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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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26th Mar 2017, 1:27 pm | #21 | |
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Re: Baird v. EMI
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It is interesting the research relating to this area. The critical flicker frequency (CFF), which is customarily designated as the boundary where the frequency of the flickering light appears to be fused (continuous light) or flickering, 50% of the time (analogous to many measured physiological variables, like the perception of a sound in audiometric testing) is said, at least, in one of the most well respected textbooks (Adler's Physiology of the Eye) to decrease with fixation away from the fovea or central vision. For example away from the fovea, at 15 degrees from fixation, their graph(data) shows that for a 2 degree white test field, a CFF value of just under 20Hz and it is about 45Hz with that same stimulus presented at central fixation. (their graph is on page 442 of the 8th Edn). The implication being, from their experimental data, that peripheral images fuse at a lower CFF. This is counter your practical experience (and mine) and your observations for a visual target, like a TV screen image, which as you state, definitely flickers more when presented to the peripheral visual fields. They actually tried to explain this away in Adler's text though (the fact that TV images flicker less with central compared with peripheral view) by stating: "Differential retinal sensitivities and spacial summations by rod & cone subsystems, as well as receptive field sizes and densities might be responsible...." But this explanation in my view was always a bit wishy-washy and lacking any real substance. One thing of note though; the CFF is definitely affected by retinal adaptation; the higher the light levels the higher the CFF. So in theory at least, perceived flicker from the TV image should be lower under dimmer room illumination, if the eye has had 15 or 20 minutes to adapt to the lower luminance level. That could be fun to verify. |
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27th Mar 2017, 9:18 am | #22 |
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Join Date: Jun 2004
Location: Burghfield, Reading, Berkshire, UK,
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Re: Baird v. EMI
In 1972, I worked for a famous (in TV terms) American company called Ampex. This was a truly international company and we used to exhibit (quad VTRs) at all the international symposia etc. One of the constant things that I heard was that the American personnel were annoyed by the flicker of the 50Hz refresh rate of the PAL/CCIR pictures compared with the 60Hz for the NTSC pictures. However, they practically all said that the flicker became much less objectionable when the offending monitor was not in a peripheral position.
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28th Mar 2017, 11:22 pm | #23 | |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,274
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Re: Baird v. EMI
Quote:
The only off-screen photo by the reporters was taken with a 2 second exposure and is not worth repeating especially when you read their descriptive text. I suspect their photo was taken from a small screen GEC receiver also photographed in the article. I have edited the text somewhat but probably the main point that was taken from the British system was the need for good DC restoration that did not exist in the American standards at the time. If you prefer to see the full text rather than my edited version then see: http://www.americanradiohistor.....937-10.pdf Peter |
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29th Mar 2017, 8:26 am | #24 |
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Join Date: May 2003
Location: Harlaxton, Lincolnshire, UK.
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Re: Baird v. EMI
Hi Peter,
I couldn't get your link to the full text to work, but this one does. http://www.americanradiohistory.com/...cs-1937-10.pdf |
29th Mar 2017, 8:58 am | #25 |
Heptode
Join Date: Feb 2010
Location: Duffort, Gers, France
Posts: 714
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Re: Baird v. EMI
The explanation that I read many years ago it that the brain has a capacity for filtering out annoying things if they are always there. For example if you live next to a busy road, after a while you don't notice the traffic any more. In the case of TV flicker, if the screen is always flickering in the middle of your field of vision then after a while your brain filters out 50Hz flicker at that point. Since you don't generally look at a TV with peripheral vision your brain never learns to filter that out. I suppose you could do a test and watch a TV for several weeks with peripheral vision and see whether you still notice the flicker.
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