If the CCD were operating in a true on-off mode, such low amplitudes would have serious aliasing, or cell-to-cell dis­tortion components. Instead the CCD op­erates as a very wide dynamic range linear sensor, where each of its 256 cells is able to yield 40 dB dynamic range, between the maximum illumination and the averaged noise. One hundred discrete illumination levels can be detected, with 50% or better certainty, and defined as signals from any of these cells. When cell-to-cell uniformity, scattering and exciter flux uniformity are considered, these defined points or levels are reduced to 50. A linear phase-interpo­lation filter is designed for an overlap of 4 to 5 cells, so that the final transition region presented to the sliced comparator is made from a statistical average of 2 cells, yielding 60 to 70% of the sample; 2 more yielding 20 to 30% and the remaining adjacent cells contributing a small amount to the sample. When all of these are averaged, 6 dB for the in-phase optical phenomenon and 3 dB for statistical noise, the resulting sample at the transition can resolve about 200 levels. The transition rise time is about one fourth that of the electrically scaled 12-kHz res­olution. However, the entire transition re­gion can still modulate back and forth at the 12-kHz rate, as the sideband frequencies are small when compared to the roughly 1-MHz cutoff of the phase-inter­polation filter. In sweeping back and forth, the transition slice region has about 1/200 of cell-to-cell alias ripple, an amount well below the intrinsic electronic noise. Thus the best-case incremental resolution would be:

4 cells X 13 µm/200 points = 260 nm

This aliasing or incremental cell-to-cell noise is well below the 1 to 4 µm scaled electronic noise, achieving 46 dB dynamic range. These numbers convert to pulse differential times, giving a 2.5 ns electrical resolution. Such accuracies have necessi­tated an unorthodox logic design, calling for very careful board layout, to prevent random crosstalk spikes. This careful control of clock jitter and its resulting low noise at the CCD output allows scan reading down to film grain dimensions.

Cell Interpolation and Pulse Generation.
The ideal 1 to 4 µm transition boundary almost doubles at the CCD be­cause of scattering, reflections and objec­tive MTF. The multilevel average signal from the cells scanned in this region is presented as a series of clocked cell pulses to a low-pass filter (Fig. 13c). This filter is tailored with a ripple function in time, which matches and complements the pro­jected optical transmission function in space. This arrangement permits many cells to be interrogated, yet still to convert into pulses very narrow boundaries, which almost clash. Without this technique, high-modulation noise would result as a tradeoff from eliminating pulse crowding dropout noise effects at high modulation. Figure 15 is an expanded section of Fig. 14. It shows the clock cell pulses (Fig. 13c) superimposed with the interpolation filter output (Fig. 13d). In practice, once the optics and filter match occur, the electrical transition appears as near Gaussian. The inflection point of greatest slope then coincides with 50% minimum to maximum transmittance and also with the correctly scaled soundtrack object image size. This optimization occurs simultaneously for pulse crowding, modulation noise and cross-modulation axis shift. Additionally, the highest width modulation at short wavelengths also occurs at this 50% point, yielding the maximum high frequency output upon PWM conversion.


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