TV sound, mono and NICAM
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 over- taken by the far superior interactive facilities provided by digital TV broadcasting.
TV FM mono system
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 oscil- lator have no effect on the inter-carrier frequency. This is because a drift in the oscillator frequencies causes the two IFs to change by the same amount, keeping the difference between them constant. Frequency drift does not have the same effect on the vision carrier because the vision carrier is amplitude modulated. Furthermore, the sound carrier benefits from the gain provided by the vision IF amplifier; and the sound process- ing circuit is a simple one.
The inter-carrier 6 MHz beat frequency is obtained from a device such as a diode which, when used over the non-linear part of its characteristic, produces the sum and difference of two separate input frequencies. The resulting 6 MHz beat frequency retains the FM information which, when demodulated, reproduces the original sound signal. Ideally, the sound inter-carrier should have constant amplitude. However, some amplitude modulation will be present, caused by the amplitude-modulated vision IF.
This interference will cause what is known as vision buzz on the audio output, unless it is removed by a limiting circuit before demodulation.