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Date: 28 January 1999
From: Simon_Mackay@BIGPOND.COM
Subject: Re: Questions related to DAB

On 25 January, Tracy Daley asked:

=================BEGIN QUOTE=========================================
2) How does the single frequency network help to increase bandwidth
efficiency?
=================END QUOTE===========================================

With AM and FM, the techniques used to modulate the signal would
require that only one transmitter can carry one carrier group for the
stated frequency. If you have another transmitter transmitting the
same material on the same frequency within range of the other
transmitter, receivers would find it hard to lock on to that
particular signal - the signals cancel out and you hear a lot of
interference. This is demonstrated by the fact that when one is using
a portable or car FM radio in a city or a hilly area and they
experience unreliable FM reception as they move - the signal
deteriorates a lotand if you notice the set's STEREO lamp flickering
frequently during this situation.

To cover a larger area or fill in areas of weak reception,
broadcasters have to obtain extra frequencies so they can use
repeater transmitters in those areas. This would be referred to as
"transmitter diversity". It therefore means that more services can't
be put on to the wavebands because of these repeaters. Therefore you
have to know what frequencies the broadcaster broadcasts on or use an
RDS-equipped radio and hope that they implement RDS with its network-
following feature. Some receivers, such as car radios installed in
prestige cars, have a "receiver diversity" system where there are two
or more aerials connected to the receiver, and the receiver chooses
the one that yields the best signal.

The COFDM method allows for multiple transmitters to transmit the
same content on the same frequency in the same area, thus allowing
for the single frequency network. The DAB receiver would be able to
make sense of the multiple streams and "shake them down" to resolve
the best signal of them all.

This means that, with the single frequency network, no extra
frequencies need to be used for repeaters or infill transmitters. It
also means more DAB multiplexes can fill the same allocated bandspace
and multiplex operators can deploy transmitters in areas where there
would be a weak signal without having to seek extra bandwidth.

I hope that this answers your question,

With regards,
Simon Mackay

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