NICAM STEREO SOUND
Two alternative systems are available in the UK for sound with analogue terrestrial TV transmissions. The longest established is f.m. mono, with a single sound channel on a 6 MHz (system I, UK) or 5.5 MHz (systems B, G) carrier, intercepted by a narrowband ceramic filter and treated the same as in an f.m. radio receiver. An alternative and better system, Nicam, is available with many transmissions, carried alongside the mono f.m. signal which is retained for compatibility.
OVERVIEW
The Nicam system is a digital one, with data conveyed in phase modulation of a low-level carrier, spaced for system I at 6.552 MHz above the vision carrier. The modulation system adopted is DQPSK (Differentially encoded Quadrature Phase-Shift Keying). The base- band signal (e.g. L and R) is sampled at 32 kHz rate with an initial resolution of 14 bits per sample. A companding system is used, with compression to 10 bits per sample in 32-sample (1 ms) blocks. For immunity to interference, parity bits are added and 45 × 16-bit interleaving is used. The frame format for this system is 728-bit frame length per 1 ms with 8-bit lumped frame-alignment word. The Nicam carrier is radiated at a point 20 dB below the peak vision carrier level. At the receiver the Nicam carrier emerges from the tuner at a frequency of 32.95 MHz. It is selected by a filter for passage to the Nicam decoder, whose first section is a DQPSK demodulator. Emerging from that as a datastream, the Nicam signal is progressively descrambled, de-interleaved, error-corrected, D−A converted and filtered. The baseband signals thus derived are amplified and passed out to loudspeakers.