Fast rise pulses are made by holding the minimum and maximum long-term averaged transmittance levels and dividing these into an intermediate level applied to one input leg of a voltage comparator. The raw filter output, fed to the other leg, gives comparator output pulses, starting and ending at the slicing points of the predetermined optical density. Additional compensation circuits are used to prevent start conditioning, IR blooming and slicing errors caused by dispersion effects from altering the averaged sliced level.

Noise From Scratches and Dirt.
Most dirt contamination and wear scratches will not exceed the optimum slicepoint density at the clear film areas. As long as these are away from the transition boundary, this noise is very small, mainly from second-order scattering and reflections of the light beam. Deep scratches in the D_max area cutting through the emulsion and giving densities less than the slicing point will reproduce as extra pulses, which could cause about the same electrical noise as average-area readings (Fig. 16). These pulses will also cause count errors on the overall scan window edge transition train, which by necessity are electronically recognized in order to prevent channel mixup. If such extraneous pulses occur, holding circuits stay active, thereby rejecting scan frames with this problem and maintaining the demultiplexed levels of the last good scan window. This technique is similar to dropout compensation in video recorders, except that in this usage, extraneous unwanted signals are removed and an almost inaudible hold or fill in occurs.

Dirt and scratch anomalies occurring to any degree in the transition zone will re­produce with intensities slightly higher than with a conventional reader. The probability of this occurring is closely equal to the ratio of anomaly size and transition zone width to channel track width. This noise has fewer events per unit time and is, therefore, more impulsive than average illumination reading. However, the overall rms level falls more than 10 dB below the conventional reading of the same track geometry.

Readback of Four Discrete Channels.
The Colortek record format resembles the conventional variable-area dual bilateral soundtrack, except that the four boundaries modulate independently of each other. Ground-noise reduction is not used, in order to maintain a fixed average center line of each modulated boundary and to permit recording down to 20 Hz and below for special applications. These modulated line boundaries, plus a dimension reference septum track, allow several ways of decoding and correcting mechanical transport noise, jitter and film weave.

One decoding method generates positive-going pulses for each edge transition. The off-going edge is from a free-run generated square wave 90° out of phase with respect to the average transition boundaries. The polarities are set so that the average image train subtracted from the reference square wave gives zero volts, when the 90° phase relationship holds. Small phase errors translate into dc offset voltages, which operate a variable-delay, gated, voltage-controlled oscillator. With correct polarities, the closed-loop system will vary its time delay to lock onto the signal PWM train. It also gives the position or time-reference transition needed to demodulate each channel edge. Changing the frequency portion of the reference oscillator corrects for magnification changes, which can occur in recording or in focusing the reproducer objective. Both of these control actions permit centering of the demodulation time reference, making the system insensitive to film weave and allowing high modulation on each track.

Individual channels are commutated as transition edge to reference square wave edge variable-width constant-current pulses. These pulses, when presented to individual channel capacitors, yield an area-proportioned voltage which is held until all channels and the reference track have been commuted and the edge transitions counted. When this end-frame count is correct, all charges are transferred to receiving low-pass synchronous filters, which remove the commutation components and smooth the waveforms. Improper counts, resulting from dirt or gross errors such as splices, will not develop a transfer signal, thus preventing the low-pass filter from being clocked to accept new charges. Thus, gross noise and spurious channel mix-up are largely eliminated.

A rough time-delay aperture equalizer is built into the low-pass filter, to help reduce ringing and pre-shoot occurring with sharp wavefronts. This degree of sophistication is necessary to ensure accurate RMS detector tracking of the expander control circuitry. This technique minimizes the audibility of small, rapid level changes which are caused by voice articulation transients.

Cross-modulation Control. Most optical low-order cross-modulation occurs from image spread or shrinkage. This causes the average area of a symmetrically modulated signal to shift as a function of modulation frequency and density. Commonly en­countered short-wavelength fill and peak rounding will cause equal distortion for unilateral, bilateral and dual bilateral recordings. Push-pull dual bilateral modulated tracks cancel this type of first order distortion, but some third harmonic and level compression still remains.