First published in 2008. Last updated: 9th June 2020.
This page contains examples of the various tuning signals and test cards used on BBC Television since the 1930s. The feature remains a work-in-progress and we’d be delighted to hear from anyone who has any additional information about any of the items covered here.
Test patterns and tuning signals
Television is a UK invention. On 27th January 1926, John Logie-Baird demonstrated that it was possible to transmit pictures using mechanical scanning apparatus. In 1929, the first experimental television service was broadcast by the BBC in collaboration with The Baird Company, using the Baird 30-line system.
In 1934 came the idea of using patterns to test the equipment. Initially, these were very simple designs, consisting of circles and lines. which tested the picture ratio.
In November 1936, Baird introduced an enhanced version of the mechanical system, capable of 240 lines. However, an electronic system from Marconi-EMI boasted 405 lines. The BBC transmitted each system on alternate weeks, from Alexandra Palace. The Marconi-EMI 405-line system was deemed superior and the BBC discontinued use of the Baird format on 13th February 1937.
1937 also saw the introduction of the first BBC tuning signal. This was used to assist viewers in adjusting the numerous controls on their TV sets to obtain the optimum signal.
Television services were suspended from 1939 until 1946 due to World War II.
With the war over, BBC TV got back up and running, for the few thousand people who could actually view it. And just as before, tuning signals were in use, prior to the start of programmes each day.
Before the end of the decade, the BBC began using patterns specifically designed to assist with aligning studio cameras and testing response. In addition to being used off-air, these designs were also broadcast, and became a useful tool for the TV trade, when setting up domestic TV installations.
The designs were hand-drawn on large pieces of card, typically 2ft x 3ft. Hence the term “Test Card” was coined. They were put to air via a camera. Originally, they were also referred to as “Opacities”.
An alternative to having to dedicate a conventional camera to airing a test card, was a monoscope. This was a special-purpose camera that had the test image painted on the inside screen of the tube. Each tube was only capable of handling one test image. However, the set-up was expensive, difficult to adjust, and was restricted to black and white output. It fell out of favour in the 1960s.
The cards later became “Transparencies” – ranging from 1ft wide, to the 35mm slides that were common during the 1960s, 1970s and 1980s. The term “Test Card” became the common term for these special designs, irrespective of the means by which they were put to air.
Test Card A was the first BBC test card. It debuted in the late-1940s and is credited with being the world’s first television test card to be transmitted. The 2.5 MC frequency grating was of most interest to engineers, hence its location in the centre of the circle. Although a great improvement on anything previously available, Test Card A and it’s successor, were criticised for not having sufficient testing features.
Test Card B no longer exists. The best remaining image is a photograph of a studio, with a partial view of Test Card B on a stand. BBC engineer George Hersee described it in his 1967 work, but it is believed that no copies of the full card remain in existence.
It was similar to Test Card A but with an extra greyscale strip below the circle. The letterbox just above ‘A’ moved to the top of the card. It was never actually broadcast, but was used for camera line-up at Alexandra Palace.
Test Card C was first broadcast in January 1948. At the time, the aspect ratio of TV broadcasts was 5:4. By the early 1950s, this had changed to 4:3. The design of Test Card C was altered slightly as a consequence.
Some BBC regional transmitter sites used slightly different versions of Test Card C. There were at least three different designs in use at the BBC. Additionally, the ITA used a another design for the ITV Network.
In 1964, two new test cards were introduced. Test Card D was adopted by the BBC and ITV for use on BBC One and throughout the ITV Network. Test Card E was used for the new 625-line BBC Two service. Test Card D and Test Card E were virtually identical, bar the frequency gratings, which were finer on the latter, to cater for the higher definition offered by the 625 service.
The frequency gratings on Test Card E were sinusoidal, and thus technically more useful than the square waves on previous test cards. However, as a result, some of the gratings looked soft and fuzzy – the lowest grating in particular. TV dealers complained about this and Test Card E was consequently scrapped, after c. one week on air. It was replaced by a modified version of Test Card C. Unfortunately, we have no examples of the special BBC Two version of Test Card C.
Test Card F, designed by BBC engineer, George Hersee, was introduced in 1967, and aired on the new colour 625-line BBC Two service. This test card features Mr Hersee’s daughter, Carole, pictured alongside Bubbles the clown.
Since the 1950s, BBC trade test transmissions were accompanied by music compiled from library music publishers. Over the years, virtually every music genre imaginable was featured. The vast majority of this music was not commercially-available in the UK. However, in 1953 and 1954 some commercially-available recordings were used with Test Card C – these were all on the Oriole label.
