the two pulses from the scanner which are varying in width relative to the sound tracks are steered to a separate circuit where it is integrated with respect to time and converted to an electrical audio signal. 
     The pulses from the scanner have only two discrete levels zero and maximum. The two levels represent no light or full light levels on the scanner active area. This provides a noise threshold wherein the noise sources on the film must have a contrast equal to the sound track area contrast or be ignored by the scanner. By its nature this eliminates emulsion residue and plastic film medium at the aberration effects on the output signal. Other sources of noise which cause unwanted pulse width modulation are required to exceed another threshold set by the integrating circuitry before conversion to audio. A potentiometer in the circuitry of the PWM electronics box can be adjusted to vary the threshold level from zero to 100%. By using 20% of the amplitude, any noise has to be 80% of the total amplitude before it will be sensed at all. 
     The improved analog sound track digitizer has incorporated a novel weave tracking circuit to follow any side to side movement of the sound track due to film processing errors or projector problems. Another circuit has been added which allows the scanner to track minimum width tracks that are as little as  {fraction (1/4+L )} the minimum standard set by the industry. An automatic gain control (AGC) has been added to the video amplifier circuit which allows for changes of  10 to 1 in light intensity and or film density. Also incorporated therein is a new circuit for comparing the video signal with a reference signal. The new video comparator has been designed to allow the user to tune the (cross-modulation) distortion inherent on a film print to zero. Additionally, new noise canceling circuits are included which work independently to remove the effect of breaks or scratches as large as  {fraction (1/10+L )} of an inch on the film and blotches of any size.   
    
    
     DESCRIPTION OF THE DRAWING 
     FIG. 1 is a schematic illustration of the novel sound system that is incorporated into the sound head housing; 
     FIG. 2 illustrates a portion of a film strip showing an analog sound track thereon; 
     FIG. 3 is an enlarged schematic illustration of the sound track seen in FIG. 2; 
     FIG. 4 is a schematic illustration of the pulse produced by the instantaneous width of the analog sound track in FIG. 3; 
     FIG. 5 is a block diagram of the reader system; 
     FIG. 6 is a block diagram of the video amplifier and comparator circuit; 
     FIG. 7 is a block diagram of the noise reduction circuits; 
     FIG. 8 is a block diagram of the film weave control circuits; and 
     FIGS.  9 . 1 - 9 . 6  show the wave forms associated with pulse width to voltage convertors. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     The analog-digitizer sound system will now be described by referring to FIGS. 1-4 of the drawings. The basic components of the system are mounted in the sound head housing of a motion picture projector system. Normally the sound head system is located immediately below the projector head. In FIG. 1 the sound head housing is generally designated numeral  10 . The sound head housing generically functions as a movie film reader assembly. Motion picture film  11  enters through an entrance opening  12  and passes around a guide roller  14 . Next it passes around capstan fly wheel  16 , guide rollers  18  and  19  and then around drive sprocket  20 . Then it passes around guide roller  22  and it exits through an opening  24  and is wound on take-up reels (not shown). 
     Light source  26  which could be a halogen lamp, light emitting diode or laser diode illuminates the back of the sound track portion of the film. Lens  34  picks up the back light image of the sound track and magnifies it 2.25 times and then projects the resulting larger image of the sound track onto the image sensor of the analog sound track digitizer  36 . Analog sound track digitizer  36  takes the form of a charge coupled scanning detector (CCD) and all electronics circuits wherein the pulse stream from the (CCD) scanner is converted to one or two channels of audio for output to the preamps, amplifiers, and speakers in theater  42  through interconnect cable  38 . Power  44  is supplied to the system through cable  48  which is combined with cable  38  to power the analog sound track digitizer  36 . 
     A portion of a strip of film  50  is illustrated in FIG. 2 having picture frames  52 , sprocket apertures  54  and a stereo sound track  56 . In FIG. 3 a portion of the stereo sound track is magnified to illustrate the analog sound track on the film strip and its clear strip having various width portions  58 , 59  and  60 . In FIG. 4, widths  58 ,  59  and  60  have been transformed into block pulse signals each having a constant amplitude A but variable widths L 1 , L 2  and L 3 . The widths L 1 , L 2  and L 3  correspond respectively to sound track width portions  58 ,  59  and  60 . The widths are produced by the analog sound track digitizer  36 . 
     Referring to FIG. 5, a block diagram of the reader system is shown with the areas outlined by dash lines that represent changes and improvements. The following is a cursory description of the reader&#39;s functions. 
