The Baird 240-line standard

[Panrock]

From a recent thread:

Originally Posted by PaulM

We don't know how BTL's transmitter worked in detail. It was designed and made for them by Metropolitan Vickers in Manchester. It's demountable valves and solid construction probably led to a pretty successful design. However, with no 'DC working' and modulator non-linearity, the sync/video ratio would have been hard to define. I think that '50%' is just a shorthand for a system that needed a greater ratio to cope with average picture level changes from near all-black, to near all-white.

Errm yes.... So it seems. In a ten year old post on this very board at https://www.vintage-radio.net/forum/...ead.php?t=9229 a respected poster there writes:

I wouldn't get to bothered about the niceties of the spec for the Baird 240-line system. I remember reading an open letter from Cossor to the BBC (but not where I read it - dammit!) that asked when the Baird transmissions were going to start conforming to the published spec. In particular they complained that the sync-vision ratio, nominally 40:60, varied and could be as high as 60:40 and that there was considerable uncertainty about the position of the frame sync pulse relative to the frame blanking. Sometimes they complained that the sync pulse overlapped the end of the blanking period giving the line timebase no time to recover from it's long period of free-running. Also the frame pulse jittered relative to the line syncs (a bit like the jitter of the head-switch point of a VCR I guess) causing the picture to move up or down one line as the phase relationship of the line and frame syncs changed.

For convenience, I append that 1935 diagram here again.

At Alexandra Palace on November 2nd, a pre-war EMI receiver was demonstrated showing a 240-line programme. To my eye, the picture actually looked sharper than the 405-line alternative (also shown). The flicker however, was most pronounced.

Steve

[emeritus]

As the extract from a 1920's Encyclopaedia of Physics shows, research carried out into flicker in the early days of cinematography established that you need a minimum of about 45 interruptions per second to eliminate flicker when viewing cine film, which in the silent era could be obtained with film shot at 16 frames per second by using a 3-bladed shutter, giving 48 interruptions per second. The volumes are undated, but must have been written before the introduction of sound film, which used the higher linear film speed provided by 24 frames/sec to get acceptable audio quality, in conjunction with a 2-bladed shutter to give the same flicker rate as silent film. Baird's 25 frames per second, non-interlaced, would inevitably suffer from serious flicker, whereas the 50 interruptions/second of the interlaced 25 frames/sec 405 line system, lay in the flicker-free zone.

[Panrock]

As regards flicker, the other determining variable would appear to be picture brilliance - or lack thereof.

For example, my approx. 12-in (diagonal) 60/120-line mirror screw also runs at 25 f.p.s. non-interlaced. Using the original dimmer modulated light, the flicker wasn't noticeable. However, when this was later increased, the flicker soon became obvious. Still, at least it's better than the 12½ f.p.s. more commonly found in mechanical displays!

Steve

[emeritus]

Yes, the curves in the encyclopaedia article confirm that. I think it is to do with the fact that the response of the human eye slows down as the light levels decrease, providing a longer integration period. I used to notice in analogue days that viewing a snowy picture though partially-closed eyes would remove the noise.

[Mr Moose]

Hello,
It wasn't only Baird that had problems with its sync signals. In 1937 Scophony had to delay release of it's large screen mechanical televisions, because of timing and phase shift problems, until the BBC had improved their sync equipment.
Yours Richard

[Brigham]

The Baird 240-line system could have benefited from a slightly longer-persistence phosphor; at the expense of resolution, of course.
It could only be a compromise. Fact is, the Marconi-EMI system was better in just about every respect.

[wd40addict]

Interesting Scophony titbit. You have to wonder if they ever did actually sell any sets given the apparent zero survival rate.

[ENGLISH VICTOR]

Panrock,
The open letter from Cossor was printed in "BBC Television the formative years"
I am too lazy tonight to get the book out tonight but that is where I expect you read the letter. It is the only place I have ever seen it.
Victor.