UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Television and Video

Notices

Vintage Television and Video Vintage television and video equipment, programmes, VCRs etc.

Closed Thread
 
Thread Tools
Old 26th Sep 2020, 12:57 pm   #1
MartinMarris
Tetrode
 
MartinMarris's Avatar
 
Join Date: Aug 2017
Location: Wye, Kent, UK.
Posts: 93
Default 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?
__________________
Martin
MartinMarris is offline  
Old 26th Sep 2020, 1:35 pm   #2
McMurdo
Dekatron
 
McMurdo's Avatar
 
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
Default 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
__________________
Kevin
McMurdo is offline  
Old 26th Sep 2020, 1:45 pm   #3
Oldmadham
Pentode
 
Join Date: Sep 2019
Location: Perth, Western Australia, Australia.
Posts: 199
Default 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.
Oldmadham is offline  
Old 26th Sep 2020, 7:32 pm   #4
emeritus
Dekatron
 
Join Date: Jan 2012
Location: Brentwood, Essex, UK.
Posts: 5,316
Default 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.
emeritus is offline  
Old 26th Sep 2020, 7:43 pm   #5
Oliver35
Pentode
 
Oliver35's Avatar
 
Join Date: Apr 2016
Location: Halesworth, Suffolk, UK.
Posts: 188
Default 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..
Oliver35 is offline  
Old 26th Sep 2020, 8:33 pm   #6
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default 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.
__________________
--
Graham.
G3ZVT
Graham G3ZVT is online now  
Old 26th Sep 2020, 9:19 pm   #7
peter_scott
Dekatron
 
peter_scott's Avatar
 
Join Date: May 2003
Location: Edinburgh, UK.
Posts: 3,273
Default Re: Visible 405-line Interlace?

Graham, you are very kind!

Peter
peter_scott is offline  
Old 27th Sep 2020, 12:06 am   #8
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default Re: Visible 405-line Interlace?

Quote:
Originally Posted by peter_scott View Post
Graham, you are very kind!

Peter

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.
Attached Thumbnails
Click image for larger version

Name:	interface board.jpg
Views:	91
Size:	97.8 KB
ID:	216475  
__________________
--
Graham.
G3ZVT
Graham G3ZVT is online now  
Old 27th Sep 2020, 11:19 am   #9
red16v
Heptode
 
Join Date: Jul 2006
Location: Winchester, Hampshire, UK.
Posts: 636
Default Re: Visible 405-line Interlace?

Quote:
Originally Posted by McMurdo View Post
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
To add, ‘movement’ in a scene is updated 50 times a second. It is quite possible for there to be movement in a scene between fields - a good example being a fast tennis rally for example where the ball is likely to be in two different physical positions from one field to the next.
red16v is offline  
Old 28th Sep 2020, 9:02 am   #10
McMurdo
Dekatron
 
McMurdo's Avatar
 
Join Date: Jun 2009
Location: Staffordshire Moorlands, UK.
Posts: 5,263
Default Re: Visible 405-line Interlace?

sorry yes I was thinking in terms of telecine for some reason or possibly antiques roadshow...
__________________
Kevin
McMurdo is offline  
Old 28th Sep 2020, 11:58 am   #11
Brigham
Octode
 
Join Date: Mar 2006
Location: Co. Durham, UK.
Posts: 1,111
Default 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!
Brigham is offline  
Old 28th Sep 2020, 1:14 pm   #12
Kyle__B
Banned
 
Join Date: Mar 2020
Location: Nijmegen, Netherlands
Posts: 142
Default 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.
Kyle__B is offline  
Old 28th Sep 2020, 1:48 pm   #13
rontech
Heptode
 
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
Default 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.
__________________
Time flies like an arrow, fruit flies like a banana
rontech is offline  
Old 28th Sep 2020, 2:11 pm   #14
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default Re: Visible 405-line Interlace?

Isn't this the point when someone invokes the Kell factor, or Nyquist–Shannon Theorem?

<ducks>
__________________
--
Graham.
G3ZVT
Graham G3ZVT is online now  
Old 28th Sep 2020, 2:30 pm   #15
Graham G3ZVT
Dekatron
 
Graham G3ZVT's Avatar
 
Join Date: May 2010
Location: Greater Manchester, UK.
Posts: 18,675
Default Re: Visible 405-line Interlace?

Quote:
Originally Posted by rontech View Post
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
I has the same perception, but to be honest, I think it was due to the poor performance of many dual standard sets on 625.

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.
__________________
--
Graham.
G3ZVT

Last edited by Graham G3ZVT; 28th Sep 2020 at 2:36 pm.
Graham G3ZVT is online now  
Old 28th Sep 2020, 7:18 pm   #16
rontech
Heptode
 
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
Default Re: Visible 405-line Interlace?

Quote:
Originally Posted by rambo1152 View Post
Quote:
Originally Posted by rontech View Post
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
I has the same perception, but to be honest, I think it was due to the poor performance of many dual standard sets on 625.

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.

Nice to know it was not just me.
__________________
Time flies like an arrow, fruit flies like a banana
rontech is offline  
Old 29th Sep 2020, 9:41 am   #17
Hybrid tellies
Nonode
 
Hybrid tellies's Avatar
 
Join Date: Jan 2004
Location: 1966-1976 Coverack in Cornwall and Helston Cornwall. 1976-present Bristol/Bath area.
Posts: 2,965
Default 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!
__________________
Simon
BVWS member
Hybrid tellies is offline  
Old 29th Sep 2020, 11:59 am   #18
rontech
Heptode
 
Join Date: Jun 2017
Location: Southport, Merseyside, UK.
Posts: 646
Default 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.
__________________
Time flies like an arrow, fruit flies like a banana
rontech is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 2:19 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.