National PCM Changeover - Scary Engineering
Brochure (4MB PDF file)
written by Keith Hayler and Russell Inman of BBC Transmission
Introduction to the brochure
by Martin Ellen
It was produced for the tenth anniversary of a 1993 project that the
participants will never forget and it gives a flavour of the considerable
challenge, as well as the great working atmosphere that existed.
The task was to replace equipment and enhance the system used to carry
the BBC's radio services between studios and transmitters. Simple
enough? Well, no not really!
Some projects are complex, some affect many millions of people, some have
got to work first time with no possibility of going back. This project
had all those things and more.
The Commemorative Colour Brochure was written for people involved in the
project, so here are some notes to put it into context.
The BBC's radio services were distributed to LF, MF and new VHF/FM
transmitters using analogue circuits provided by the
Following work started in 1965 by BBC Research Department, the first
digital distribution system was introduced by the BBC. It was for
television sound and worked by inserting a bitstream in the sync pulses
of the vision signal - called Sound-in-Syncs or SiS. Because
separate music circuits were no longer needed, it reduced costs a great
deal and it was one of many examples of BBC engineers saving money that
could be used for programme making. An
Emmy and a
Award for Enterprise was received jointly by BBC Research Department
and Designs Department as a result of this innovation, which was
used by other broadcasters worldwide. This work led
to the development of a 13 channel PCM system for the BBC's radio
A new distribution system was introduced using a
13 channel PCM
coder in London feeding PCM decoders at main transmitting stations.
The system used 13 bits per sample with linear coding and the resultant
6336kbit/s multiplex was conveyed on analogue "vision" circuits provided
by the Post Office, as well as on the BBC's own microwave links.
Some of the channels were paired to provide stereo feeds to VHF/FM
transmitters, some provided mono feeds to LF/MF transmitters and some
provided contribution feeds between studio centres.
This was a major
advance which gave nearly everyone in the UK good quality stereo
The television SiS system used analogue companding and 10 bits per
sample to squeeze sound information of sufficient quality into the
limited capacity of the sync pulses. There was less constraint on
the PCM system for distribution of the radio services as
plenty of linearly coded channels, using 13 bits per sample, could be accommodated within the
capacity of a vision circuit. (15kHz music circuits and 5.5MHz
vision circuits were effectively the only choice of bearer circuit at the time.)
Telephone networks started to move towards digital technology
and the standardised bitrate of 2048kbit/s (designed for 30 voice
channels) looked attractive for conveying broadcast quality sound.
Given the potential cost of such circuits and the need for stereo,
companding was required to put six mono channels (or 3 stereo) of music
quality into a bitstream of 2048kbit/s. BBC Research Department
met this challenge by inventing "near instantaneous" companding, which
gave better sound quality compared with instantaneous non-linear
companding such as "A-law". The system was developed by Designs
Department and given the name NICAM - Near Instaneously Companded Audio
Multiplex. (NICAM as used on TV came later.)
Although 2048kbit/s circuits started to become available from the
Post Office, the tariff was too high as it was related to multiples of
relatively expensive low bitrate (e.g. 64kbit/s) data circuits rather
than the inherent data capacity of broadcast quality vision circuits,
for which the BBC had a long term tariff agreement.
Demands on the BBC's radio distribution/contribution network continued
to grow and 13 channels became insufficient, so planning started on a
way to increase it. The first stage involved increasing the
bitrate sent over the 5.5MHz vision circuits from 6336 kbit/s to
8448kbit/s. The use of this standardised bitrate offered the
potential for migration to digital bearer circuits (see also
The birth of digital transmission and distribution).
An unusual form of mux/demux was produced by Designs Department which
initially enabled 12 linear channels from the 13 channel PCM system to
be multiplexed with one 6 channel NICAM coder. This met an initial
need for more channels (13 up to 18 mono channels that could be paired
for stereo). The mux/demux also offered the possibility of
migration to 8 linear+12 NICAM and then 4 linear plus 18 NICAM.
However, in reality the transition was made straight from 12+6 to 24
NICAM in 1993.
Meanwhile the original Mark I NICAM equipment was made obsolescent by
the design of Mark II NICAM
far more compact.
For 20 years virtually the entire population of the UK had been
listening to BBC radio programmes that passed through the 13 channel PCM
system. It was a resilient, duplicated system that was very
reliable, but it now needed to be replaced by Mark II NICAM, providing
24 music channels (12 stereo). Detailed preparation was needed for
this complex task involving studio centres and main transmitting
stations throughout the UK.
With several bays of equipment involved at
each site, obviously a great deal of system design and testing was
required. Various backup arrangements were in place but there was no
going back - it had to work - and it did.
Failure could have meant the loss of all the BBC's national radio
services for a significant period. So, no pressure then!
A most enjoyable event was held to celebrate the tenth anniversary of
the PCM changeover and this 42 page
Commemorative Colour Brochure
was produced for the occasion (4MB PDF file). A far more detailed
account of the project starts on page 32 and you might find it helpful
to read this first.
Brochure added to this web site.