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Long-distance time base correction!

by Dave Buckley

Up to 1984, Television Training Department (TVT) was based at Woodstock Grove (WG), a building at the end of a cul-de-sac of the same name, just off the pointed end of Shepherds Bush Green and alongside Kensington House.

TVT had a small television studio which went colour in 1975. At the same time, the department's video tape format was standardised on low-band Sony U-matic format (SUM), a robust ĺ inch tape cassette system.

WG video tape area consisted of two SUM machines which could be remotely controlled from the technical position in the production gallery. During training programmes, one machine would record the studio output while the other would play-in inserts via a time base corrector to ensure synchronous working into the vision mixer.

There was a problem though - when TVT ran regional courses (e.g. at Plymouth), two spare SUM machines would go with the technical staff plus the time base corrector from the VT area.

This meant that, if the WG studio was required for training programmes during the time the regional course was taking place, then VT inserts would have to be taken non-sync on the vision mixer.

However, sometime in the late 1970ís/early 1980ís an item appeared in a Weekly Information Sheet to the effect that three transcoders had been installed in the Standards Convertor Area. These normally converted between PAL and SECAM or vice-versa, but there was a comment that, if set to PAL to PAL, the unit would act as a Field Store Synchroniser (FSS). It was this that made me sit up and ring Lines Bookings!

This produced the information that, of the three units, one was permanently booked to News (now thereís a surprise); the second to International Control Room, while the third was bookable.

As the WG studio had two reversible vision circuits (plus associated music and control lines) to TC, what if the raw output from the replay SUM was sent up one vision circuit, into the FSS and the output sent back on the other video circuit, and the studio genlocked, synchronous VT replay would be possible, albeit with the signal travelling at least two miles, if not more!

And this is exactly what happened on quite a number of occasions when TVT undertook regional courses and the VT time base corrector had been taken away. And the fact that the WG studio was genlocked, made it effectively a TC studio running on 6A pulses!

There was one stipulation from Lines Bookings - if an urgent call came in for the FSS during our booking, then TVT would get about 10 minutes warning before the facility was taken away. In fact, in all the times TVT booked the FSS, this happened once!

For more information on the Woodstock Grove Training Studio and associated areas, see BBC Engineering No. 110 (July 1978), pages 5 to 10.


Dave Buckley, Television Training Department, 1969 - 1993 (and then freelance until 2003)

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