Broadband transmission on limited bandwidth digital line

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. The translator includes a ROM addressed by the most significant bits of the 14-bit words. Stored in the ROM at each such address is a table of 12 instruction words. The instruction words direct a switch to select each bit for the 12-bit word, whether to invert it, and whether to include it in parity.