In 1954, some pieces from the Xavier Cugat Orchestra and the David Carroll Orchestra included vocals. Tracks featuring vocals were a rarity during trade tests. For a period in the 1970s and early 1980s, some of the tapes included pop music. Any such music played with the test card would have been an outtake recording, produced outside of the UK. The BBC could transmit this material as library music, as the recording would not have been released commercially in the UK.
The test card was also accompanied by a test tone on many occasions. Until the 1970s, the tone would be inserted for short periods at regular intervals during the day. By the 1980s, music was played with the test card for much of the day, with tone being restricted to short periods each morning, when the test card first appeared.
There were different tones for BBC One and BBC Two: BBC One used 1KHz, and BBC Two 440Hz. The use of different tones on each channel was mainly just so that engineers (and viewers of a certain disposition) could easily identify each channel.
Originally, the tones originated at Broadcasting House (the home of radio). We’re not sure exactly how they were generated there. In later years (around the start of the 1980s), some units were built that took in 5MHz from Rubidium oscillators, and synthesised the above audio frequencies precisely. The kit was still there as late as 2009 and still worked.
However, since the introduction of the ‘VALID’ test signal (colour bars with a rotating central pattern and synchronised blip on the sound, made by Vistek (now ProBel)), the BBC uses 997Hz for the main programme stereo, and 440Hz for the second stereo – often used for Clean Effects. The problem with precise 1KHz is that only some of all the possible digital levels are used. 997, however, is such an odd number, that it cycles through all the levels, thus fully exercising any DACs. But it is close enough to 1KHz so as not to be audibly different.
Test Card F was broadcast from a 35mm slide, using a Rank Cintel slide scanner. It was actually a dual-layer slide – one layer for the monochrome areas, the other for the colour parts – very carefully aligned and sealed in a glass slide-holder. There was no BBC One-branded slide version of Test Card F. The ‘BBC 2 COLOUR’ test card slide was used on BBC One and BBC Two. However, when Test Card F was aired on BBC One, a ‘BBC 1’ caption was electronically superimposed, covering up the ‘BBC 2 COLOUR’ text.
The Rank Cintel slide scanner was a large and expensive piece of equipment, but probably of more importance, technically, is that it would be near impossible to keep two scanners aligned identically to match each other. Like all standards, their absolute accuracy is not as important as the fact that everybody agrees. Hence, it was far better to use just one scanner, and overlay a ‘BBC 1’ caption on the feed that was used for the BBC One transmissions, as this resulted in a consistent test signal for both networks.
Test Card F was generally broadcast for longer periods during daytime hours on BBC Two than on BBC One. The latter’s schedule often being filled by programmes for schools and children. The BBC One caption overlay was a little crude by modern standards. It was produced by a dedicated unit that had the relevant parts from an ‘Anchor’ machine (an early form of electronic caption generator, before the introduction of Astons and Ryleys).
In 1980, the BBC began an experimental Ceefax In-Vision service. Gaps in the schedule which previously would have been occupied by trade test transmission, were now being filled by a selection of teletext pages. The In-Vision broadcasts – offering a digest of news, sport, travel and TV listings – were largely restricted to short, early morning slots, with the test card continuing to play the more prominent role during gaps in the programme schedule.
With TV sets becoming more reliable and members of the trade now having test signal generators available to them, the days of the trade test transmission were numbered. The BBC was also keen to promote its teletext service and BBC editorial teams wanted to make better use of the dead airtime.
In early May 1983 daytime trade test transmissions were replaced by Pages from Ceefax. The test card was now largely restricted to brief early morning slots, shortly after transmitters came back on air following overnight closedowns. Weekdays, Test Card F would be broadcast for seven-and-a-half minutes, between 5.52am and 6am on BBC One (preceded by a period of pulse and bar, accompanied by tone). It would also be broadcast for seven-and-a-half minutes on Saturday and Sunday mornings, before the first programme of the day. The test card would also have a seven-and-a-half minute outing on BBC Two each day, prior to the start of programmes or Pages from Ceefax.
