A Two-Channel Picture Coding System: II--Adaptive Companding and Color Coding

The two-channel picture coding system previously reported is improved by adaptive companding and extended to component color coding. The basic system divides the spectrum into lowand high-frequency spatial components. The former are coarsely sampled and finely quantized, while the latter are finely sampled and coarsely quantized using a companded, randomized quantizer. The adaptive version uses a companding factor, larger than one, which multiplies the highs before coding and divides the result after decoding. This substantially raises the signal-to-noise ratio, which is calculated for both the adaptive and nonadaptive cases and confirmed by computer simulation. The color version uses an achromatic highs channel as in the basic monochrome system, plus a three-color lows channel coded in a perceptually uniform color space. With 3.5-4.3 bits/pel for 512 × 512 monochrome and color still images, the nonstatistical version produces images free of visible defects under normal viewing conditions. Projections are made for system parameters capable of achieving the very high quality now being proposed for production component coding systems, but with substantial reduction in channel capacity.