NICAM
The system of transmitting sound for TV broadcasting employing a sound FM inter-carrier provides a good quality sound given good quality sound amplification at the receiver. However, it is incapable of producing hi-fi quality and unable to carry stereophonic sound. Stereo sound has been successfully transmitted by VHF radio broadcasting using analogue mod- ulation of an FM carrier. Such a system does not readily commend itself for TV broadcasting because of its bandwidth requirements. It is not pos- sible to add a second sound carrier without causing unacceptable inter- ference to either the vision or the primary 6 MHz sound carrier. To avoid this, compromises would have to be reached, and this would defeat the original aim of stereo hi-fi sound transmission.
After years of research and development, BBC engineers came out with a radically new sound system for TV broadcasting using state-of-the-art technology. The system became known as NICAM 728 or NICAM for short. NICAM stands for near instantaneous companded audio multiplex and 728 refers to the data rate of 728 kbps. It provides two completely independent sound channels so that dual-language sound tracks may be transmitted in place of stereophonic sound. It can carry data in one or both channels and is completely separate and independent of the existing FM monophonic sound channel.
In the NICAM system, A and B channels are digitised at a sampling rate of 32 kHz. The digitised analogue sound signals are grouped into data blocks of 704 bits each and the data blocks are organised in 1 ms frame structure (Figure 17.4). Each data block is preceded by a frame alignment
word (FAW) to inform the receiver of the start of each frame. The total frame is then used to modulate a 6.552-MHz carrier, which falls just out- side the normal 6 MHz FM sound but remains within the total TV channel bandwidth of 8 MHz. For stereo transmission, the two sound channels are multiplexed, digitised and transmitted in turn.
The basic outline of the NICAM stereo system is shown in Figure 17.5. The two analogue sound channels, A and B, are pre-emphasised before going into an analogue-to-digital converter (ADC) with a sampling fre- quency of 32 kHz and a Nyquist rate of 30 kHz resulting in a maximum audio frequency of 15 kHz. Groups of 32 samples of each channel are grouped together to form the basic data segment of the system. The sam- ples of each segment are then converted into 14-bit codes. This is followed by a 14-to-10 companding network which compresses the 14-bit codes into 10 bits without any significant loss of quality. The error detection parity bit is then added, resulting in 11-bit samples. Next, the channel data segments are organised into data blocks. Each block consists of two segments, one from each channel, a total of 2 X 32 X 11 bits = 704 bits.
These 704-bit chunks of data form the basic block (sound + parity) of the NICAM broadcast data frame. Frame alignment and control bits (24 bits in total) are then added to 704 + 24 = 728-bit frame.
The time duration of each frame is 1 ms resulting in a bit rate of 728/1 ms = 728 kbps.
Framing is followed by interleaving and scrambling. Interleaving is nec- essary to ensure that error bursts are distributed among several samples
which are far apart. Scrambling avoids the uneven distribution of energy which follows the process of modulation. The companded, interleaved and scrambled data frame is then used to modulate a sub-carrier that is
6.552 MHz above the vision carrier (Figure 17.6). The modulation technique selected for NICAM is differential quadrature phase shift keying (DQPSK) which was described in Chapter 8. This type of modulation is very eco- nomical in bandwidth requirements. It is used here in order to squeeze yet another signal in the tightly packed 8 MHz bandwidth allocated for each TV channel. Before transmission, the modulated NICAM carrier is passed through a sharp cut-off, low-frequency filter to ensure the NICAM fre- quency spectrum does not overlap with the analogue FM carrier, a process known as spectrum shaping. The NICAM carrier at +6.552 MHz is broad- cast at the low level of -20 dB with respect to the vision carrier, represent- ing a power ratio (peak vision carrier to NICAM carrier) of 100:1. The analogue 6 MHz FM sound carrier is retained for compatibility and both sound carriers are added to the video signal for UHF transmission.
We shall now describe the new techniques introduced by the NICAM system in detail.