Down the Tubes

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Introduction

The year 1986 was notable for the FIFA World Cup in Mexico in the summer, during which Maradona claimed the 'Hand of God' was responsible for the hand goal which put England out of the contest. Earlier in that year the Chernobyl Power station had exploded causing a major nuclear disaster.

The World Cup was already underway when we commenced the day shift one morning in the BBC Communications Centre in Bristol. This technical area at the time was a hub for the BBC's vision, sound, and data circuits between London, Cardiff, Plymouth and Southampton. In particular, BBC1 and BBC2 was received was London and fed to Cardiff, Plymouth and the Mendip transmitter station. Additionally, the Centre enabled Points West opt-outs on both television networks by switching the local studio into circuit to Mendip.

Network Failure

Part-way through the morning the BBC's transmitters in Southwest England lost their line feeds and went to their reserve feeds. Plymouth phoned to complain of the loss of both television networks, and Wenvoe Monitoring and Information Centre (MIC) confirmed that Caradon Hill, Huntshaw Cross and Stockland Hill were on RBS (Re Broadcast Standby). Enquiries revealed that all vision circuits between Plymouth BBC and the BT Repeater Station at Forder Battery (outside Plymouth) had been lost. A digger working on the new Plymouth Parkway - a new dual carriageway carrying the the A38 through Plymouth to the Saltash Bridge - had sliced through the vision tubes and completely cut BBC Plymouth off from the BBC network. Coincidentally Television South West, the regional ITV franchise, had escaped this fate by the IBA self-providing all its inter-regional circuits and was therefore not reliant on BT.

BBC TV Analogue Network South West England 1986

On the BBC's instructions, BT Forder Battery plugged the incoming BBC1 and BBC2 circuits into the amplifiers feeding Caradon Hill and Stockland Hill, an action which bypassed BBC Plymouth and restored programmes to the transmitters.

Regional Communications Structure

In those days each BBC Region had a Communications Manager responsible for the provision of temporary and permanent circuits, of any description, for use by the BBC in the Region. Reporting to him were Assistant to Comms Manager (formerly Communications Systems Engineer) and Communications Planning and Operations Manager. The latter was responsible for the provision of Outside Broadcast circuits from BT or the mobile Comms department (Radio Links) and the staffing of the Communications Centre.

The A to CM possessed expert knowledge of all regional permanent circuits, had good professional relationships with BT counterparts throughout the Region, and also had a grasp of BT's networks.

Temporary solution

The A to CM in Bristol was an experienced engineer, having worked in the South and West for many years. When this report reached him he remembered the 405 line days before Plymouth Broadcasting House was extended for colour operation. At one time, he recalled, the original BBC-GPO cable containing a vision tube and audio pairs went in an entirely different route than later cables. A phone call to BT confirmed that this cable was still terminated in both BT Forder Battery and BBC BH Plymouth although unused and unconnected. He gathered together two pairs of vision equalisers - a Cauer and a Bode - and left for Plymouth.

BBC Vision Equalisers

The staple diet the BBC used as equalisers were the Bode and the Cauer. Fundamentally these equalisers were based on the Bridged-T design which preserved the source and load impedances.

The Bridged-T attenuator and fader were first designed in the early days of radio transmission, where the load impedance was built around 600 ohms. This figure is derived from the characteristic impedance of an infinitely long transmission line i.e. two wires run parallel to each other on telephone poles. It was found in the early days of the GPO that such a line had a characteristic impedance of 600 Ohms at around 800 Hertz.

The Bode and the Cauer (the latter known coloquially as 'Fairy fingers' and based on the Zobel network) were versatile pieces of equipment. With some knowledge of the effect of various frequencies on the vision signal, by looking at a 'Pulse and Bar' test signal, fine correction could be applied to bring vision signals into spec.

Switching Centre Ops Bays 1980s
Switching Centre Ops Desk 1980s

In the Ops Bays photo, the Bode equaliser is shown in the right bay, a large white panel with 15 black knobs. Two newer electronic Bodes are shown above - two silver panels side by side.

