How it all began 

In a companion note - “The birth of digital transmission and distribution” - we looked at the early forays of the BBC into digital distribution and particularly the setting up of what came to be known as the 68PAL project – a pilot project looking at the opportunities offered by the digital distribution of television and radio signals.

The purpose of this note is to provide a little more information, for those who are interested, on the technical realization of the terminal equipment developed for the trial. The reader is warned that a reasonably technical background is required to get the most from this article!

Outline specification 

The outline specification of the 68PAL terminal equipment was as follows:

  1. We wanted to be able to carry two full broadcast-quality TV signals (BBC One and BBC Two), including associated sound, within a standard 140Mbit/s multiplex.  We also wanted the equipment to be transparent to any data signals carried in the field interval of the TV signal (mainly used for transporting Teletext information) but there was no requirement to carry the Insertion Test Signals (ITS)[1] (Click the number in square brackets to see the "footnote" in the right pane).
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  2. We aimed to get a very high degree of ‘transparency’  – with no perceptible impairment to either sound or vision signals with at least three coding and decoding processes operating in tandem.
     
  3. The system was required to be tolerant of degraded or disturbed analogue sources and, in their presence, should not demonstrate appreciably greater subjective impairment than wholly analogue circuits.
     
  4. We also wanted a standard 8.448Mbits/s interface to be available to carry other services, including a number of audio channels that could be used, for example to distribute our FM radio channels.
     
  5. We wanted to be able to add or remove one or more of the major components of the asynchronous multiplex without having to fully de-multiplex and decode the entire 140Mbit/s bit stream.
     
  6. Comprehensive monitoring of key transmission parameters was needed, including the measurement and reporting of transmission error rates or any framing losses or input signal losses.
     
  7. As the equipment was likely to be moved around reasonably often, then it needed to be reasonably transportable. Full portability was recognized early on as an unachievable objective!
     
  8. Monitoring and supervisory requirements were required to be good enough for use in operational environments and we wanted to include built-in diagnostics to ease maintenance. Although the initial equipment was not designed or built to the full specification normally required of operational equipment, it was designed to minimize the amount of additional work required to get there.

The Structure of the Multiplex

The 140Mbit/s signal (actually 139.264Mbit/s) comprised two identical “packages”, each of 68.736Mbit/s[2]. Standard commercial equipment was used to combine and separate the 68Mbit/s packages (see Figure 1).

The BBC designed and built the multiplexer used to assemble each individual 68Mbit/s package. The multiplex comprised:

  • A 53.2Mbit/s tributary[3] for the video.
     
  • A 676kbit/s tributary for the stereo TV sound
     
  • An 8.448Mbit/s tributary for additional audio and data signals.
     
  • A 4.096Mbit/s tributary allocated for error-protection but which could also be made available for additional data should error-protection proved unnecessary (they were optimistic days!).
     
  • The small remaining capacity was used for various control, signalling and synchronization purposes.

 

Figure 1 gives an overview of the transmit terminal. All the signals are asynchronously multiplexed together using a technique called plesiochronous, or near synchronous, multiplexing.   (Click links such as plesiochronous to see further information in the right pane). The nominal bit rates of the different tributaries are independent of each other and of the resultant 140Mbit/s signal - avoiding the need to have to synchronise the data-rates of each of the multiple sources[4].

All signals are multiplexed into a fixed output frame containing 378 6-bit words. Each frame is around 33μs long. Each of the multiplex tributaries has a fixed allocation of words (see Table 1). This gives a considerable amount of flexibility including the ability to over-write the entire frame or, by over-writing the relevant words, any single tributary within the frame.

The hardware implementation was also based on a parallel 6-bit word format that had the advantage over serial operation of requiring a lower clock frequency and a significant reduction in design difficulties related to signal timing. High-speed devices were only required for the final serial-to-parallel conversion required to generate the 68Mbit/s serial bit-stream.

