The history of the BBC trade test transmission (part 4/4)

First published in 2008. Last updated: 4th August 2020.

Our history of the BBC trade test transmission concludes.

The other electronically generated test card

In 1971, the BBC introduced a modified version of the Philips PM5544 test card. This was the first electronically generated test card to be used by the BBC. It appeared on BBC One and BBC Two from time-to-time, often popping up in slots where Test Card F would normally have been used.

Test Card G was used mainly when the slide scanner used to broadcast Test Card F was undergoing maintenance, or when the slide scanner failed. It is believed that Test Card G was phased out on network BBC One and BBC Two following the permanent introduction of the electronically generated version of Test Card F in 1984. However, regional centres (such as BBC Scotland) continued to broadcast Test Card G well beyond May 1984.

Test Card G was regularly referred to by BBC announcers as ‘the colour test card’.

In the examples shown here, the one featuring the lighter-weighted font for the ‘BBC 2’ text is thought to have originated in Birmingham. The network version is believed to have used a similar font weight – the only perceptible difference between the London and Midlands versions of the test card was that the black vertical reflection test line near the top of the circle was slightly further to the right on the network version (as demonstrated in the late-1970s image of the network BBC One version of Test Card G).

One of the images shown above shows Test Card G minus the familiar vertical, coloured bars either side of the circle. The test card hardware included controls to allow this element – and part of the centre cross – to be switched on/off. We’ve also been told that when the equipment overheated, some of the on-screen elements would disappear.

The channel identification text was switchable on the Test Card G generator. The ‘BBC 1’ caption was displayed in the black rectangle at the top of the circle. However, only one Test Card G generator was available to the presentation department in London. This posed a problem if it was needed at the same time on both channels.

Granted, it was a very expensive piece of kit (in 1971) – and one for each channel might’ve been difficult to justify. But a similar argument was also at play here to the one that resulted in the same Test Card F source being used on both channels. For consistency of technical output, it was better that the same hardware was used to play out Test Card G on both channels.

The hardware used at Television Centre in London, for network playout, generally had the ‘BBC 2’ caption selected. So, when played out on network BBC One, the presentation department usually overlaid a ‘BBC 1’ caption on top of the ‘BBC 2’ text generated by the Test Card G hardware. In the example shown above, the same grey background used for Test Card F can be seen. This wasn’t always the case though. The overlay background colour was often recoloured black for Test Card G transmissions on BBC One.

BBC Scotland was still using its TCG generator in the 1990s. The examples below were shown in 1992 (BBC One) and 1993 (BBC Two) respectively. In both cases, they were shown at closedown, when the BBC Scotland programme schedule finished up earlier than network. Unusually for a late-night appearance of the test card in this era, it was accompanied by music.

On two occasions in 1997 – 21st June and 5th October – BBC One and BBC Two transmitters remained on air throughout the night to test the contingency transmission facilities at BBC Pebble Mill in Birmingham. In the event of a major problem at BBC Television Centre in London, which prevented BBC One and BBC Two being transmitted from there, control of both networks would switch to Birmingham.

On BBC One, we got to see Birmingham’s Test Card G generator – complete with ‘BBC 1’ text at the top of the circle. Unfortunately, we don’t have a copy of the unusual test pattern that was aired on BBC Two as part of this contingency test.

Programme transfers

During the 1970s and until 1983, BBC Two radiated a test card (mostly Test Card F) for much of the day. For a small portion of this downtime, the network distribution of BBC Two was used to send programmes and other material to the regional centres. This was referred to internally as ‘programme transfers’.

The main purpose of this process is thought to have been feeding future network programmes to the regions, for situations where they opted out of the network schedule and showed displaced network programming at a later time. It’s believed that this was achieved by using the opt-out switches at LO SWC (London Switching Centre). CAR (Central Apparatus Room) fed the programme to LO SWC. It is claimed that BBC Scotland were the biggest user of this facility.

In those days, the signals for BBC One and BBC Two were fed to the main London transmitter at Crystal Palace and also on network distribution circuits to the major regional broadcast centres (Cardiff, Bristol, Birmingham, Manchester, Glasgow, Belfast etc.). Most of the time, the regional centres would route the London signals to their local transmitters but occasionally, the region would ‘opt-out’ and feed their own material to their transmitters. Local news bulletins/programmes are the classic example.

Where the BBC Two network distribution was being used for programme transfers, some switching had to occur to ensure that viewers at home could not see this programme material. The precise nature of the switching is unclear but we do know that viewers in many parts of the country reported seeing Test Card G during the programme transfer window.

Test Card G generators were only available at the following BBC regional centres: Glasgow, Cardiff, Birmingham and London. Whatever switching arrangement was at work, the Test Card G output from these four centres managed to cover BBC Two at all transmitter sites across the country.

The national centre in Northern Ireland, located in Belfast, did not have a Test Card G generator; however, Test Card G still appeared in Northern Ireland during programme transfers. It is not clear which mainland regional centre’s Test Card G output was routed to Belfast/Northern Ireland – though anecdotal avidence suggest BBC Scotland’s Test Card G output made it across the Irish Sea.