For those who would like to know of the other equipment in the bays, here goes. At the top of the right-hand bay are two Designs Department UHF tunable receivers. Below these are two Decca 4011 UHF receivers - the tuning knobs look positively archaic today. Below this is an RSA (Remote Signal Analyser) which could measure differential gain when looking at a chrominance staircase test signal. Below this are two Noise Boxes, they were able to measure Unweighted, Luminance Weighted and Chrominance Weighted Noise on a vision circuit.

On the left-hand bay there is a Sync Gauge at the top. Coloured lamps on it would light up if there were any pulses errors. This was a clever little box which in my experience never let me down. By feeding any PAL signal into it, one glance would immediately alert you to an error and what type of pulse it was. This was very important for VT machines. It didn't like Sound in Syncs though.

Moving down, below a row of 50v fuses was a Natlock tone comparator. The five Natlock tones between 800Hz and 1600Hz could be lined up on these meters and fed to the Sync Pulse Generator to lock pulses to London. Next down is a RACAL counter for measuring the Natlock oscillator frequency fed to the SPGs. Beneath this is another Remote Signal box to measure differential phase.

The second photo is of the Ops Desk back in the 80s. Top left is the SPG panel. There were two pulse chains, A and B. The VTs and Studio A used Chain A as this was never adjusted. Chain B was used by Studio B for Points West and could genlock in 5 seconds maximum, but this caused a VT on the chain to throw its toys out of the pram. It could also occasionally upset Studio B's EMI 2001 cameras. On the desk nearest the wall is the Immediate Access Switcher mentioned in the article. This controlled the programmes going to Wales, Mendip and the South West as well as the contribution to London. Above the waveform monitors some Trimming Equalisers (white units) can be seen. Each had three knobs to adjust gain, chrominance level, and chrominance phase. These equalisers were on all incoming feeds and were most useful. The rest of the desk contained various monitoring controls, the telephone switchboard and talkback panel.

Radio Link van

Now for the final photo. On display is a Cauer equaliser - bottom right, a white panel on which are 17 knobs. Referring to Zobel wiki links above and comparing with this picture will give a good idea what the knobs were for. To operate both this and Bode equalisers it was necessary to have an understanding of the component frequencies of a vision signal and to know which octave was required to correct a disparity. The two other items worth mentioning in this photo are the Pulse and Bar/Non Linear Staircase test generator top right, and the grey Mark I Microwave Associate SHF link control panels seen on the left.

Back in business

The old vision tube was re-terminated at both ends and equalised, and plugged to a Protection Circuit to Bristol. BT locked this circuit off so that it could not be automatically switched away to make good an existing vision circuit. BBC Plymouth were once again re-connected with the BBC Network via Bristol Comms Centre. BT confirmed that this arrangement was going to last for quite a period since they had to wait for the roadworks to end before re-routing and reinstating the axed tubes. But what about Plymouth opt-outs?

The solution was simple. Before each opt Plymouth would offer studio line-up to Bristol Comms Centre via the Protection Circuit, and Comms Centre would hot switch them into circuit, and out again at the end of the opt. Comms Centre could switch to either of the networks so all opts on the networks would be covered.

This arrangement was most successful, and it was good to see how the switching could be paced for minimum disturbance.

Failure of an opt-out

All went well for a couple of weeks until one evening. We opted Plymouth to the BBC1 South West transmitters at 6.30pm and Spotlight South West went on air. Around 6.40pm the PY-BS 'P' circuit suddenly disappeared and didn't return. The South West transmitters shrugged their shoulders, set off alarms and did what they were supposed to do - switch to RBS. At this point I took executive action and switched Points West to the transmitters. Why? Because the transmitters were already taking Points West via RBS, so feeding that to line would produce no further disruption to viewers, and Points West was the Plymouth standby programme. Anything was better than network. We phoned Bristol Studio B who made an on-air apology to South West viewers and that was that. The Network Router - which was called the IA (Immediate Access) Switcher - was put in extended mode, meaning that Studio B had control of Mendip and the South West transmitters and could opt them all back smoothly.

Spotlight South West were not amused. They wanted someone's head on a plate, but we could not find out why a locked off circuit had been seized. BT clammed up. There was going to be an inquest.

