The family of response curves, as R6 is adjusted, is shown in Fig. 7. When optimally adjusted, the square wave response of the ribbons becomes very smooth and free of overshoot.

It is necessary to provide a means to limit peak modulation of the ribbon in order to maintain a specified minimum and maximum width of light or dark areas of each individual soundtrack. A soft peak clipper is provided at the input of the drive circuit with dc coupling so that precise boundaries may be established for a modulation of each track (Fig. 8).

A pair of pole-zero networks at the input of each drive amplifier assembly corrects the 12 dB/octave falloff of the ribbon-amplifier combination above the resonant frequency of the ribbon. The resulting frequency response of a typical total system, as read with a silicon solar cell, is shown in Fig. 9. The peak and valley at about 16 kHz are caused by the second harmonic of the ribbon, which is not fully damped by the magnetic field.

The sound of high-fidelity signals fed into this amplifier-ribbon system and monitored through a high-quality solar cell and amplifier is free of the usual resonant ribbon sound. Even impulse noise from phonograph records is handled smoothly.

Ground-noise reduction is not used. It would serve no useful purpose due to the manner in which the tracks are read by the CCD scanner. To preserve the integrity of the dynamic range, great care is taken to ensure that the light-valve ribbons never clash. Assuming that the master magnetic recording is not compressed, the average modulation level will be substantially lower (peak excursion, ±0.008 in) than in a conventional system. Figure 10 shows the performance curves of a four-track stereophonic valve RA 1563-B and of the re­producer.

The Colortek CCD Reproducer
The Colortek CCD Reproducer em­braces entirely new technology, principally intended to reproduce a multiplicity of discrete soundtracks, while also reading conventional Academy variable-area and variable-density tracks with improved quality. It uses a charge-coupled-device (CCD) cell array scanning a narrow focused slit, which only reads the moving edge boundaries of variable-area recordings. In contrast to a conventional reader, it is virtually blind to all other areas of the film, which are subject to dirt and scratches, making this reproducer substantially less susceptible to film-born noise. The system converts four recorded tracks to pulse-width-modulated (PWM) trains. Film weave, optical-dirt buildup, exciter-lamp aging, image spread and shrinkage, which causes bad cross-modulation distortion, are all electronically recognizable on the scanned pulse trains. These anomalies are automatically compensated for without a servo, moving parts or adjustments. When this system is used with its complementary compander, it provides the necessary alignment and drift-free operation. A dynamic range of 80 dB has been attained, together with 40-dB channel separation. The frequency response of the present scanning system is Hat from 20 to 12 kHz. When CCDs optimized for this system become available, the upper limit will be improved.

Optical and Scan Geometry.
The overall arrangement is similar to a conventional focused-slit illumination-averaging reader. Instead of using a mechanical slit optically projected on the film, there is a 17 µm aperture in a monolithic array of 256 image-sensing cells of the CCD. This device, together with its optics and support circuitry, is relatively large and is configured so that it can take the place normally occupied by the exciter-lamp assemblies of the standard American sound heads. A sub-miniature halogen-cycle exciter lamp and its condensing optics take the place of the photoreceptor in the sound drum. This grouping (Fig. 11) places the emulsion in its conventional position facing the CCD scanner's objective lens which has an intermediate-sized aperture.

Dirt and scratches on the base side are thus projected with slightly less amplitude, yielding an overall reduction of noise, be­cause they point toward the relatively large illumination source.

The halogen-cycle exciter lamp has a built-in first stage aspheric condenser lens as part of its envelope. An external infrared heat shield (necessary for the film-plus-CCD system) and secondary correction lens render an illumination uniformity within 10% over an area measuring 0.4 X 4 mm. This linearity is less than desired for low-distortion average-area reproduction. However, it will be shown that boundary or edge-scan methods are much less sensitive to this phenomenon. The slight tradeoff results in a miniature assembly that can be mounted entirely within the sound drum. There is also a higher optical efficiency, which allows the filament to operate at just sufficient temperature to maintain the halogen cycle. This long-life operator produces high infrared energy, which in turn would cause blooming or image smearing in the CCD. This is corrected by interposing cold and spectral-limit filters. (Theoretically, the absence of infrared radiation makes it unnecessary to have a redeveloped silver track. The spectral sensitivity curve of the CCD scanner is such that a dye track of appropriate density, hue and commensurate sharpness should yield satisfactory sound reproduction.)

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©1978 SMPTE Journal
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