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Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc. |
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26th Sep 2020, 12:57 pm | #1 |
Tetrode
Join Date: Aug 2017
Location: Wye, Kent, UK.
Posts: 93
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Visible 405-line Interlace?
"Interlaced scanning is used with a picture repetition rate of 25 c/s because this gives fifty frames a second. .... the flicker is the same as with fifty pictures a second, that is, it is unobservable. Actually, the result is not quite the same, for it is possible to detect interline flicker upon close observation, while if the eyes are moved rapidly while looking closely at the picture one set of lines seems momentarily to disappear. At the normal viewing distance, however, these effects are hardly observable." W.T. Cocking, "Television Receiving Equipment", Third Edition 1950, page 18.
Is that true? If you get close to the screen, can you "see" interlace?
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Martin |
26th Sep 2020, 1:35 pm | #2 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,271
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Re: Visible 405-line Interlace?
I was always taught 50 fields, 25 frames. The scene is only updated 25 times a second but the screen is scanned 50 times per second. It's a subtle difference.
You're not supposed to see interlace, thats the idea, but as the screen size increses, you can certainly start to see the lines. Poor interlace (either by design or fault) can result in line pairing, exaccerbating the line structure. There's a thread somewhere about 'spot wobble' which adds to the subject AFAIK flicker is always more apparent at the periphery of vision, because the eye's persitence of vision is mainly effective in the centre. But that's a medical subject and I'm just repeating what Ive read elsewhere
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Kevin |
26th Sep 2020, 1:45 pm | #3 |
Pentode
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 199
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Re: Visible 405-line Interlace?
I have never really been able to "discern interlace" as such with 625 line TVs.
If you move your head fast, or wave your outspread fingers in front of your eyes you get a "strobing" effect, which may be similar to that described. "Line structure" is clearly visible if you sit a bit closer to the screen, but it doesn't seem to look any different to how sequential, non interlaced scans would look. Interestingly, before I saw 405 line TV, I assumed there would be an obvious difference between that system & the 625 system, both in vertical resolution & line structure visibility,but sitting in a guest house "TV room", back in 1971, viewing a dual standard BW TV, damned if I could see any difference! That was the last dual standard TV I ever watched, as far as I can recall. |
26th Sep 2020, 7:32 pm | #4 |
Dekatron
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,339
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Re: Visible 405-line Interlace?
Screen brightness also affects the perception of flicker. At low intensities the response time of the eye increases, so the eye integrates two or more consecutive frames and does not perceive flicker that would be apparent at higher brightnesses.
Last edited by emeritus; 26th Sep 2020 at 7:42 pm. |
26th Sep 2020, 7:43 pm | #5 |
Pentode
Join Date: Apr 2016
Location: Halesworth, Suffolk, UK.
Posts: 188
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Re: Visible 405-line Interlace?
I can’t say I’ve noticed. I have the same book, and it’s very interesting, although I still believe Cocking was mad. I started to build the set he designed for WW in 1947- the timebases alone use 6 valves. A complicated design for the home builder..
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26th Sep 2020, 8:33 pm | #6 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,713
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Re: Visible 405-line Interlace?
The PC graphics cards that I have persuaded to output a 405 line waveform does not correctly implement the eight broad pulses that are in the broadcast specification, and consequently the Cossor 916 that I was using in my experiments showed no sign of interlace whatsoever.
Peter Scott designed a circuit using a couple of TTL chips to insert the broad pulses artificially, and once I had got the timing adjustments correct, it was obvious if I observed the 10'' tube at close quarters, that the vertical resolution doubled, and the gap between the lines all but closed. The temporal nature of interlace is obvious to me if I alter my gaze vertically, or wave my hand in front of my eyes. Peter always points out that his monostable circuit is crude, I call it brilliantly simple.
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26th Sep 2020, 9:19 pm | #7 |
Dekatron
Join Date: May 2003
Location: Edinburgh, UK.
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Re: Visible 405-line Interlace?
Graham, you are very kind!
Peter |
27th Sep 2020, 12:06 am | #8 |
Dekatron
Join Date: May 2010
Location: Greater Manchester, UK.