Principles used for coding the Television pictures


General

Two main techniques were used to reduce the bit-rate required to carry the TV picture: Differential Pulse-Code Modulation and sub-Nyquist[5] sampling. We shall return to these below.

 A third technique was also tried but discarded. This involved coding only the active period of the signal that carried the picture information, and removing the synchronizing information that makes up almost 20% of the analogue signal. A new set of synchronising information needs to be re-introduced at the output of the decoder. This proved impractical to implement given the range of analogue input signals (often far from theoretically perfect) that could be encountered in practice. 

The theoretical requirement

In order to digitize an analogue TV signal without substantially affecting its quality then any reasonably comprehensive digital textbook will tell you that the signal must be sampled at more than twice the highest frequency it contains and that those samples need to be converted to a digital code-word of 8 to 10 bits to ensure reasonable fidelity of the conversion process. This generally results in a bit-rate of around 140Mbit/s for a single PAL-coded TV signal - about twice the bit-rate we needed to achieve in practice! 

Differential Pulse-Code Modulation (DPCM) 

The first technique used to reduce the bit-rate was Differential Pulse-Code Modulation. The principle used is to transmit differences between the picture sample and a prediction of that sample, rather than transmit the sample value itself. If good predictions can be made of the expected value, then the differences will be small and the data-rate required to transmit these differences can be reduced[6]. In the algorithm used in the 68PAL equipment the digital words used to represent the differences are 6-bits (1 sign bit used to indicate positive or negative differences together with a 5-bit magnitude). Difference magnitudes are coded non-linearly so that small differences (which occur most frequently) are accurately represented. Large differences are not so accurately represented, and thus the re-constructed signal will be distorted, but since these large differences are usually associated with unpredictable pictures (picture ‘cuts’ or lots of movement) then the distortions are subjectively less important.  

The predictor would ideally be based on previous sample values but since these are not available at the decoder (these are what we’re trying to avoid sending!) then the re-constructed sample values (based on previously transmitted differences) must be used instead. Identical predictors are used at the sending and receiving ends that take as their input the decoded signal (implying that the coder must also include a local decoder to drive the predictor – see Figure 2 which provides an outline diagram of the operation of the DPCM coder). Note that the DPCM decoding equipment is a duplication of the adder and predictor shown in the coder diagram.

 

The prediction algorithm uses the two immediately preceding samples from the same television line as the current sample, three samples from an adjacent area of the picture in the previous line and three samples, again in an adjacent picture area, from the previous field. The choice of the optimum number of prediction terms, and their various weighting factors, was based on a statistical analysis carried out on representative picture data. 

Although it proved an effective method for reducing the bit-rate, the use of this technique did not come without its consequences - see Tadpoles.

Sub-Nyquist Sampling 

The sampling rate used was twice the colour sub-carrier frequency of the PAL signal (8.86MHz or 2fsc). This is insufficient to ensure that the sampling does not introduce distortion - a frequency of at least 11MHz is required. The result of this was that distortion components were present (these are known as alias components) in the frequency range 3.37MHz to 5.5MHz. Because of the nature of the TV signals the energy in the signal tends to be clustered at multiples of the TV line frequency (15.625kHz). By arranging for the alias components (which will have a similar structure) to sit in between these wanted components, a comb-filter (a filter with peaks and troughs every 16kHz above 3.37MHz) can be used to minimize the effect of aliases whilst also minimizing the impact on the wanted information. See Figure 3 for a more pictorial description of this process. 

In practice, the best results were obtained by initially sampling the signal at four times the sub-carrier frequency (4fsc, which is well above the minimum alias-free frequency), applying comb-filtering to the 4fsc samples to remove components at frequencies which would otherwise form aliases at exact multiples of line frequency, forming the sub-Nyquist sample values (which in practise amounted to no more than throwing away every other sample!) and finally comb-filtering the 2fsc samples to remove the aliasing introduced by removing the unwanted samples.

