ECOC 1975 - ECOC 2013

185
TELEVISION TRANSMISSION EQUIPMENT AND SYSTEMS
P H Fell and A H Kent
Introduction:
The system described here has been envisaged as a general purpose
transmission link suitable for use both in a CCTV/CATV application and also
as part of an integrated multiservice system for use in an expanded local
network. These integrated systems provide a.means whereby all the services
cormnonly specified for the popularly termed "wired city" may be offered.
The use of fibre-optics for these systems has been outlined elsewhere 1 .
The transmission system to be discussed here is the final connection in
one of the schemes described.
Description of TV/Broadband Transmissioh Link
The main design objective in this particular link is simplicity and cheapness
since each individual subscriber will have a receive terminal. The
quality of transmission necessary is that for a CCTV system 2 rather than
the more stringent Broadcast Link specification. With this in mind, the
ways in which a link of 5-10MHz bandwidth could be implemented were
considered. The use of digital transmission was dismissed since it would
require a complex and costly converter at the subscriber's terminal, and
its main advantage, easy regeneration of the signal, would be of little
benefit. The great majority of links will be short; 70% of all
subscribers are within 2.5Krn actual route distance of their local exchange.
The linearity of the specially developed high-radiance small area LEDs 3
was found to be sufficiently good to allow transmission by direct analogue
modulation of the LED light output intensity by a baseband video signal,
and so this was the form of modulation chosen (Fig. I). One video and two
sound channels are transmitted in this particular scheme. The TV sound
channel is transmitted as F.M. on a 6MHz carrier in line with the broadcasting
standard, and the second sound channel is also F.M. at 10.7MHz.
Since these are cormnonly encountered carrier frequencies, ceramic filters
and I.C. demodulators are readily available.
The received signal after detection drives a baseband picture monitor,
and the two sound channels are filtered, demodulated and fed to audio
amplifiers and speakers (Fig.2).
Transmitter
The specially developed high-radiance LED is in the output stage of a
simple feedback amplifier, whereby the standing diode bias current of
about 100mA is modulated linearly to a depth of 50% by the video input
signal. The LED is provided with a short tail of fibre bonded to it for
connection to the main cable fibre.
Detector
The choice of detector photodiode lies between a PIN diode and an
avalanche diode. While some increase in sensitivity is obtained by virtue
of the internal gain of the avalanche diode, it is not considered
sufficient in this relatively low bandwidth high signal/noise system to
justify the extra cost involved.
P H Fell and A H Kent are at Plessey Telecommunications Research Ltd.