How NICAM Works
In order to provide mono "compatibility", the NICAM signal is transmitted on a subcarrier alongside the sound carrier. This means that the FM or AM regular mono sound carrier is left alone for reception by monaural receivers.
A NICAM-based stereo-TV infrastructure can transmit a stereo TV programme as well as the mono "compatibility" sound at the same time, or can transmit two or three entirely different sound streams. This latter mode could be used to transmit audio in different languages, in a similar manner to that used for in-flight movies on international flights. In this mode, the user can select which soundtrack to listen to when watching the content by operating a "sound-select" control on the receiver.
This is the spectrum of NICAM on PAL system. On the SECAM L system, the NICAM sound carrier is at 5.85 MHz, before the AM Sound Carrier, and the Video bandwidth is reduced from 6.5 MHz to 5.5 MHz.
NICAM currently offers the following possibilities. The mode is auto-selected by the inclusion of a 3-bit type field in the data-stream
- One digital stereo sound channel.
- Two completely different digital mono sound channels.
- One digital mono sound channel and a 352 kbit/s data channel.
- One 704 kbit/s data channel.
The four other options could be implemented at a later date. Only the first two of the ones listed are known to be in general use however.
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