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At&t Bell Laboratories

Bell Telephone Laboratories

David W Petr

John B Sharp

George W Schramm

Jeffrey Neumann

A data transmission system that can controllably transmit data in three different modes between connected data devices. The first mode transmits data at a 64 Kbps rate. The second mode transmits data at a 56 Kbps rate plus control signals at an 8 Kbps rate. The third mode transmits data and control messages at a rate of up to 50 Kbps between connected terminals or other data devices. For all modes, the data is transmitted by inserting it one byte at a time into an information field of successive ones of cyclically reoccurring time multiplexed frames.

Multimode data communication system

Accumulation of distortion possible from multiple digital code conversions is eliminated in an ADPCM coder that converts PCM incoming signals, e.g., .mu.-law PCM to linear PCM and, then, to a quantized n-bit differential PCM output signal. Elimination of distortion accumulation in subsequent code conversion stages is realized in the coder when the differential PCM output is not on the most positive or most negative ADPCM quantizer steps by controllably modifying the coder differential PCM output so that a next subsequent coder would generate the same differential PCM value. Accumulation of distortion in subsequent code conversions when the differential PCM coder output is either the most positive or most negative quantizer step is eliminated by controllably modifying a decoder output so that the next subsequent code converter will choose the same ADPCM code value.

Digital code converter

An ADPCM coder (100) converts a linear PCM input signal representative of PCM encoded speech or voiceband data into a quantized n-bit differential PCM output signal. Samples of the PCM input signal are delivered to a difference circuit (11) along with a signal estimate of the same derived from an adaptive predictor (12). The resultant difference signal is coupled to the input of a dynamic locking quantizer (DLQ). A quantized version of the difference signal is delivered from the output of said quantizer to an algebraic adder (17) where it is algebraically added with the signal estimate. The result of this addition is coupled to the input of the adaptive predictor, which in response thereto serves to generate the next signal estimate for comparison with next PCM sample. The adaptive quantizer has two speeds of adaptation, namely, a fast speed of adaptation when the input linear PCM signal represents speech and a very slow (almost constant) speed of adaptation for PCM encoded voiceband data or tone signals. The decoder (101) operation is essentially the inverse of the coder, and similarly has two speeds of adaptation.

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Red Bank, NJ, United States of America

David W Petr