Designs Department - Technology

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The birth of digital transmission and distribution

A personal memoir from David Birt and Rhys Lewis
 

How it all began

Today, we tend to take digital transmission and video compression for granted. Multiple TV channels are routinely delivered at high-quality (or so it’s claimed!) where before it was only possible to deliver a single analogue channel. It wasn’t always that way! In the early 1980’s, we found ourselves in Transmission Section contemplating the challenge of the 68PAL project – aimed at delivering two digital TV and multiple audio signals, using what now seems to be an exorbitant bit-rate of 140Mbit/s, between London and Birmingham. But more of that project anon, first a little background...

Background
 

The BBC’s Research (RD) and Designs Departments (DD) were amongst the earliest explorers of the digital world, having recognized early on the benefits of digital technology for use in the distribution^[1] network (see The True Benefit of Digital). And, as ever, the lower bandwidths of audio signals meant that audio distribution led the way. In 1970 the Sound in Syncs (SiS) system, invented at RD, and transformed into operational success by DD, first went into service. This system embedded the TV sound signal into the synchronising pulse of the video signal avoiding the need for separate circuits to carry the sound. The original system served the BBC well for over 15 years, and along the way became the internationally accepted norm for TV sound distribution, before it was eventually replaced by a two-channel (stereo) version which is still in use today!

Following the early SiS work, DD went on to develop, and put into operational use, some 13-channel PCM^[2]  equipment which allowed 13 linearly-coded (no digital compression) music channels^[3] to be sent over an analogue video bearer. This system was used across the UK for many years to feed FM Radio services to the BBC’s transmitters (and they were still the BBC’s in those days!). The equipment was followed in the early 80’s by first the Mk.1 NICAM^[4]  equipment (2 compressed music channels in 676kbit/s), and subsequently the Mk.2 NICAM equipment (6 compressed music channels in 2048kbit/s). Apart from tariff savings, the move to digital brought very significant benefits in terms of reliability, stereo performance and sound quality when compared with the previous analogue distribution links.

By 1982, Transmission Section had also supplied terminal equipment for an experimental digital video fibre-optic link which had been installed between the BBC’s studios at Lime Grove and Television Centre. This allowed two uncompressed TV signals to be transmitted between the two sites using a bit-rate of 280Mbit/s – a design challenge which pushed the capability of the digital integrated-circuits available then to their (and indeed our) limits!

The 68PAL Pilot

In parallel with this work in the BBC, British Telecom (BT) were well advanced with their plans for a national digital trunk network based on interfaces at a set of particular hierarchical levels (See Magic Numbers). BT were keen to persuade the BBC to move to digital distribution of it’s signals (its analogue links were fast disappearing) whilst the BBC had an interest in assessing the benefits (or otherwise) of moving to digital distribution in the hope that this would provide cost savings in the longer term.

A pilot digital transmission scheme was set up as a collaborative venture between the BBC, who would be providing the coding/decoding equipment, and BT who would provide the two-way links using 140Mbit/s interfaces (see “Magic Numbers…”). The links were fibre-optic from Broadcasting House in London to the BT tower, microwave between the London and Birmingham BT towers, and thence coaxial cable into the BBC’s premises at Pebble Mill.

The main objectives were:

• to enable both BT and the BBC to assess the technical and operational problems of using digital bearer circuits for broadcast services,

• to prove the equipment design, including establishing the principle that digital compression was an acceptable means of distributing TV channels. This latter point may seem strange today - but in those early days there were doubters.

BBC Communications Department was heavily involved in initiating and managing the overall pilot scheme whilst the provision of the terminal equipment was a collaborative project between Designs Department and Research Department. The system was operational from January 1984, and was exhibited at the IBC Convention at Brighton in September of the same year.

The BBC's primary interest was to distribute two TV channels (we only had BBC One and the relative new-boy BBC Two in those days) bi-directionally between London and Birmingham. Provision for a number of audio and data channels was also required. This could be conveniently achieved by splitting the 140Mbit/s interface to BT into two similar ‘packages’ – each package used a bit rate of 68.736Mbit/s (twice the 34Mbit/s hierarchical rate). The video signal into the equipment was in the prevalent analogue-PAL format, and hence the equipment, and the pilot, came to be known as “68PAL”.

The key to achieving the pilot’s objectives was the method of compressing a PAL-coded video signal conceived by Research Department working with their counterparts from other broadcasters, universities and telecommunication organisations across Europe. The bit rate required for the video signal was reduced from a notional 140Mbit/s or so (for an uncompressed PAL signal sampled at an ideal frequency) to around 53Mbit/s in order to fit within the available 68Mbit/s and leave space for other signals. The additional space was occupied by a flexible mix of up to 24 channels of NICAM audio, data, telephony and error protection to bring the “package” size up to the full 68Mbit/s.

The pilot ran for some 6 months or so – and carried the BBC’s live services for much of this time. It was certainly the first time in the UK that live video signals had been distributed digitally - and may well be a world-first! We both well remember that this included a nervous period during Wimbledon fortnight where a very careful eye was kept on the output as we had been advised by our colleagues in R&D that, during the early prototyping work at Kingswood Warren, the digital compression had caused tennis balls to fragment or even disappear entirely from time to time. We’re glad to report that this did not happen for the real thing!

How it all ended

In summary, although the operation was a success, unfortunately the patient died! The equipment and the pilot worked extremely well, but the equipment never went into production. The main reason was to do with tariffs. At the time, the BBC had a very favourable long-term deal with BT for the well-established analogue video distribution network that had a number of years left to run. There was therefore no immediate financial incentive to begin the transition to digital distribution, and the technical performance and reliability of the analogue network was satisfactory. It was also apparent that the onward march of technology (perhaps not quite so relentless then as it seems now!) would, over the course of some years, lead to further performance improvements in video coding and also help to reduce the cost and size of the terminal equipment.

The pilot did however demonstrate that video compression was a feasible method for signal distribution.

The technology was licensed, and we believe that it was used by other broadcasters overseas in a variant which offered a further bit-rate reduction to 34Mbit/s or less by not transmitting the line and field blanking. Although such a possibility had been researched within the BBC and had been proposed, there had been some concern about susceptibility to the effect of tolerances within the source analogue signal. To accommodate such variances would have required further development work that would have delayed the pilot.

For some additional technical detail of the design of the terminal equipment see our companion article: “A Technical Overview of the 68PAL Equipment”.

You can also find some additional "snapshots" of life in Western House at the time of 68PAL in David Birt’s entry under Reminiscences "Third Time Lucky".

David Birt
Rhys Lewis
April 2005

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