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National PCM Changeover  -  Scary Engineering

Commemorative Colour 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.

1950s The BBC's radio services were distributed to LF, MF and new VHF/FM transmitters using analogue circuits provided by the Post Office.
1960s 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 Queens 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 services.
1972 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 services.

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.

1980s 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 which was far more compact.

1993 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!

2003 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.
2006 Brochure added to this web site.