Difference between revisions of "Down the Tubes"

From mb21 TX Wiki
Jump to: navigation, search
(BBC Vision Equalisers)
Line 29: Line 29:
  
 
The [http://en.wikipedia.org/wiki/Zobel_network#Bode_equaliser Bode] and the [http://en.wikipedia.org/wiki/Zobel_network#Cauer_equaliser 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.
 
The [http://en.wikipedia.org/wiki/Zobel_network#Bode_equaliser Bode] and the [http://en.wikipedia.org/wiki/Zobel_network#Cauer_equaliser 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.
 +
[[File:SwitchingCentreOBBays.jpg]][[File:SwitchingCentre_Ops_Desk.jpg]]
  
 
==== Back in business ====
 
==== Back in business ====

Revision as of 10:16, 29 June 2014

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. SwitchingCentreOBBays.jpgSwitchingCentre Ops Desk.jpg

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.


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