     The timing and clocking of the circuits is provided by a crystal oscillator  101  and a timing generator  102  which provide all synchronization and control pulses for all parts of the circuit. CCD scanner  103  receives an image of the variable area sound track and converts this optical image to one or more pulses each having a width in time related to the measured width on film of the particular sound track being scanned. Generally two pulses are generated representing the two stereo channels found on standard movie theater prints. The two pulses are then routed to the Video Amplifier And Comparator Circuits block  104  where they are cleaned up and standardized for application to the Pulse Width To Voltage Converters  105  where the width of the pulse is converted to a related voltage level. This happens 40,000 per second which is the rate at which the CCD camera updates its scan of the sound tracks. FIGS.  9 . 1 - 9 . 6  show the wave forms associated with the Pulse Width To Voltage Converters  105 . FIG. 9.1 is a 25 us square wave on the timing generator  102  which establishes the time period assigned to the right and left channels during each scan of the sound track. FIG. 9.4 shows the right and left scanner pulses conditioned by the Video Amplifier Comparator Circuits  104  which are converted to voltage levels as shown in FIG. 9.5 and FIG. 9.6. FIG. 9.2 is a pulse which resets the right and left Pulse Width To Voltage Converters  105  after each scan of the sound track. Before the converters  105  are reset by reset pulses (see FIG.  9 . 2 ), the voltage levels they hold are transferred to Sample And Hold Circuits  106  where the right and left channel voltage levels are held during the next scan. The output of the Sample And Hold Circuits step by step become the right and left channel audio output waveforms which are filtered of high frequency sampling components by filters  110 . Noise Comparators  108  compare the current sample at the output of Pulse Width To Voltage Converters  105  with the stored previous sample at the output of the Sample And Hold Circuits  106 . If the amplitude of the new scan exceeds the stored sample of the previous scan by a predetermined amount and rate, the Noise Comparator  108  senses it and outputs a pulse which turns on the Transient Noise Gate  109  which in turn bypasses the noise pulse to ground. 
     Audio Amplitude Envelope Detectors  115  and  116  each produce a voltage level which follows the long term amplitude profile of the right and left audio output channels  118  and  119  respectively. These voltage levels, one common to each channel, are used to automatically set the sense level of the Noise Comparator  108  for each channel so that the noise detector threshold is raised for high output levels and lowered for low or zero levels. Secondly the two voltage levels are gated by Channel Select Gate  117  and directed to the Video Amplifier And Comparator Circuits  104  where they are used to establish the proper comparator reference dc level with respect to the maximum width of the pulses generated at the CCD scanner. 
     An added circuit  111  monitors the side to side movement of the two channel pulses, generated by the Video Comparator  104 , with respect to the Channel Gate signal (See FIG.  9 . 1 ). The two channel pulses must exactly straddle the positive going edge of the Channel Gate square wave (See FIGS. 9.1 and  9 . 4 ). If the channel pulses move in either direction such that one or the other of the channel pulses intersects the positive going edge of the Channel Gate square wave, the circuit in  111  will shift the positive going edge so that neither channel pulse can encroach on the channel gate period of the other pulse. The side to side motion referred to here is caused by side to side Weave Motion of the film as it passes the CCD scanner. 
     FIG. 6 shows the Video Amplifier And Comparator Circuits. The circuits are composed of a video amplifier section with amplifiers A 1 , A 2 , and A 3  and a comparator section with comparator device A 4 . The video signal from the CCD scanner  103  is input to A 1  where it is amplified and coupled to A 2  for additional gain. The output of amplifier A 2  is connected to the positive input of the comparator device A 4 . A peak detecting circuit  122  is also connected to the output of amplifier A 2  which generates a negative feedback dc voltage and has a fixed threshold. This feedback begins reducing the gain of amplifier A 1  when the output of A 2  reaches a desired level. This AGC (automatic gain control) action holds the output pulses from A 2  at a fixed amplitude even though the amplitude of the input pulses to A 1  is changing. The output of A 2  takes a third path to the inverting input of amplifier A 3 . A 3  inverts the polarity of its pulses and has a gain adjusting variable resistor  124  called Reference Level Adjust. A 3  output becomes the inverting input source for comparator A 4 . The inverting input of amplifier A 3  has two additional input sources, a dc offset adjust  126  and a control signal input  128 . The dc offset adjust is used to establish the dc operation levels of the two inputs to the comparator A 4 . The resulting effective combination of outputs A 2  and A 3  with respect to the input of comparator  104  and the dc offset produced by the Reference DC Offset pot  126  can be seen in FIG. 6.1 where the positive pulse train from A 2  is superimposed over the negative pulse train from A 3  and with the addition of the dc offset which positions the two pulse trains with respect to each other such that they cross each other at a zero voltage level  130 . The resulting output of the comparator A 4  can be seen in FIG. 6.2. The use of the inverted signal of A 3  as a comparator reference signal for the output signal of A 2  keeps the cross points  130 , where the comparator output changes state, to always be at zero potential even if the amplitude or the dc level of the input pulse train were to change. Additionally, the cross point speed of the two signals is doubled because the pulses are moving in opposite polarities, this reduces cross point jitter noise. The remaining input, Control Signal  128  is a dc voltage derived from the sound track reader audio output signals which represents the overall amplitude profile of the outputs. This dc level is used to correct the cross points at the input of the comparator A 4  when high amplitude signals are read from the sound track. Finally, the Ref DC Offset pot  126  when made available as a manual adjustment allows the user to tune out a particular kind of distortion found in variable area sound tracks called Cross Modulation Distortion. 