Test Card F goes electronic
With in-vision teletext now dominating non-programme airtime, what of the future for Test Card F? Well, back in the autumn of 1982, BBC Engineering announced plans to create an electronic version of Test Card F. This short article from their in-house magazine outlines the proposal:
The BBC has radiated Test Card F during trade test transmissions for a number of years but its future use became uncertain partly due to the difficulty of obtaining quantities of high quality slides. Consultations with the receiver trade via the British Radio and Electronic Equipment Manufacturers’ Association (BREMA) revealed that many service engineers prefer Test Card F to other purely electronic patterns. This is thought to be because the central picture of a young girl gives a subjective assessment of picture quality and aids the setting of colour saturation.BBC Engineering
For future use, Designs Department are producing an equipment which stores the test card in digital form in order to overcome the problems of slide duplication and the need for a slide scanner. Data will be held in YUV form using eight bits and 13.5/6.75 MHz sampling frequencies. All of the geometric patterns will be derived from computed picture samples calculated by programs which take account of the requirements for different rise times in the corner diagonals, the circle and the frequency gratings. This data, which describes an essentially perfect test card, will be combined with the central picture samples obtained by a YUV ‘frame grab’; techniques for noise reduction on this central Image will be explored.
Captions for network identification will be added to the complete test card from separate integral digital generators so that the one equipment will deliver several differently annotated outputs, all of which are coded PAL. The first equipment is expected to be in service by mid-1983.
However, whatever the reasons, the electronic test pattern did not materialise on air until May 1984. The timing of the deployment of the new equipment was perhaps unfortunate, what with Pages from Ceefax now replacing virtually all daytime trade tests. But, the new kit had many other uses within the BBC too.
Contrary to much of what is written online about the introduction of the electronic version of Test Card F, it did not immediately permanently replace the slide format. The electronic pattern was introduced on an experimental basis in May 1984. The slide version, and Test Card G were used on air for a while longer. Although Pages from Ceefax had largely replaced the test card, Ceefax was prone to breaking down occasionally. In such circumstances, the test card stepped in to cover.
No major design alterations were made for the electronic version of the test card. There were a couple of cosmetic changes: a new font for the designation letter; and the colour bars along the top, which now completely replaced the black and cyan blocks.
Internally, most PAL coders inserted a couple of lines of colour bars within the vertical interval. Known as IRS bars (Insertion Reference Signal), this allowed for quality checks at any time, and not just during line-up, similar to the ITS (Insertion Test Signal) on transmitted signals. The coder for the slide version of the test card was modified to insert a fat band of colour bars at the top of the picture but had the disadvantage of hiding the top of the arrow-head that pointed to the edge of the picture. When the electronic test card was created, the colour bars were part of the frame store that made up the image and so they were extended down to the top white bar of the frame to look tidier. The arrow-head was overlaid on top of the central magenta bar, thus restoring that feature.
Electronic Test Card F was showcased at the 1984 International Broadcasting Convention (IBC). And BBC Engineering publicised the development in their quarterly magazine, in autumn 1984:
Test Card F has been radiated by the BBC during trade test transmissions for a number of years: it is also widely distributed within Television Centre providing a quick and convenient means of confirming the correct operation of numerous items of video equipment. Its future use had become uncertain, partly owing to the difficulty of obtaining high-quality slides but also because of the need for a dedicated slide scanner. The latter is of course expensive in terms of capital and requires routine monitoring and adjustment if excellent results are to be obtained.BBC Engineering
The digital Test Card F generator displayed at IBC (international Broadcasting Convention) replaces both slide and scanner whilst requiring no routine maintenance. The equipment contains a read-only picture store conforming to the 4:2:2 coding standard (CCIR Recommendation 601), having the luminance signal sampled at 13.5 MHz and the colour difference signals at 6.75 MHz. A single 4U-high 19-inch rack takes mains and pulse inputs to deliver digital and analogue YUV test card together with a PAL signal from an integral coder.
All of the geometric patterns were generated by computer techniques with reference to the original drawings. Edges were both horizontally and vertically anti-aliased to give the most accurate realisation of Test Card F. The central picture of the girl was copied from a slide by means of a YUV ‘picture grab’. and the resulting data were inserted into the geometric pattern during the computer preparation process. The combination of the real picture and computer-generated patterns provides a stable, high-quality source for a well-known test signal.
The original central picture has been retained as it provides valuable information for assessment of flesh tones, overall saturation, luminance-chrominance timing and picture monitor convergence etc.
The autumn 1982 article mentioned that captions for network identification would be added to the test card via separate integral digital generators. However, during its first few months of on-air use, the new electronic test card was broadcast without a channel identification caption. A rather crude, temporary solution eventually materialised. However, it wasn’t long before they were replaced by the proper channel logos.
There was a minor glitch with the channel logos, when they were first added: they weren’t vertically centred between the white lines above/below them. This was addressed in 1985, but, unfortunately, for BBC Two, the horizontally centring was knocked off slightly. The change is apparent when comparing the above stills, from late-1984 and mid-1989.