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Re: Visible 405-line Interlace?
The latest iteration. The four additional pots allow me to apply colour bars, and accurately set a good 1vP-P grey-scale into 75 Ohm with 30% sync The test-point above the blue pot is vertical sync to externally trigger my scope. The 7805 that was on the board has now gone, it's now powered from pin 9 of the VGA connecter, It took me a while to realise I could do that, but I had to modify the VGA switch I use to pass through the 5v to one of the outputs.
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27th Sep 2020, 11:19 am | #9 | |
Heptode
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
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Re: Visible 405-line Interlace?
Quote:
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28th Sep 2020, 9:02 am | #10 |
Dekatron
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,271
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Re: Visible 405-line Interlace?
sorry yes I was thinking in terms of telecine for some reason or possibly antiques roadshow...
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Kevin |
28th Sep 2020, 11:58 am | #11 |
Octode
Join Date: Mar 2006
Location: Co. Durham, UK.
Posts: 1,117
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Re: Visible 405-line Interlace?
The double refresh rate of broadcast television is what makes the movement look so fluid.
In television drama which includes both studio and outdoor film inserts, it's one of the things that makes the inserts look 'old'. Seemingly, present-day drama directors discard every second frame, even in 1080p HD, and repeat the first one instead, to mimic the old 'mechanical projector' technique. A little like adding the 78rpm shellac hiss to music on CD! |
28th Sep 2020, 1:14 pm | #12 |
Banned
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
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Re: Visible 405-line Interlace?
Interlacing is visible if your TV is sharp and you feed it a direct signal (RGB Scart or baseband on a b&w set). If something in the image only exists on one line, it'll be extremely distracting. Production companies would film with this in mind, no thin vertical stripes etc.
I wouldn't worry about it on some fuzzy old 405 over VHF. |
28th Sep 2020, 1:48 pm | #13 |
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
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Re: Visible 405-line Interlace?
I was puzzled when I first had a 625 line TV in about 1965. The contrast and general "crispness" on 405 line pictures always seemed better than on 625 line version. This was most apparent on old black and white cinema films which were shot on nitrate stock.
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28th Sep 2020, 2:11 pm | #14 |
Dekatron
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Re: Visible 405-line Interlace?
Isn't this the point when someone invokes the Kell factor, or Nyquist–Shannon Theorem?
<ducks>
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28th Sep 2020, 2:30 pm | #15 | |
Dekatron
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Re: Visible 405-line Interlace?
Quote:
As a case in point when I took a series of off-screen photographs of the Apollo 11 mission from our Bush TV135 set, I used BBC1 on 405, even though the coverage was carried on BBC2.
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-- Graham. G3ZVT Last edited by Graham G3ZVT; 28th Sep 2020 at 2:36 pm. |
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28th Sep 2020, 7:18 pm | #16 | ||
Heptode
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
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Re: Visible 405-line Interlace?
Quote:
Nice to know it was not just me.
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29th Sep 2020, 9:41 am | #17 |
Nonode
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
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Re: Visible 405-line Interlace?
Some dual standard black and white televisions use to give good 405 line pictures with good contrast and sharpness whilst on 625 the pictures tended to look grey, washed out and some suffered weak sync on certain picture scenes. They also suffered vision on sound effects on 625. Our GEC rental set was a good example of this and was why we continued to watch BBC1 and ITV on 405 rather than on 625.
A lot of this was I think down to the use of mean level AGC for both systems and AC video coupling to the CRT with no DC or black level. The two best dual standard black and white TV's that produced good pictures on both 405 and 625 were the Philips 210 and the RBM A640 which used DC coupling from the Video output to the CRT with the RBM A640 using a kind of black level correction circuit. Its interesting to note that the RBM A640 also had an interlace control in their frame timebase circuit. Some engineers told me that this was because RBM could not produce a TV with good interlace!
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29th Sep 2020, 11:59 am | #18 |
Heptode
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Re: Visible 405-line Interlace?
The set to which I referred was rented from Visionhire and was made by Bush. After about three years it could be changed and The 22" ( I think ) Philips was much better as you say.
I can't recall the Model Numbers though.
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