NICAM reception
At the receiver, the tuner converts the vision carrier and the FM sound inter-carrier to an IF of 39.5 and 33.5 MHz, respectively. The NICAM carrier is 6.552 MHz away from the vision carrier, so it is converted to an interme- diate frequency (IF) of 39.5 – 6.552 = 32.948 MHz (normally referred to as 32.95 MHz.
This is demodulated by a DQPSK detector and applied to the NICAM decoder, which reverses the processes carried out at the transmitter to recreate the 14-bit sample code words for each channel. This is then followed by a digital-to-analogue converter (DAC), which reproduces the original analogue two-channel, left and right, sound waveforms.
The basic elements of NICAM sound reception in a TV receiver are shown in Figure 17.9. Following the tuner, a special surface acoustic wave
(SAW) filter provides separate vision and sound IF outputs. A sharp cut- off removes the two SIFs, 33.5 MHz for mono and 32.95 MHz for NICAM, from the 39.5 MHz vision carrier. The SAW filter provides a separate path for the FM and NICAM carrier IFs. In the sound IF demodulator, the
39.5 MHz pilot frequency is used to beat with the FM SIF and with the NICAM IF to produce 6 MHz FM and 6.552 MHz DQPSK carriers. Sharply tuned filters are then used to separate the two sound carriers. The FM car- rier goes to a conventional FM processing channel for mono sound and the 6.552 MHz NICAM phase-modulated carrier goes to the NICAM process- ing section. This consists of three basic parts. The DQPSK decoder recovers the 728 kbps serial data stream from the 6.552 MHz carrier. The NICAM decoder descrambles, de-interleaves, corrects and expands the data stream back into 14-bit sample code words. Finally, the DAC repro- duces the original analogue signals for each channel.