     The circuit in FIG. 7 is one of two identical circuits, one for the right and the other for the left channel. The output pulses from the Video Amplifier And Comparator Circuits  104  are applied to the input of the Pulse Width To Voltage Convertor  105  where the width of the input pulse produces an output corresponding voltage level. At the end of the pulse width to voltage conversion period Sample And Hold Circuit  106  samples the voltage and stores the value. The output of Sample And Hold circuit  106  connects to the inputs of three comparators  134 ,  135  and  136 . The Pulse Width To Voltage Converter  105  is then reset in preparation for the next pulse from the Video Amplifier And Comparator Circuits  104 . The next pulse input  116  is then converted to a new voltage level depending on the width of that pulse. The output of the Pulse Width To Voltage Converter  105  is connected to the remaining input of comparator  134 . Comparator  134  compares the stored voltage sample from the Sample And Hold Circuit  106  with the last voltage level generated by the Pulse To Width Voltage Converter  105 , if the last voltage level taken is a noise pulse that exceeds the value stored in the Sample And Hold Circuit  106  plus the dc value contributed by the resistor network  140  and  141 , comparator  134  will output a trigger signal to the one-shot multivibrator  143  which in turn applies a fixed width pulse to the gate of transistor  145  which in turn shorts the audio output line  118  to ground preventing the noise pulse from reaching the audio output terminal of the reader. A dc control signal representing an amplitude profile derived from the same channel audio output signal is introduced to the positive input of comparator  134  with the result that comparator  134  is made less sensitive to noise pulses when the output audio signal level is high and more sensitive at low or zero signal levels. The fixed pulse width which is output from one-shot multivibrator  143  is established at a value which will cancel transient noise pulses caused by breaks in the film in the sound track area which are a maximum of a tenth of an inch wide. Since not all noise pulses have the same duration, detected noise pulses shorter than the one-shot multivibrator  143  period are detected by comparator  135  when they go to zero. Comparator  135  then sends a reset pulse to the one-shot multivibrator  143  to terminate its output pulse early thereby allowing transistor  145  to release the audio output line  118 . The resistor network at transistor  145  functions as follows. When one-shot multivibrator  143  fires, transistor  145  is instantly turned on and capacitor C 1  begins to charge. If the pulse from one-shot multivibrator  143  is short, capacitor C 1  will have only a small charge and transistor  145  will turn off quickly. As the width of the one-shot pulse increases capacitor C 1  will receive a larger and larger charge with the result that for longer noise pulses transistor  145  will not turn off rapidly to kill the noise pulse but will turn off more slowly the wider the one-shot pulse becomes. This reduces the noise circuit turnoff thump at the end of long noise pulses. Finally, comparator  136  senses a zero voltage condition at the output of the Sample And Hold Circuit  106  which indicates that a blotch exists on the film which completely cuts off any light passing through the sound track area. Normally there is a dc bias at the output of the Sample And Hold Circuit  106  because the clear area of a variable area sound track never closes completely. This is called the bias line and is used to reduce negative signal swing clipping in sound track readers. If the minimum bias disappears an abnormal blockage is indicated. At this point the positive comparator input voltage from the Sample And Hold Circuit  106  is compared with a minimum reference set by resistor network  147  on the negative comparator input. The output of comparator  136  drops to zero if the comparator  136  positive input drops below the reference set by resistors  147  on the negative input. The output of comparator  136  is connected to one input of a nand gate  149 . The other input to the Nand gate is a 1.25 mhz clock signal. The output of the Nand gate  149  is connected to the switched capacitor output filter  110  clock input terminal. This clock signal is required for audio output switched capacitor filter  110  to function. When comparator  136  output drops to zero the 1.25mhz clock signal for the audio output filter  110  is interrupted causing filter  110  output to freeze thereby holding the last level before the loss of the clock signal and not allowing the output signal to drop to zero with a resulting output noise pulse. 
