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THE TRANSMISSION GALLERY

TALES FROM A COLD FIELD

Being fairy-stories told to the author as a young engineer
© Ray Cooper, 2005 (2nd revised edition, Jan 2006)

And now, in colour.

Colour came to BBC2 in the late summer of 1967. Everyone was quite confident: the transmitters were only a few years old, most of the teething troubles had been got out of the way (it was felt), and did not the advertising blurb for these transmitters say that they had been designed with colour in mind?

When the first colour signals were piped down the line for test purposes, we soon became disillusioned. The 'colour in mind' that the manufacturers had was obviously not of the PAL variety. The picture looked as though it was being viewed through a fine Venetian-blind. The source of the problem was soon located to clamps within the video-corrector chassis - evidently the clamping action was much too hard, and since they were clamping on the back-porch of the waveform, which was where the colour-burst resided, they tended to push alternate lines up and down in lift as they tried to clamp to odd bits of the swinging burst that is a feature of the PAL system. Ironically they probably would have worked perfectly well with an NTSC signal, whose burst does not swing.

The modifications needed to cure this were quite simple, however - the clamps were 'softened' at subcarrier frequency by including a parallel tuned circuit in series with them.

Initially, tests were only radiated out of programme hours. It was only natural however that the studios should wish to get their hands in, and so after a while programmes in colour started coming up the line. Since transmitter modifications across the country had not been completed, these could not be radiated in colour - so the colour information was removed with a switchable filter at the transmitter input. This of course didn't stop the station staff watching any programmes in colour on their monitors.

One of the first to be seen was 'Bonanza', and very good it looked after years of monochrome. But of the home-made product, perhaps the first and finest was 'Late Night Lineup', with Joan Bakewell ('The Thinking Man's Crumpet'). Late at night, all station staff would crowd into the control room to critically assess the colour balance and, er.. the legs..

It was about this time that a local Birmingham department store (and I shall not shame them by naming them) announced that they had acquired a colour television receiver, and it was displaying colour pictures, to be seen in their Home Entertainment department. Since I knew full well that Sutton was not in fact radiating colour information of any kind, I was intrigued and went along for a look myself.

It was an odd machine, about the size of a drinks cabinet and obviously converted, possibly on a laboratory basis, from a production American model - it had a large round 'double-D' picture tube. And yes, the pictures did seem to be coloured. Unfortunately, all of the colour was spurious, and caused by gross misconvergence. That didn't stop a good many onlookers admiring it.

By the time we were allowed to actually radiate colour signals, we knew that our goose was cooked. If the drive stability in monochrome had been poor, then in colour it was going to be awful. Colour signals suffer from more distortions than do monochrome ones, and two of these are linearity-related - 'differential gain' and 'differential phase'. Quite tight limits had been set for these, which the drive was totally unable to live up to. When we connected test gear and viewed these waveforms, the distortions could be seen creeping about on a continual basis, with occasional large (sometimes very large) jumps. The only way to keep the gear anywhere near spec. was to adjust it more or less continually. This isn't practical in the real world, so we did the best we could, which was to check it every hour or so, and adjust as necessary. Sometimes it had just crept: on other occasions there had been a big jump. To illustrate, the limit on differential phase distortion at that time was ten degrees: jumps of forty degrees were commonplace, and on one occasion I personally claimed the all-comers record of eighty-five degrees, which I suspect is close to the theoretical maximum possible.

The reason for all of this became apparent - the gear may have only been three years old or so, but it was physically worn out from over-adjustment. Whole items in the modulator assembly had to be refurbished. And we were not alone. To illustrate:

Sutton held the (one) base spare diode modulator assembly (we were fairly centrally located in the country). There was a desperate call one day from Pontop Pike, where these units were also used. 'Please send us the spare modulator, we can't get ours to work at all'. Being kind-hearted we did so, after extracting a solemn promise that they would then immediately send the faulty unit to Sutton for refurbishment - which they did.

I must explain about these units. The cavities, which were the bits that got adjusted the most, were tuned by sliding a set of earthing spring fingers along the inner conductor of a co-axial cavity: this inner conductor was a flat bar of brass, heavily silver-plated. It is to be expected that after much adjustment, the plating wears down and the bar has to be re-plated. With the returned Pontop units, not only had the plating gone - most of the bar had been worn away as well, and the spring fingers were not contacting anything at all, just moving along inside a groove that they themselves had chewed. Possibly the rather abrasive air around Pontop (there were still many coalmines operating there in those days) may have speeded the wear.

Gradually the problems were got on top of. With refurbished units, and less vigorous adjustment, the units would last a few more years before they needed attention again. But in reality, the only cure was replacement of the drives with more modern models.

Positively the Last Open Day Ever >

 
mb21 by Mike Brown
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