Digital transmission of commentary-grade (7 kHz) audio at 56 or 64 kb/s

A two-band coding system has been constructed for the purpose of providing commentary grade (7 kHz bandwidth) speech or music transmission at 56 or 64 kb/s. The lower band, 0 to 3650 Hz, is coded with 4 bit adpcm and the upper band, 3600 - 6800 Hz, is coded with 3 bit or 4 bit apcm. The quality of the coded signal makes the method useful for news and sports broadcasts, and possibly for AM remote music broadcasting. The audio sounds better than that produced by two conventional alternatives: 3200 Hz bandwidth with 8-bit coding and 7000 Hz bandwidth with a single 4-bit coder. The system may be used in any place with access to a 56 kb/s Dataphone Digital Service port or to other 56 or 64 kb/s lines. The power consumption is approximately 12 watts in the present form; it could be reduced by a factor of at least 2 by hardware optimization.