ABSTRACT
Compressed or data rate reduced digital audio first appeared on the analogue terrestrial broadcast services of Europe in the later part of the 1980s. Based on an earlier similar system invented by the BBC, NICAM 728 (Near Instantaneous Companding and Audio Multiplexing) is capable of delivering near CD quality sound in stereo, mono or dual mono (for dual language transmission). NICAM is still found in PAL I, B, G and H
systems mainly in Europe, and shares a commonality with the MAC packet family common in Europe on the DBS system. There are three main stages of processing: analogue-
to-digital (A-D) conversion, near instantaneous companding and multiplexing of the data stream. In the A-D process, preemphasis is added to the signal according to CCITT J.17. Then it is sampled to 14-bit resolution at 32 kHz sampling rate, the samples being coded in two’s complement. The sampling frequency of 32 kHz was chosen for its commonality with other contribution circuits and MAC systems. To prevent aliasing, a 15 kHz filter is introduced before the A-D stage. The signal then undergoes a near instantaneous companding
process, which is achieved by assigning five companding coding ranges that can be signalled by just 3 bits. For details of the exact method, refer to the BBC R&D document RD 1990/6 by Bower.2 The companding effectively reduces the data rate to 704 kbps, comprising 64 10-bit samples each with a parity bit. The coding information indicating the companding applied to all 32 samples in each channel in each 1-ms block is signalled to the receiver in a 3-bit scale factor code, which is applied using ‘Signalling-in-Parity’. The bit stream is assembled of the 704 sound/data bits which
are bit-interleaved, 11 bits of additional data, 5 bits for control information and an 8-bit frame alignment word, totalling 728 kbps.