 

 

Dealing with frame interval signalling information

The description above outlines the handling of the picture information. A different mode of operation is required for information carried within the TV signal during the field synchronization information – the so-called Field Interval Signalling (FIS). During the non-displayed field period, data and test signals are inserted onto the blank TV lines. These do not have the same degree of predictability as the pictures and do not lend themselves to the techniques used to reduce the bit-rate of the video signal! Of these, the incoming Insertion Test Signals (ITS) were blanked and reinserted after decoding using a standard commercial ITS inserter. For other FIS information (such as Teletext) the approach taken was to re-quantise the incoming 4fsc data from 10-bits down to 3-bits per sample (in the FIS coder shown at the top of Fig. 2). 3-bit quantizing is adequate for the data information as the data itself only represents digital (0 or 1) values. 

Pairs of the 4fsc 3-bit samples are collected into 6-bit words at a 2fsc word rate and passed to the output data switch for transmission (see Figure 2). At the same time, the other pole of the data switch (to the left of the quantiser block) feeds dummy data into the predictor to prevent misbehaviour.  

At the decoder end, during the field interval, the FIS 3-bit samples are converted back to 10-bit values and a data selector bypasses the DPCM decoder and passes the samples directly to the output Digital to Analogue Converter to re-generate a close approximation of the original Teletext signals. As in the coder, dummy data is fed into the decoder predictor during this period to keep the predictors at both ends of the system in the same state at the end of the FIS period and at the resumption of picture transmission. 

The means for signalling to the decoder that the incoming samples are for FIS rather than video relies on the fact that only 63 of the 64 possible 6 bit codes are used for DPCM video data. The unused code word is used exclusively to signal that a block of FIS data follows. This signal together with validity bits is used to operate the data switches between the video and FIS modes of operation. The length of the block of FIS data is pre-determined, so there is no need to signal the end of the block. 

TV Sound 

The TV sound was carried using the, by then well-proven, BBC standard two-channel NICAM transmission equipment which multiplexed the two audio channels into a bit-rate of 676bit/s[7].

The other tributaries

The 8448Mbit/s multiplex 

An 8448kbit/s tributary could be used to carry a flexible mixture of audio and data signals using existing commercial or the BBC’s own NICAM-based audio distribution systems.  

Error correction

4096kbit/s was allocated for error protection although this could also have been used for additional audio or data channels in situations where the benefits of error protection were not required[8].  

A Reed-Solomon error correcting code was used. These codes work by treating the digital bit-stream as multi-bit symbols and, in our case, each 6-bit digital words became a symbol of the code.  The code works by adding a number of check (or parity) symbols to the transmitted signal allowing errored symbols to be recognised and corrected in the decoder. The analysis and correction is carried out on the symbols rather than individual bits. These codes are very powerful and particularly good at dealing with errors that occur in bursts[9]. They are now commonplace in CD-players and the like. However, this commonplace use has relied on the introduction of large-scale integration techniques that were not available to the project. In those days, the implementation of the codes was quite a challenge – involving a significant number of integrated circuits to implement (45 at the coder and 130 in the decoder) and a very clever engineer who was up to the somewhat esoteric mathematics involved in the correction algorithm! 

The error correction system proved vital to the field trial – not only because the performance of the new link was unknown – but also to allow its error statistics to be compiled in both the short and the long term. A further microprocessor-based unit in the de-multiplexer not only gave an LED bar-graph indication of the current error rate, but could also be linked to an external data logger to record long-term information. 

On a spare 68Mbits/s channel from Birmingham to London (the return paths were not used during the pilot) the error statistics were permanently monitored to provide a picture of the nature of errors on the link. 

Click to see more details of the code.