Meanwhile, we put our heads together to work out what could be done if a similar incident re-occurred. I arranged for our Graphics department to produce a 35mm slide "This programme will continue in sound only until vision is restored", or something like that. And yes, it was the good old Letraset days, no electronic picture stores. Studio A let us use their slide scanner, and we arranged for Plymouth to feed their studio output on an audio circuit to Bristol for each opt. This audio circuit was married with the slide scanner and coded Sound in Syncs together with an ITS signal. If, therefore, the 'P' circuit was lost again, then we would switch to this standby and Plymouth could decide what to do, but at least we would hold the transmitters.

As it happened, we never needed to use it. A few days later we heard about what had happened.

There was a Mexico World Cup match scheduled for 7pm. At 6pm it was discovered that there were not enough Goonhilly to London vision circuits available to carry this match. BT told the BBC that the only remaining protection circuit was locked out for Plymouth and they would be on air shortly. A hasty top level meeting took place in Television Centre in a 3-inch carpet office. If one was ever summoned to a meeting in an office with that thickness of carpet, you knew it was important. The Head of News and Head of Sport locked horns. The beast in charge of Sport won the contest, and Head of News backed off with a mighty snort. By which time Spotlight was on air.

It was 6.40pm. BT were instructed to take the circuit.

The Fallout

BBC Plymouth were almost in orbit over this. The Regional Television Manager was in a foul mood. A plume of smoke seemed to rise from BH for days. Memos went to and from London. Meetings were scheduled. Then a decision was made. A phone call came from on high to the Comms Manager and he was told to sort it.

Bristol Communications had taken delivery of new Mark II microwave links which were a considerable improvement over Mark I both in range and specification. The Mark I links were collected from the stores, dusted off and taken to Plymouth and Caradon Hill over a weekend. A link was established from Plymouth BH (see BH Plymouth) to the roof of the transmitter building at Caradon Hill. Transmitter department had been instructed to provide switching on the BBC1 and BBC2 TIE panels to enable the link to be put in circuit. BT provided a spare link to Plymouth Forder Battery to feed Stockland Hill.

Every thing worked fine, and lasted for a couple of months until normal service was restored.

Maradona had won.

See also : BBC Eng Info


--Oldcommsboy 14:27, 20 June 2014 (PDT)

Comments

Mark Carver writes:

Interesting that TSW escaped the problem owing to IBA provided circuits ? I wonder what they were, I don't ever recall seeing SHF dishes on the roof of TSW's Derry's Cross studios, and I don't think either Forder or Caradon could be seen from the there, though Plympton was possibly LOS from the roof of the theatre next door ? (Later, as Chris knows, when Westcountry TV tookover in 1993, their entire network from Wenvoe southwards was IBA/NTL digital circuits between all the Tx sites and opt out studios in the region).

I recall a similar BT circuit failure here in the south region, 1988 or 89 I think.

It was a weekday evening about 22:10hrs. I was watching a repeat of The Singing Detective on BBC 1 Rowridge. Suddenly the feed failed, and after a few seconds of thrashing about, the RBS feed of CP, via Hannington jumped in. I switched over to BBC 2, that looked the same, a rather fuzzy and 'ringy' image, but good enough. TVS had a 'News at Ten' caption up, with ITN via audio only, that made sense because ITV didn't use Sound in Sync for audio, until NICAM came along 89/90ish,so audio was provided on landlines, separate to the vision circuits.

Anyway, TVS found an alternative feed of ITN, because seconds later the video was restored. C4 was just black and silence. Back then C4 went via the local ITV companies on its way to the transmitters,because the ITV companies sold the advertising in their patch. It took about 10 minutes for TVS to stick up an apology card and music on C4, but they were still able of course to run the ads in the appropriate time slots.

Everything returned to normal about an hour later.

The IBA Engineering info prog the following week featured the failure. I didn't realise until then, that there was more havoc for TVS in the South East (Dover/Bluebell Hill/Heathfield side of their region. Everything normally received at their Maidstone studio, came via Southampton, and fed from there back on BT links via the BT Tower in London. So Maidstone lost both incoming ITV and C4 feeds. The IBA at Croydon were able to receive ITV off air from Hannington, and fed that to the BT Tower, for onward transport to Maidstone, and BT routed C4 directly from their Charlotte St centre to Maidstone.

So C4 from Rowridge was ads but no progs, and from Bluebell Hill progs but no ads !!

The IBA however never explained how TVS Southampton had managed to restablish their network feed of ITN so rapidly ?