In a letter (dated January 1980) to a viewer who enquired about the seemingly random appearance of Test Card G on BBC Two Wales, Charles Hope of the BBC’s Engineering Information Department explained that on occasions where programme material was sent down the line from London, Test Card G “originated from our Cardiff studios and was fed to both the Wenvoe and Mendip transmitters. Because the programme material from London will include sound, we also have to originate test card music from Cardiff.

“The pattern is a slight modification of the original Philips design. The modifications make the pattern more suited to the British television system than the Continental system for which it was designed.”

Interesting that Charles suggests the BBC Wales Test Card G transmission also went out on the Mendip transmitter, which was/is in the BBC West region.

Other test signals used by the BBC

Test Card F is the test signal that most people would associate with the BBC. But other test patterns/signals were radiated by BBC TV, alongside TCF.

The gentle brightness change from black to white on the sawtooth signal, if viewed on an oscilloscope is a linear ramp, with a gentle slope up, and a sudden drop to black at the end. This was (perhaps erroneously) perceived to be similar to the teeth on a saw blade. The sawtooth was a common sight on BBC One and BBC Two in the 1970s and early 1980s, when the transmitters were switched on in the morning, and at closedown.

The composite pulse and bar was used for aligning video circuits with an oscilloscope and not by eye on a monitor. From the early 1980s until the early 1990s, it was often the first thing to appear each morning when the BBC One and BBC Two transmitters came back online, following the overnight closedown. A time switch was used to trigger the pulse and bar each morning – typically half-an-hour before the appearance of Test Card F – allowing sufficient time for the transmitters to warm up.

From the early to mid-1980s, a black and white version of the pulse and bar was often used. However, a colour version was also in use. Chrominance was added to the standard monochrome test signal to try and accurately and easily show chrominance amplitude, and the composite pulse showed chrominance/luminance delay. It is believed that the original pulse and bar generators didn’t even use station subcarrier, but just an approximate 4.43MHz oscillator.

Until the late-1980s, during monochrome programmes, the colour burst was switched off. A colour television would detect that, and disable its colour decoder circuit, so eliminating any colour noise, which would otherwise be very apparent over a black and white picture. However, in later years, Tektronix produced the TSG271 test signal generator which always had a burst on its output, and the phase of the subcarrier on the pulse and bar signal was as specified for that in the ITS signals (120B0 – nearly, but not exactly, magenta).

The original pulse and bar design included an additional feature not shown here. In the white area on the right-hand-side, and not far from the centre of the screen, there was a black vertical line, running the full length of the screen. However, this line was later split into four ‘dashes’ of equal length. These dashes were used as a means of identifying the BBC regional centre where the signal originated. The presence of four vertical dashes (the maximum) meant the signal was being broadcast by BBC Northern Ireland in Belfast. No dashes (as in the examples above) indicated that the pulse and bar originated in London.

The streak test wasn’t transmitted regularly by the BBC. But, we happen to have one off-air recording. If the low frequency performance of a circuit is poor, then a constant brightness level, which is basically a DC voltage, will end up with a slope on it. By having different width white bars, different LF time-constants can readily be seen. This was useful for aligning flying-spot slide scanners and telecine machines.

From c. 1984, a new test signal appeared. The Open University test signal comprised a black screen, with white text reading ‘OU’ in the top left and top right corner of the screen. It was accompanied by tone. The signal was transmitted around Open University programmes, from a special transmission suite, used specifically for Open University broadcasts. The announcer had complete control over all sound and vision and was also responsible for playing out the Open University programmes from this suite.

The OU test signal would normally appear for a short period prior to the first OU programme each morning, and in the gap following the end of the last early morning OU programme, and the appearance of Test Card F shortly before 9am, when the main BBC Two Network Control area came fully online. The signal was merely a means of keeping the transmitters on air.

We’re not clear about the significance of the combinations of “OU” text. In the above examples, the one featuring “OU” in the top left corner only was broadcast at closedown on BBC Two. The one with “OU” top left and right went out on BBC One at closedown. There may also have been a variant with “OU” top right only. If you know anything about the meaning behind this, please get in touch and let us know.

An electronic switch was set to put out pulse and bar half-an-hour or so before the first Open University programme each morning, allowing time for the transmitters to warm up. The automated system would then switch to the OU suite, which would be sending out the OU test signal. In a situation where the announcer had overslept, resulting in the first OU programme not being broadcast or delayed, the only thing being broadcast would be this fairly innocuous test signal.

The OU transmission suite was used mainly for early morning and late-night Open University broadcasts. Eagle-eyed viewers will have noticed a slight frame roll during any transition between BBC Two Network Control and the OU suite. If you ever wondered why this occurred – well, here’s the explanation.

In those days, there were two complete transmission chains – all the way from the network mixer output, via Ceefax and ITS inserters, and SIS encoders, with an auto-changeover between the two on detection of an SIS decoder fault (which would also cover failure of the Ceefax or ITS inserters, and the whole circuit from the mixer output).

But, this auto-changeover was done with real relays. Thus the U-links on the inactive chain would be swung across from the main Network Control area output, to the OU suite, and the changeover forced manually. While everything was synchronous, it was the rather large hole in the signal resulting from the relays changing over that resulted in the frame roll.

RELATED WEBSITES: MHP – The Test Card Gallery and The Test Card Circle.




Posted by The Clean Feed Team

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