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TBC-6 Timebase Corrector
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In order for a C Format Video tape recorder to produce colour pictures some means of
stabilizing the off-tape colour (chroma) signal was needed. For while the bandwidth capability of the format was quite good enough to handle the full 5 MHz of broadcast television, even the battleship engineering standards employed in the tape transports was not able to provide a stable enough colour composite video signal. In addition, for editing or playout of tv programmes over the network the output video signal has to be exactly synchronized with the rest of the network.
A 'cheap and cheerful' colour adapter like the one for the VPR-5 shown elsewhere on the
site or that fitted to the VPR-20 were just about OK for in-the-field, non-critical use. Indeed domestic video recorders all have something not too dissimilar inside them. But for serious use in the broadcast studio, a 'proper' T.B.C. (timebase corrector)like the one described here is essential. This is because not only will this machine completely take apart the 5Mhz composite television signal, store the colour and black and white elements in a digital memory and re-combine them locked to a stable crystal reference, it can also synchronize (lock) the whole video recorder with the rest of the television station or network. This is very useful as it is vital to have all the video sources in a TV studio synchronized so that they may be switched or mixed together without any picture disturbances or colour anomalies. |
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The above image shows the VPR-6's TBC-6 front panel opened and the video input board
partly pulled out for access. This piece of equipment on its own is another rather epic excercise in video engineering. Costing around £10,000 in 1984, it contains a mixture of state-of-the-art analog and digital electronics on four of the largest multi-layer printed circuit boards you will probably ever see. (Cray computer boards excepted.) And to use any C Format video recorder to its full extent, or even get colour pictures from the machine you will need one of these complex and sometimes quirky heavy grey boxes. |
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Timebase correction of television pictures was probably the first widespread use of
digital signal processing. Two decades ago this was a pretty epic undertaking in terms of electronics and cost, as a TBC such as this probably contains quite a bit more circuitry that the video recorder itself. The service manual for the TBC-6 comes in several volumes and I once counted 48 separate circuit diagrams...
The above image shows the TBC-6 pulled out of 'the rack' with it's top cover removed.
The switched-mode PSU is on the left, the two very necessary cooling fans are on the right
and the four main PCB's sit in the nesting frame in the middle. To save having to use extender boards when servicing this equipment all the PCBs are interchangeable in the nesting unit. So you just take the top cover off the unit and just swap the board you wish to work on with the one previously sitting in the top slot. Of course this presupposes that you know which board or boards need to be worked on in the first place... |
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Timebase correction basically consists of digitizing the analogue off-tape composite video
signal, storing the data in RAM and working out what the ongoing timing errors are. The 'timing error profile' information is then used 'in reverse' as it were to read the data out of the memory either under control of an accurate internal timing reference (typically and ovened crystal) or synchronized to 'station syncs'. The data is then converted back to video and new colour burst and syncs are generated (all correctly colour framed and phased of course). Drop-out compensation is also part of this processing, where missing lines are replaced with previous lines held in the memory. (But unlike the later analog cassette formats, the 1-Inch C Format was a very low drop-out and robust format to start with.)
Digital audio (as in a CD player) samples audio signals at fourty four thousand times a
second, this machine samples the video signal at seventeen million times a second. Not that difficult these days apparently, but 8 bit 'flash' video A to D converters running at 17Mhz, was once very exotic technology indeed. |
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It could be argued that this rather large chip actually started the whole digital video
'revolution'.
In the late 1970's TRW introduced the TDC-1007J, 8 bit 'flash' analogue to digital
converter for composite video signals. (It was probably originally done for the US military.) While very expensive and quite large single 'chip' at perhaps over £1,000 in 1980, it was actually a rather simple and cost effective way of digitizing a composite video signal and made the widespread use of digital timebase correctors (at least by broadcasters and high-end professionals) viable. It also 'opened the floodgates' to all sorts of new digital video effects units (DVE's) such as those made by Quantel et al, indeed it probably made that company. The Ampex TBC-6 on this page uses a later version of this original TRW chip.
This example shown above is one of a pair to be found in an IVC TBC 2002 timebase
corrector (which was actually made by an embryonic Quantel and is featured elsewhere on this site). It's serial number is 0067 and it was made in the fourth quarter of 1979, the last time I tried it out it was still working happily. |
