A class of image quantizers called model-testing vector quantizers is introduced. These are finite-state quantizers with the additional ability to make a decision about whether the current input conforms to the assumed finite-state model or not. The nonconforming component is coded at relatively lower bit rates by utilizing well-known linear and nonlinear properties of the human visual system. An adaptive spatial and intensity quantization procedure is used that removes the perceptually irrelevant information from the nonconforming component. The differential brightness discrimination of the eye is also utilized. Resulting coded images are indistinguishable from the originals at bit rates that range from 1.0 to 3.5 bits/pixel for a wide variety of 512*512 images. These bit rates can be regarded as upper bounds for the perceptual entropy of the corresponding images.>

It is shown that postfiltering circuits based on higher order LPC (linear predictive coding) models can provide very low distortion in terms of special tilt. Thus, they can provide better speech enhancement than circuits based on the backward-adaptive pole-zero predictor in ADPCM (adaptive digital pulse code modulation). Quantitative criteria for designing postfiltering circuits based on higher-order LPC models are discussed. These postfilters are particularly attractive for systems where high-order LPC analysis is an integral part of the coding algorithm. In a subjective test that used a computer-simulated version of these circuits, enhanced ADPCM obtained a mean opinion score of 3.6 at 16 kb/s.>

Adaptive differential PCM (ADPCM) is known to provide high quality digitization of telephone bandwidth speech at 32 kb/s. For ADPCM systems operating at sub-standard rates such as 24 or 16 kb/s, a postfiltering technique has been shown to provide a simple means for enhancing speech quality [1]. This paper proposes an algorithm which adapts the degree of postfiltering to the local performance of the ADPCM coder. The adaptation requires no extra information from the transmitter. As a result of the adaptation, segments of speech which are reproduced relatively well by the coder are only mildly postfiltered. At 16 kb/s, the adaptively postfiltered system provides good communications quality with most telephone speech inputs.