The compander.
Figure 3 shows a comparison of the noise spectra of magnetic tape, magnetic stripe on film, and the clear area of a photographic soundtrack. It can be seen that, for optimum results, a compander designed for magnetic studio recording will, at best, give a compromise in performance when used for photo­graphic sound recording. This is due to the clearly audible difference between clear film and a silent magnetic track, where the greatest divergence falls in a region in which the ear is very sensitive. Of course background noise will be less with a re­corded soundtrack than with clear film. However, in practice, the difference be­tween modulated film and magnetic tape is about 20 dB worse for the film.

A special compander has been developed for the specific requirements of the Colortek system. Its purpose is to render the optical soundtrack equal to, and perhaps better than the magnetic master. Its signal compression ratio is 2.3:1 in order to obtain the desired dynamic range and SNR. The compander uses a single-band system which has the advantage of correct operation regardless of the operating level. RMS level sensing is used to keep down spurious noises due to dirt or gross damage. Under normally existing conditions a SNR of 45 dB is considered good. When using one-quarter tracks (as is done with the Colortek system) the SNR would be worse. Our compander achieves an SNR that exceeds 90 dB per quarter track. The control circuitry for the compander has been designed so that prints from the negative obtained with its use can be played in a monophonic as well as a stereophonic mode. Due to the novel design of the reproducer, crosstalk between tracks is down more than 40 dB.

The multi-purpose control track.
Figure 4 shows the track layout of the Colortek system and the position of the normal 0.084-in wide reading aperture. Note that, in its final form, a fifth narrow track is laid down in the exact center of the recording area. It is 0.002 in wide and is modulated "snake-track" or push-pull fashion ±0.002 in. In other words: this track, called the control track, is of constant width. It serves four essential purposes.

1. It enables the CCD scanner to recognize and compensate any side weave of the soundtrack, whether it originated in printing or by alignment errors of the sound reader. The distance between the reference edge of the Film and the center of the scanning slit should normally remain constant as the soundtrack passes through the reader. If mechanical weave is present, this distance will vary accordingly, and the control track will vary its position with regard to the center of the scanning device. Printing weave will produce the same effect. In transversal scanning by the CCD scanner these length differences are transformed into time differences, and the appropriate electronic logic makes it possible to compensate for these and lock the scanning process to the center of the control track.

2. The control track may also serve as an image spread monitor. Because the normal width of the control track is precisely fixed, image spread (or contraction) will be read out by the CCD scanner as a change in width. This is identical to a zero-axis shift on which a cross-modulation test can be based. By using these data, the electronic slicing level of the reproducer can be altered so that the system will see the correct 2-mil dimension of the control track, thereby assuring perfect symmetry and correcting, within limits, deviations caused in printing or processing or through changes of rawstock. Such adjustments reduce distortion from these causes to negligible proportions by automatic image-spread monitoring.

3. The control track can be used to trigger special effects such as lights, smells, sounds, or auditorium speaker groups as encoded on the control track by pulse bursts. The code commands are generated by a touch-tone keyboard, similar to that of a telephone, and are read out in the same way. The system has an encoding capacity of up to 100,000 commands. Of this capacity, less than 100 have been assigned initially.

4. Another use of the control track is the general automated operation of the theater. Commands can be recorded for the operation of house lights, curtains, screen for­mat changes, etc.

Finally, another use of the control track can be envisioned. By looking at the control track by means of a secondary reader placed ahead of the CCD scanner (or of any other optical scanner, for that matter) it is possible to sense bad splices or gross damage to the soundtrack and to suppress the audio momentarily.

The Light-Valve Electronics

The requirement for full frequency response and improved transient tracking for the Colortek system has required a redesign of the electronics driving the light valve ribbon.

Normal practice is to drive these ribbons through a transformer in a current drive mode with a damping resistor mounted in the galvanometer assembly. Ribbon resonance is commonly between 8 and 9 kHz with a 4-dB peak at this frequency. A 0.5-ohm damping resistor is used to achieve this result. Figure 5 shows the standard connection.

This resonance may be reduced or eliminated with a resonant filter having exactly complementary response, with tight tolerances in values or frequency shifts. Otherwise, this resonance may cause quite large errors in frequency or time response.

A much more stable solution is to pro­vide an adjustable negative resistance at the terminals of the ribbon. In this manner the same resonant system which causes the frequency response errors is involved in their correction.

The circuit used to provide this negative impedance is shown in Fig. 6. A four-terminal configuration is used to eliminate cable and contact resistance effects. Rl and R2 provide voltage feedback through pin 4 of the connector, thereby serving to maintain zero impedance at this point if R6 is at the bottom of its travel. R6 provides an adjustable positive current feedback using R7 as a current metering resistor. R3 and R4 are chosen to reduce the effect of cable and contact resistance in the pin 1 circuit.

R6 is adjusted to provide maximum bandwidth without transient overshoot.


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©1978 SMPTE Journal
HTML transcription and graphic reproductions ©2001 The American WideScreen Museum