Diagnostics, maintenance, and monitoring 

As a diagnostic aid, an integral test signal generator was included. The test signal occupied 32 picture lines and comprised a pedestal and ramp. The various feedback loops could be broken to provide open-loop operation at the flick of a switch. This is essential for fault finding because in closed-loop circuits it was nigh on impossible to determine what was going on when everything went haywire. The test signal also facilitated digital signature analysis. For readers unfamiliar with this technique, it was a Hewlett-Packard invention. It consists of a pseudo random counter whose clock is derived from the system under test. This counter is started and stopped by chosen events in the system under test using probes that connect to test points. The displayed alphanumeric count is unique (almost) to the part of the system under test. Once these displays have been noted for a working system they can be used to verify quickly the correct operation of further cards built at a later date, or to identify a fault area. 

Each system also contained a microprocessor-based monitoring card. This card ran the same algorithms as the equipment in which it sat – but could only do so at a relatively slow rate. Sampling points were introduced at key system points that took “snapshots” of the data being processed. The microprocessor collected these snapshots and verified them against its own model of the video coding algorithms. The same idea was used later in Mk2 6 channel NICAM but seems to have fallen out of favour now that microprocessors are able to operate at the speed necessary to do all the high-speed processing themselves! Monitoring information captured at the sending end (such as the condition of the signals at the various input ports) was sent to the receive terminal by using the slots allocated to justification words (see Box - Plesiochronous multiplexing) when they were not needed to carry real data, and so did not entail any overhead in bit rate. 

Construction

The construction was modular and based on ‘4U’ cards housed in standard width crates. Each terminal bay (transmit or receive) was 1.25m high and incorporated fan-assisted cooling since the power consumption approached 500W - See Figure 4.

Figure 4 – Rhys Lewis with the 68PAL coder (on the left) and decoder

 

Figure 5 – A Solderwrap card (wiring side)

Most of the cards used an automated wiring system known as “Solder wrap” (not heard of before or since!) – see Figure 5. It is very much like the more familiar Wire Wrap (used extensively at that time for prototype work) except that the wiring was done by machine, and the joints were soldered as well as wrapped. 

Standard card layouts were used which were pre-drilled to accept the integrated circuits. A special wire was used, which was around 36swg, self-fluxing, and carried in bundles supported by special plastic pillars. 

The typical problems encountered during manufacture included: shorts from solder splashes, wiring errors (due to human error on the original drawings) and reversed devices since these had to be manually inserted before the solder wrapping began. Given a certain amount of dexterity, a pair of fine tweezers, 20-20 eyesight and considerable patience, it was possible to repair or modify Solder Wrap cards but it was not uncommon for such intervention to add to the toll of broken wires! 

This build form was not suitable for all types of cards and so some, which used a lot of high-speed logic or which were critically dependent on timing delays, used early multi-layer printed circuit boards (PCBs). These were only just becoming possible through the use of Computer Aided Design (CAD) techniques for generating PCB artwork (rather than the sticky black tape and large sheets of paper that were in common use before then!) and more sophisticated manufacturing processes for the PCBs themselves. Three-layer PCBs were used with the middle layer used as a ground plane. This allowed the tracks on the outer layers to be treated as transmission lines with well-defined transmission properties. High-speed inter-crate connections within a bay relied on the use of balanced signals transmitted via twisted-pair ribbon cables. 

Performance

Picture quality 

As intimated above, the picture quality was good. Subjective tests were conducted using the CCIR 5-point quality grading method. The results are shown in Table 2. 

These results show that the video coding scheme did not introduce any significant impairment of the original PAL signal. Tests also showed that no change in impairment could be observed between pictures processed through one codec, or through three codecs in cascade.

The impact of errors 

With error correction disabled, the subjective effect of random errors was judged to be “perceptible but not annoying” at a BER of about 1x10– 6. When the error correction was enabled, the effects of errors remained “imperceptible” for a BER up to about 1x10 –4. A similar performance was also obtained for the sound channels. 

