A Brief Word On Analogue Tv - Sinclair Microvision MTV1 Service Manual

A brief word on analogue TV

Fast-scan analogue TV broadcasts were made on the VHF and UHF bands. The signals were
basically amplitude modulated, though filtering was used to attenuate substantially one sideband
(making the signals into 'vestigial sideband' or VSB), a move intended to reduce the necessary RF
channel bandwidth by almost half and, incidentally, reduce the transmitter power consumption by a
similar amount. In most countries negative modulation was used, with the peak white
corresponding to minimum instantaneous transmitter output. This meant that the sync pulses were
the strongest thing in the signal, which is why receivers could lock to even fairly noisy signals.
Sound was sent as an FM signal offset a few megahertz from the vision carrier.
● System I sound offset 6MHz (mono or PAL) - UK/Eire
● System B/G offset 5.5MHz (mono or PAL) - CCIR (Western Europe)
● System D offset 6.5MHz (mono or SECAM) - OIRT (Eastern Europe Block)
● System A offset 4.5MHz (mono or NTSC) - NTSC countries (and Argentina PAL)
A multi-standard set like the MTV1 had to be able to receive on VHF (Bands I & III) plus UHF
(Bands IV & V), work with a 50 or 60Hz frame rate (with the concomitant different number of lines
and line frequency) and be able to demodulate sound on any of the subcarrier frequencies. (For
simplicity and perhaps due to lower perceived demand, the MTV-1 does not demodulate 6.5MHz).
In many parts of the world terrestrial television channels are now transmitted only via digital
methods, commonly DVB-T or ASTC or variants thereon. The MTV1 is not able to receive these
transmissions, however it is possible to create your own local signal source that acts as a very
short-range analogue TV transmitter. The simplest approach is to use an ordinary VHF or UHF
modulator (also called a 'game adapter') to produce a low level RF TV signal, then attach a small
microwave amplifier IC (eg the ERA series from MiniCircuits, which are simple 3-terminal devices
and straightforward to use – see www.minicircuits.com/pdfs/ERA-1+.pdf) feeding a suitable length
of antenna. A modulator on its own doesn't produce enough RF to be received at more than a
hand-span but with an amplifier as suggested the range will increase to a metre or three.
Suitable video sources include digital set-top boxes, streaming-service receivers (eg Roku, Fire,
Chromecast...) and laptop/desktop computers. Most of these have no analogue outputs, instead
using HDMI to output high-definition signals. Inexpensive HDMI-to-composite converters are
readily available and do an excellent job, although you may have to fiddle with the settings in the
source device to make it output a 4:3 format video signal as opposed to the now almost-universal
16:8 widescreen. If a 4:3 screen is fed with a 16:9 signal, everything onscreen looks stretched
vertically.
Raspberry Pi computers have analogue video (and, in most cases, audio) outputs and can be set
to produce PAL or NTSC-format video. This may provide a useful source of video that can be fed
straight into a modulator.
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Sinclair MTV1 service manual V2.1b