Abstract: To insure timing recovery, T carrier requires that the signal contain no more than fifteen consecutive zeros and a "1"s density of at least one out of eight. When a 15 kHz audio program signal is converted into 12-bit words and four words inserted into six 8-bit time slots of the T1 line, timing recovery requirements have been met by using, in addition to an even parity bit, a dedicated "1" bit.Here, instead of the usual eight chords, the compression characteristic used in generating the 12-bit words has only 7 chords, each represented by a combination of three chord bits that always includes at least one "1". This frees the dedicated "1" bit for additional coding detail. Centrally locating the chord bits within the 12-bit word further allows a mix of voice and program channels in the one channel bank.In a disclosed implementation, the analog samples are first converted to 14-bit linear PCM words, and then digitally translated into 12-bit compressed words.

Abstract: A reduction in idle channel noise and crosstalk in a mid-riser-biased successive approximation encoder is achieved through the use of two polarity decisions. Encoder (401) assigns a sampled analog input signal to the closest one of a multitude of discrete signal levels or code steps. A comparator (109) provides this assignment by successive comparisons of the sampled signal with a series of reference signals (110). Each comparison produces a binary digit. The first comparison, or polarity decision, is not transmitted and instead coupled to feedback circuitry (401) to reduce any dc component in the analog input signal to substantially zero. A second polarity decision is then made using a non-zero offset reference signal (402) corresponding to an intermediate position on a code step, typically the midpoint. The non-zero offset reference signal is applied along with subsequent reference signals to the comparator to determine the closest code step.