The electronic Test Card F was a revelation, as it was perfectly linear and had a flat grey background, unlike the old slides, which always had a bit of shading. Digitising an image was quite difficult in those days and rather than try to scan the original photo again, the BBC’s research department (who created the electronic version) worked with the computer graphics workshop at TV Centre and grabbed an image of the current test card slide on Quantel Paintbox. They took the data file back to their base at Kingswood and incorporated the central picture into the computer-generated pattern.
However, so that there was a clean and precisely-defined white circle around the picture, they had to zoom in a little to hide the old one on the slide. Despite their best efforts, the picture on the electronic test card always looked a little desaturated (pale).
According to Barney Wol’s website (no longer online), when the widescreen Test Card W was being put together (and the revamped 4:3 card that was named ‘J’), the original 2.25″ slide of Carole Hersee was rescanned.
The kit that produced the electronic test card also had the ability to add an on-screen countdown. Since the 1970s, a ten-minute countdown to the next programme was regularly displayed over the test card. The updated version of the countdown was still used to count into Ceefax AM (though it was not used every day). We believe its use was discontinued by the early 1990s. Though there is evidence of the countdown in use as late as October 1996:
Occasionally, there were technical issues with the ‘in-vision’ Ceefax transmission: we are unable to verify the cause of the issues but there were certainly problems with the Ceefax transmission system over the years – particularly in the 1980s. And perhaps the in-vision teletext generator misbehaved occasionally too. Whatever the reason for the lack of availability of Ceefax, Test Card F or Test Card G (more on that shortly), would be brought in as a substitute. The test cards also stepped in during some periods of industrial action, where Ceefax content (particularly news, weather, sport, business and travel) was unavailable, although apology captions were also occasionally used in such situations.
The scheduling of Test Card F
There was quite a lot of change in policy over the years regarding the on-air use of the test card. And on top of that, lots of ad hoc scenarios. But here are some general points:
- Contrary to popular opinion, in the 1960s, 1970s and 1980s, Test Card F was not broadcast overnight, after BBC One and BBC Two closed down. That changed in 1989 with occasional experimental overnight broadcasts of (encrypted) specialist programming, for British Medical TV. And also from 1992, with the launch of BBC Select (and later also Nightschool TV and BBC Focus) – more details on this below.
Until November 1997 – with the exception of nights where BMTV, BBC Select, Nightschool TV, BBC Focus or The Learning Zone was being broadcast, the BBC One and BBC Two transmitter network was shut down each night, around fifteen minutes after the last programme.
When the announcer had bid us farewell, the screen would fade to black. We’d typically have thirty seconds of black and silence, which would be followed by ten minutes of tone on BBC One and two minutes of tone on BBC Two. Why the difference in duration between the two channels?
Well, late-night regional news and weather forecasts were common in all BBC One regions in the 1970s and 1980, with the exception of London (network). Apparently, regional centres in England could not opt back to network if there was no valid network feed to opt back into. So, network had to stay online until all the regions had opted back in. This was not the case for the regional centres in Belfast, Cardiff and Glasgow, which regularly stayed up later than network.
Local continuity in the English regions was discontinued in 1980, due to BBC cutbacks. However, the practice of ten-minute tone at closedown on BBC One continued until November 1997. This was presumably out of courtesy to the national regions in Belfast, Scotland and Wales, who may have been closing down minutes after network. By staying online, network will have allowed the national Ceefax service to continue in the regions. If network went offline for the night, the Ceefax feed to the regions would have been lost. Ceefax was still a national service until 1997.
Regional continuity was much less common on BBC Two, and so there was no need for a lengthy period of tone – just enough to wake up anyone that may have fallen asleep.
Prior to the complete cessation of overnight transmitter shutdowns, there were a small number of occasions over the years where freezing weather conditions meant that the transmitters needed to be kept switched on overnight. In these situations, it was not uncommon for Test Card F to be transmitted although we do recall at least one occasion c. the mid-1980s where Pages from Ceefax and music was transmitted.
- Until the late-1960s, the test card was not transmitted between 1pm and 2pm. Black and tone was shown at lunchtime if there were no programmes.
- The test card was not transmitted on Sundays, until 1974 (precise date unknown). Instead, short gaps between programmes were filled by black and tone. It was presumably felt there was no need for trade tests, as shops were closed and service engineers would not be working on a Sunday.
There were few Sunday closedowns on BBC One, except during the summer months. BBC Two was normally off air completely on Sundays during the day until the early 1970s, when Open University transmissions came along. Even then, there were often large gaps between the final OU programme and the first BBC Two programme of the day, so the transmitters were shut down. Later, when the gap between OU and the first BBC Two programme was short, black and tone was used.
