|
Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
|
Thread Tools |
8th Nov 2016, 8:29 pm | #1 | ||
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,534
|
The Baird 240-line standard
From a recent thread:
Quote:
Quote:
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
__________________
https://www.radiocraft.co.uk |
||
8th Nov 2016, 9:35 pm | #2 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,346
|
Re: The Baird 240-line standard
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.
|
8th Nov 2016, 10:05 pm | #3 |
Nonode
Join Date: Feb 2005
Location: Worcestershire, UK.
Posts: 2,534
|
Re: The Baird 240-line standard
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
__________________
https://www.radiocraft.co.uk |
8th Nov 2016, 11:54 pm | #4 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,346
|
Re: The Baird 240-line standard
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.
|
9th Nov 2016, 11:30 am | #5 |
Octode
Join Date: Jul 2008
Location: Gloucestershire, UK.
Posts: 1,043
|
Re: The Baird 240-line standard
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 |
9th Nov 2016, 1:52 pm | #6 |
Octode
Join Date: Mar 2006
Location: Co. Durham, UK.
Posts: 1,117
|
Re: The Baird 240-line standard
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. |
9th Nov 2016, 2:13 pm | #7 |
Octode
Join Date: May 2009
Location: Stevenage, Herts. UK.
Posts: 1,518
|
Re: The Baird 240-line standard
Interesting Scophony titbit. You have to wonder if they ever did actually sell any sets given the apparent zero survival rate.
|
14th Nov 2016, 12:44 pm | #8 |
Hexode
Join Date: Feb 2008
Location: Sydney, Australia
Posts: 356
|
Re: The Baird 240-line standard
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. |