     In order to provide separate isolated paths for each channel a control signal, Channel Gate  151 , which originates from the master Timing Generator  102  in FIG. 5 is used as a gating signal to route the two channel pulses produced at the output of the Video Amplifier And Comparator Circuit  104  to their proper individual paths. This Channel Gate signal is fixed in time at a 50% duty cycle, half of the total period is allotted to the right channel and half to the left channel. However, the two channel pulses generated by the CCD scanner  103  and conditioned by the Video Amplifier And Comparator Circuits  104  are not locked to the Channel Gate signal and will vary in position with respect to it. A problem exists if the two channel pulses from the Video Amplifier And Comparator Circuits  104  shift position with respect to the Channel Gate  151  and actually collide with it. This is usually caused by the film weaving as it passes the sound track reader particularly with high signal levels. When this happens, depending on which way the shift took place, portions of one channel will spill over into the other channel causing distortion and clipping. To prevent this the circuit in FIG. 8 was included in the design of the reader. The basis for this corrective measure asserts that if the two channel pulse signal from the Video Amplifier And Comparator Circuits  104  does not have a predictable location within the Channel Gate period then the Channel Gate must move in order to align with it. Referring to FIG. 8, the signal from Channel Gate  151  triggers a pair of one-shot multivibrators  153  and  155  on its negative transition which is the beginning of the CCD scan period. These one-shot multivibrators have periods which will set the limits for the amount the Channel Gate  151  will be allowed to shift to accommodate any shift of the two channel pulses. The values were set at 5% of the total scan period. One-shot multivibrator  153  limits the excursion of the left channel of 5% into the right channel period and one-shot multivibrator  155  limits the right channel to 5% into the left channel period. The circuit functions as follows (See FIGS.  8 - 8 . 7 ). The signal from Channel Gate  151  has a 50% duty cycle which is fixed so in order to move it, it will have to be regenerated. When the signal from Channel Gate  151  goes negative the two one-shot multivibrators are both triggered. One-shot multivibrator  153  in turn triggers RS flip flop  157  so that its output transitions negative. One-shot multivibrator  155  transitions high, see waveforms  8 . 2  and  8 . 3 . The other input to the weave control circuit is from the Video Amplifier And Comparator Circuits  104 . This signal is inverted and tied to one input of an AND gate  159 , the other input to the and gate  159  is the output of one-shot multivibrator  155 . 
     Now assume a relationship between Channel Gate  151  and the two channel pulses that places the right channel pulse 3% into the left channel period, see FIG. 8.4. In this situation when Channel Gate  151  goes negative it triggers one-shot multivibrators  153  and  155 , one-shot multivibrator  153  triggers flip flop  157  changing its output to negative and holding it for the duration of the one-shot multivibrator  153 &#39;s period, see waveform FIG. 8.2. One-shot multivibrator  155  changes to a high level enabling one input of the AND gate  159  (see FIG. 8.3) while the inverted right channel pulse from comparator circuits  104  enables the other input to AND gate  159 , (see FIG.  8 . 3 ). This configuration holds the output of AND gate  159  high until the right channel changes to a low level wherein the output of the AND gate  159  also goes low and changes the flip flop output to a high level enabling the left channel time period. Next assume that the left channel is 3% into the right channel period, see FIG. 9.6. Again the falling of the Channel Gate  151  triggers both one-shot multivibrators. In this case, one-shot multivibrator  153  triggers flip flop  157  changing its output to negative and holding it for one-shot multivibrator  153 &#39;s period. One-shot multivibrator  155 &#39;s output enables one input of the AND gate and the inverted right channel pulse from circuits  104  enables the other input to the AND gate  159  holding the output of the AND gate  159  high until the right channel signal goes negative which changes the flip flop a 57  output to the high state enabling left channel about 5% before the 50% transition of the Channel Gate  151 , see FIG. 8.7.