An unexpected effect

One of the other practical problems encountered was that of the noise immunity of some of the circuitry. Impulsive interference was the cause of this with noise induced from transient events[10] causing mis-operation of the circuitry. In particular, some of the high-speed logic parts used suffered from a relatively high sensitivity to this effect with particular problems for “clock” signals - since these provided a common synchronising signal for a large proportion of the circuitry. 

The effect of such impulses was the appearance of the “tadpoles” described earlier, gently swimming about the picture, though usually disappearing after a few seconds. Quite calming and therapeutic really, as long as you didn’t have to fix it! Investigation revealed that the effect was also present on analogue distribution equipment in the laboratory but that the ‘spike’ of impulsive noise cause was not very visible (a single pixel of white or black). In the digital equipment the processing amplified the effect of a single error and made the impact considerably more visible.  We minimised the impact of the effect, though never removed it entirely, by careful modification including the use of balanced clock signals. 

It works!

After many hours of tests and de-bugging, the first full system demonstration took place at Kingswood Warren in December 1982. By the following April we had several working crates for testing. In June 1983 a note in David’s laboratory book reads “today we sent ‘68’ to the [British Telecom] tower and back – all worked fine”. The system entered full-scale operational use between London and Birmingham in early January 1984. 

The 68PAL equipment was first demonstrated to the world at large on the BBC’s stand at the International Broadcasting Convention in Brighton in September 1984. It attracted much interest, in particular for a comparator that we’d built to allow people to see where the artefacts occurred! The comparator isolated the codec artifacts so that they could be viewed separately. The method used was to subtract the decoder’s output signal from the original input signal to the coder – leaving just the differences. This could be done with very high precision in the digital domain. The visitor was presented with three push buttons situated beneath the monitor screen. The first of these routed the original input signal to the monitor. The second routed the comparator output (showing just the coding artefacts) to the monitor. The third routed the decoder output to the monitor. The ability to view the artefacts alone, “told” the viewer what to look for in the way of impairments in the decoded picture as it would be seen in the home. Without such sensitisation, the impairments were in fact quite difficult to spot! 

The sort of things which were just noticeable were a degree of edge “busyness” on the castellation round the edges of the standard analogue test card (Test Card F), and perhaps an occasional “twinkle” at the edge of a bright, fast-moving object such as a skier. The worst effect was a “twittering” observable on horizontal boundaries between saturated colours. This was not seen often on normal pictures but was most evident on the Teletext pages that were transmitted as the normal vision signal on BBC2 during the day (remember, this was before the advent of Daytime TV!).  

…and in the end! 

After many months of successful operation the pilot came to an end. Although the technical performance of the equipment was not in any doubt, the economic case could not be made for a move to digital distribution at that time. Since then of course the advent of more and faster processing in a chip has led to the development of much cleverer algorithms for reducing the bit-rates required for video and audio transmission. It would now be uncommon to discover an analogue link in use in the distribution infrastructure – but, perhaps, that’s the subject of another story! 

People 

A considerable number of people were involved in the inception and implementation of the 68PAL affair. The bulk of the work in Designs Department was done by Rhys Lewis, David Birt and John Robinson with able support from John Weston and many other colleagues – whether engineering, management or support staff. 

It would also be unfair not to mention our colleagues in the BBC’s Research Department – 68PAL was after all their baby! Of particular note are the contributions of Nick Wells, Jonathan Stott and Keith Slavin (Keith was the engineer who designed the error-correction system). 

Our apologies go to those whose names we’ve managed to miss through ignorance or forgetfulness.

 

A dedication

During the writing of this article we learnt of the death of Gordon Parker who was the Head of Designs Department at the time of the 68PAL work.  It would be hard to forget the early morning visits from Gordon during the work. He came bouncing in through the door, jingling the coins in his pocket, with a cheery “Good Morning” and a word for everyone.  He had an unmatched knack for what is today called “walking the floor” and taking the pulse of a project, or the department, without any apparent need for formal grilling. This article is dedicated to Gordon’s memory.