decoder has a complementary weighting curve to the encoder. The decoder level
sensor weighting is identical to the encoder network, the result being an encode-decode
response which is flat over the audio band.
necessary property of this system is the ability to decode monophonic signals
summed from four individually companded tracks. Listening tests have
demonstrated that a single expander can satisfactorily decode combined multitrack
sound with only minor crossmodulation of signal envelopes, the final result
being a significant improvement over the limited dynamic range currently
reproduced from motion-picture soundtracks.
The Control Track
narrow track is provided in the middle of the soundtrack group (Fig. 4). This
track has four functions:
A position reference to provide information to be used to correct film weave.
Film-weave errors are primarily present below 100 Hz and can be quite large in
the 5 to 30 Hz band. Since each track is unilateral, correction for weave
reduces low-frequency noise and avoids wasting amplifier power and loudspeaker
voice coil travel with subsonic noise signals.
A means of monitoring image spread to allow exposure and developing of
soundtracks to be controlled optimally at each step through measurement and control
of track width.
A command and data track to control ancillary effects in the theater and
establish correct playback speaker format for each film automatically.
4. A means to
control any other necessary functions such as smells, lights, curtains,
theater equipment and to carry identification information.
This control track
is typically 0.002 in wide. It is modulated in snake-track fashion, that is,
from side to side, with constant line width. Modulation is ±0.002 in in
amplitude using signals above 300 Hz. This track acts as an accurate position
reference for errors below 100 Hz, even while it is being modulated.
requirement of this system is for compatible monophonic playback. The coupling
of this control signal into mono playback is therefore of concern. The
modulation of the control track is 30 dB under the combined level of the four
variable-area tracks. Furthermore, a snake track couples into a light-level
sensitive sound system only at the level of the push-pull crossmodulation
products. Normal practice holds the single-ended crossmodulation products at
about —40 dB. The push-pull products are usually 20 dB lower. Hence, the
overall level of control tones present in mono playback is 90 dB below peak
program level. This level is sufficiently low
to be imperceptible under monophonic theater listening conditions. With the
stereo playback system, each track is electrically separated by a position-sensing
commutator system, and thus no crosstalk is present from the control track.
For convenience and economy the existing
telephone touch-tone codes are used as command signals. A brief review of the
touch-tone system is in order, to give a basis for understanding how this
system is applied to motion-picture projection use.
The standard touch-tone keyboard
has four rows and three columns arranged in a matrix. Each row and each column
has a unique tone associated with it. Each button depressed results in two
tones, one unique to each row and the other unique to each column. Note that
the bottom row has two symbols other than "O," one marked with a star
(*) and the other with a number symbol (#). Tones between 697 Hz and
1477 Hz are used to convey the information.
A large number of unique codes may
be required for picture and print identification as well as for elaborate
effects work, and yet, it may sometimes be necessary to give a series of
closely spaced commands. A two-tier system has therefore been selected, a
two-digit system for up to 100 rapid or commonly used commands, and a
five-digit system for up to 100,000 commands, identification numbers, etc.
Since these commands may be
manually generated with rather uneven push-button actuation, an actuate code
must be used at the end of each word. This "actuate" command also
indicates to the decoding circuit the number of characters to be expected in
the word. The * indicates that a two-digit word has been transmitted and the #
indicates that a five-digit word has been transmitted.
The frequency band above 1500 Hz is
available for additional codes and other features such as automatic cue and
Replay Modes and Compatibilities
Modes of the Colortek Track. Basically, three read-out modes of the
Colortek track are possible. They are: (1) read-out by the CCD scanner, as
described; (2) read-out by a standard or customary optical sound reproducer; (3) read-out by
means of a solar cell.
The video output
of the CCD scanner is arranged, as described, to determine the electrical
equivalent of the image edge, which is then sliced to ensure its accurate
retention. After dc restoration or squaring, the circuitry deals with pulse
trains at approximately four times the scanning rate. At this point, a
monophonic output is available, being the electrical sum of all channels. (The Veritone
low-frequency system mentioned below and an Academy filter are fed from this
point, for replaying conventional soundtracks.) Thereafter, for stereophonic
reproduction, logic is performed to
unscramble the composite signal into four discrete outputs. These outputs go to
four expander circuits and balanced line drivers to feed the main fader. At
this juncture, one has a direct replacement for conventional four-track
magnetic prints. Although the present reproducer is arranged to commutate four
discrete tracks, a small alteration to this section of the circuitry will
enable it to reproduce n tracks, where n could be 6, 8 or more.
When two additional channels are required,
as is the case when it is desired to play 6-track Todd-AO 70mm film reduced to
the 35mm format with a Colortek track and using the CCD scanner, this is made
possible by the use of a 4-2-4 matrix, such as the Sansui QS System. Under these
circumstances, the encoder is introduced before the recorder, and the decoder
after the reproducer. This method has the advantage of being able to play the
same print as either 1, 4, 5, or 6 channels of sound, thereby satisfying the
single inventory requirement. This technique uses the left and right tracks (1
& 3) for the matrix, leaving the center and surround (2 & 4) for discrete
use, thereby assuring positional integrity. It has been found to be mandatory
to retain the solid discrete center, in order to guarantee satisfactory
reproduction for the vastly variable acoustical environments that prevail in
Other than using the CCD scanner,
it is possible to play Colortek recordings through a standard reproducer with
the Academy rolloff incorporated, but this will not give an ideal reproduction.
Therefore, an alternative simple overall read-out unit has been developed,
which has a 1-mil optical slit and a solar cell as part of the sound head. The
electronics contain one preamplifier per projector, mounted in the sound head,
and one central control box. This consists of a power supply, expander,
switching logic for the system selection (i.e., Academy or Colortek), projector
selection and line amplifier with balanced transformer output to feed the main
fader. By this method, the wide dynamic range and frequency response of
Colortek are available as "Hi-Fi Mono."
Reproduction of Academy Tracks. Standard
variable-area and variable-density tracks can be easily reproduced by the
Colortek CCD scanner. Any pulse train from the center slicing comparator yields
an average voltage, which is proportional to the transparent area of the track.
This voltage is derived from a scan-rate synchronous low-pass filter, similar
to those connected to the multi-channel commutator. Its output is taken
through an Academy filter. Variable-density data are taken from the same filter
connected to the input of the center slicer comparator. The CCD delivers an
overall window envelope, with amplitude proportional to cell illumination.
Both formats, variable-area and variable-density, together with the Colortek
multi-channel track, can be logic recognized and correctly presented to the overall theater sound system without operator