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.
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.
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
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.
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
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.
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
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.
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.
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.
is adjusted to provide maximum bandwidth without transient overshoot.