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AssigneeAmerican Telephone and Telegraph Company, AT&T Bell Laboratories
InventorsNevio Benvenuto, Guido Bertocci
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In an ADPCM coder and decoder including a so-called locking-unlocking adaptation speed control, the adaptation speed is locked to a very slow, almost constant, speed of adaptation for voiceband data and partial band energy signals, i.e., tones and tone like signals, and is unlocked to achieve a fast speed of adaptation for speech. When a so-called partial band energy signal is being inputted, the adaptation speed is biased toward the unlocked state and when a transition occurs from a partial band energy signal to another such signal, the adaptation speed is set to the totally unlocked state and coefficients of an adaptive predictor are set to prescribed values. This is done in both the coder and decoder to minimize generation of impulse noise in the decoder output.
A coder for converting digital samples in a first code to digital samples in a second code, a first code input sample being supplied to a difference circuit along with a sample estimate of said first code input sample to obtain a difference sample indicative of the difference therebetween, adaptive predictor means for generating a plurality of coefficient signals to produce said sample estimate, adaptive quantizing means supplied with said difference sample and being responsive to a scale factor for producing a quantized second code sample and for producing a quantized version of said difference sample, means for adding said quantized version of said difference sample with said sample estimate and supplying the sum thereof to said adaptive predictor, means for dynamically controlling said adaptive quantizing means speed of adaptation including means for producing a first speed of adaptation state when the input sample represents speech and for producing a second speed of adaptation state when the input sample represents encoded voiceband data or partial band energy signals, said coder being characterized by,
A decoder for converting digital samples in a second code to digital samples in a first code comprising inverse adaptive quantizer means for receiving input samples in said second code and providing at its output a quantized version of a difference sample that was originally encoded from said first code into said second code, adaptive predictor means for generating a plurality of coefficient signals to produce a sample estimate, means for algebraically combining an estimate sample and said quantized version of said difference sample to obtain a reconstructed sample in said first code, means for dynamically controlling said adaptive inverse quantizing means speed of adaptation including means for producing a first speed of adaptation state when an input sample represents speech and for producing a second speed of adaptation state when the input sample represents encoded voiceband data or partial band energy signals, said decoder being characterized by,
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