The birth of digital transmission and
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...
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
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
equipment which allowed 13 linearly-coded (no digital compression) music
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
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
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.
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.
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.
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.
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
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.
did however demonstrate that video compression was a feasible method for signal
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.
additional technical detail of the design of the terminal equipment see our
companion article: “A Technical Overview of the 68PAL Equipment”.
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".
In this article ‘transmission’ is used to describe the link between the
transmitter and the listener or viewer and ‘distribution’ is used to
describe the BBC’s internal links carrying signals from the studio or other
broadcast centre to the transmitter.
PCM – Pulse-Code Modulation, a popular early technique used in digital
A music channel has a bandwidth of at least 15kHz and requires sampling with
an accuracy of 14-bits or better. By comparison a voice channel has a
bandwidth of only 4kHz and requires 8-bit sampling accuracy, a television
channel has a bandwidth of 5.5MHz but also requires sampling with 8-bit or
NICAM – Near-Instantaneous Companded Audio Multiplex, a technique invented
at BBC RD for near-lossless encoding and multiplexing of audio signals.
Subsequently, NICAM came into the public’s ken through its use to transmit
the stereo version of television sound.
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