near instantaneous companded audio multiplex (NICAM)
Various methods are used for sound transmissionon television. While in some European countries two- channel sound with sound carriers and secondary sound carriers is preferred, there is NICAM (Near Instantaneous Companded Audio Multiplex), a method developed by the British BBC in the 1980s for the transmission of high-quality digital sound.
NICAM, which is specified in the European standards under ETSEN 300 163, operates on a digital basis and digitizes the audio signal in pulse code modulation( PCM) with 14-bitresolution and a sampling rate of 32 kHz. The digitized audio signal is compressed and transmitted in data packets, each with 32 quantizations per second. The NICAM channel with a data rate of 704 kbit/s is available for transmission.
The NICAM signal is an independent audio signal containing audio signals for stereo, two-channel mono, mono with data channel, or for one data channel only. It is transmitted over a separate subcarrier in addition to the audio carrier over which the mono signal is broadcast. The subcarrier frequency depends on the television standard on which NICAM is used and can be 5.85 MHz for the B/G system or 6.552 MHz for the I system. The NICAM data is transmitted in a 728-kbit/s data stream, which in turn consists of 728-bit long NICAM data packets, so that one data packet is transmitted every millisecond.
On the receive side, the corresponding NICAM service is shown in the display. Due to the different transmission services, one could transmit a three-language mono broadcast with NICAM: the first language via the audio carrier, the second and third via the NICAM subcarrier in two-channel mono.