Programme gaps of 15 minutes were often classified as an Interval, and were filled using a slide and music.
- From June 1983, Test Card F would appear for seven-and-a-half minutes prior to the start of the first programme of the day on BBC One (which was Ceefax AM on weekdays, at 6am). BBC Two was a little different. The first programme of the day there was quite often an Open University broadcast. In those situations, the OU transmissions were preceded and followed by black and tone (or, from 1984, the special OU test signal – more on that later). Test Card F would appear seven-and-a-half minutes before 9am (weekdays). The periods of black and tone/OU test signal and tone were replaced by Test Card F by the late-1980s.
- On BBC One, Test Card F appeared before the first programme of the day on Saturdays and Sundays, for c. seven-and-a-half minutes, having been preceded by a period of pulse and bar – unless the first programme was an Open University production, in which case black and tone or the OU test signal and tone was used. Towards the end of the 1980s (date TBC), Pages from Ceefax appeared for 15 minutes prior to the first programme on Saturdays/Sundays, again, unless the first programme was for the OU.
- Saturdays/Sundays on BBC Two usually meant starting the day with Open University programming. OU broadcasts would usually have been preceded by black and tone (or the OU test signal and tone, from 1984). Towards the late-1980s, OU test signal and tone was replaced by Test Card F and tone.
Where there were no Open University programmes at the weekend, BBC Two would come on air shortly before 9am, with pulse and bar initially, followed by seven-and-a-half minutes of Test Card F, before giving way to Pages from Ceefax at 9am.
By c. 1994, even where the first programme of the day on Saturdays/Sundays was for the Open University, Ceefax pages were broadcast, for c. 15 minutes prior to the start of the programme (preceded by a period of Test Card F).
- Between 1989 and 1992, the BBC broadcast specialist programming for British Medical TV overnight. The service was encrypted. A similar overnight encrypted service, targeted at various special audiences, was broadcast between June 1992 and December 1994 on BBC One and BBC Two. These programmes were usually broadcast between 2am and 6am, under the banner BBC Select. The service aired at least twice-a-week on each channel.
Where a BBC Select programme was being broadcast, the channel in question would remain on air throughout the night. The BBC Select broadcast would be preceded and followed by Test Card F and tone. A similar arrangement applied for the Nightschool TV (January 1993 – October 1995) and BBC Focus (September/October 1995) overnight services on BBC Two.
- On Monday 4th January 1993, BBC One’s Business Breakfast moved to a 6am start. Consequently, Ceefax AM moved to 5.45am. Test Card F was now broadcast for fifteen minutes from 5.30am each weekday. Pulse and bar was no longer transmitted when the transmitters were coming online.
- At 2am on Monday 9th October 1995, the launch of the overnight Learning Zone signalled the end of overnight closedowns on BBC Two on Sunday/Monday/Tuesday/Wednesday/Thursday nights, leaving Test Card F relegated to the early hours of Saturday and Sunday mornings only. The Learning Zone was suspended during holiday periods, such as Christmas and Easter – weekday closedowns did occur during these periods.
- Following the launch of BBC News 24 in November 1997, Test Card F transmissions on BBC One came to an end. Rather than closing down, BBC One now broadcast BBC News 24 overnight. Weekend Test Card F remained unaffected on BBC Two for the time being, appearing around half-an-hour prior to the first programme of the day and giving way to Pages from Ceefax after fifteen minutes.
In early 1998 (precise date TBC), the practice of weekend overnight transmitter shutdowns on BBC Two was discontinued. For a number of months, Test Card F and tone filled BBC Two’s downtime; however, by December, Pages from Ceefax was being broadcast through the night, accompanied by tone.
- Since 1998, the test card has put in the odd appearance on BBC One and BBC Two (in various guises – Test Card F, Test Card J and Test Card W), mainly as part of overnight RBS tests on the analogue transmitter network. RBS tests were carried out once-a-year, on BBC One and BBC Two.
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).
Much like the arrangement for Test Card F, there was one Test Card G generator serving both network channels in London. So, BBC One had to use an overlay for its channel identification. And in this example, 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.
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.
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.
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. The test cards shown here were broadcast from BBC Pebble Mill as part of a test of this contingency arrangement.
With thanks to Dave Baldwin for the 1980s/1990s recordings from which many of the stills were obtained. Thanks also to Andy Emmerson, for the black and white test cards. Our thanks to Andrew Nairn for his input regarding test card usage.FEATURE IMAGE:
PICTURED: BBC Test Card J. SUPPLIED BY: The TV Room. COPYRIGHT: BBC.