TABLE OF CONTENTS Back in Chapter 1, we briefly touched on sound systems for TV. Here the subject will be examined more closely, with particular regard to NICAM for terrestrial analogue TV broadcasts. There remains one other facility, namely teletext provided by analogue terrestrial TV broadcasting. Teletext is a system of broadcasting information in the form of pages of text and graphics. Teletext information is specially coded and transmitted during the unused scanning lines at the start of each field. It is quickly being overtaken by the far superior interactive facilities provided by digital TV broadcasting.
As described earlier, the monaural sound is transmitted on its own frequency-modulated RF carrier with 50 kHz maximum deviation at a level 10 dB below that of the vision carrier. In the USA, the spacing between the sound inter-carrier and the vision carrier is 4.5 MHz above the vision carrier. For the UK, it is 6 MHz above the vision carrier frequency and we will use that for descriptive purposes.
At the receiving end, a sound intermediate frequency (SIF) of 33.5 MHz is obtained. This low-level constant-amplitude signal goes to the vision demodulator where it beats with the vision carrier to reproduce the frequency-modulated 6 MHz signal which is then selected by a filter, amplified and demodulated. The inter-carrier technique has several advantages. One of these is that tuning errors and drift in the local oscillator have no effect on the inter-carrier frequency. This is because a drift in the oscillator frequencies causes...
