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Television Standards Converters, Modulators etc Standards converters, modulators anything else for providing signals to vintage televisions.

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Old 27th May 2006, 8:44 am   #1
jim_beacon
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Default Baird 240 line standard details wanted

Hi,

in the interests of pushing Kat's standard converter to the limits, does anyone have the details of the Baird 240 line system? There is some limited information on the Aurora website, but I'm looking for synch pulse timings and the like (about all I know so far is that the full 240 lines are visible, it is 24 frames per second, non-interlaced, with a 50% synch level).

The next question, if I manage to persuade my video card to go that slowly, does anyone have a TV that will display the results?

Thanks

Jim.
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Old 27th May 2006, 8:50 am   #2
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Default Re: Baird 240 line standard details wanted

If nothing else gets found earlier, I will get a book out of the Library Stores once my exams are over - I think there was a basic description of it in there. If anyone else has it and wants to check, it was Television Theory and Practice by Reyner.

My intention is to try and turn a Woollies tenner-TV into a monitor for standards between 30 and 625 lines. I guess it may be a bit of a task!

Sam
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Old 27th May 2006, 8:55 am   #3
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Default Re: Baird 240 line standard details wanted

The earliest pre-war sets were dual standard 240/405. A very few may even have the switching still present. 6kHz line scan may be possible on a modified 405 set but on most sets with flyback EHT you may well have trouble getting both width and EHT right at the same time. The frame rate should not be a problem. The sync separator may need to be modified too. Otherwise you could use a 'scope as discussed in a thread many months ago.

I'm sure that Darryl will give you the details of the 240 line standard that he used to when designing the original Aurora. Just PM him and ask. I wouldn't be surprised if he got the details from David Boynes (Fernseh in the forum).
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Old 27th May 2006, 9:01 am   #4
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Default Re: Baird 240 line standard details wanted

Well, if you do find out Jim, would you mind posting the information here as I'd like to know too!

Sam
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Old 27th May 2006, 9:37 am   #5
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Default Re: Baird 240 line standard details wanted

I can't remember all the details of the Baird 240 line system., but here are some details that come to mind.
The line frequency is 6Kc/s and the frame rate is 25c/s.
The sync pulses were deeper than the 405 system being 50% instead of 30% of the total waveform.
The frame sync was a single broad pulse.
Other details I'll try to find are the duration of the line and frame blanking periods.

DFWB.
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Old 27th May 2006, 11:16 am   #6
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Default Re: Baird 240 line standard details wanted

I've found the information of the Baird 240 line system.

The sync pulses are actually 40% of the video waveform.

The frame broad pulse duration is 12 lines. The frame blanking period is 20 lines. Therefore, the frame pulse is 2milliseconds and the frame blanking is 3.333milliseconds.

The line pulse width is 8% of the 166.6 microsecond line period = 13.33 microseconds. The line blanking is 10% of the line period = 16.6 microseconds. The is no front porch. The leading edges of the line pulse and blanking are coincident.
It's the same for the frame blanking and sync.

DFWB.
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Old 27th May 2006, 12:07 pm   #7
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Default Re: Baird 240 line standard details wanted

This drawing is dated Sept 35.
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Old 27th May 2006, 1:40 pm   #8
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Default Re: Baird 240 line standard details wanted

Hi,

Sorry, I couldn't resist...

A look at the log file after starting X with verbose logging reveals the following when validating modes:
Code:
(II) NVIDIA(0):   Validating Mode "1800x220p":
(II) NVIDIA(0):     1800 x 220 @ 25Hz
[...snip details of mode...]
(II) NVIDIA(0):     Mode is valid.
I've not looked at the output of the card yet, but if the computer says it's valid it should be working.

I'll leave Jim to post his X modeline first, though - see if it matches mine...

Regards, Kat
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Old 27th May 2006, 2:25 pm   #9
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Default Re: Baird 240 line standard details wanted

Hi,

I have a reference to what I presume must've been an electronically-generated version of Baird's 240-line system, that being 243 lines, with a line-rate of 6.075kHz. Can anyone fill in the blanks?

Regards, Kat
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Old 27th May 2006, 3:43 pm   #10
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Default Re: Baird 240 line standard details wanted

Hi Kat,
The 243 line standard was an electronic system that EMI intended to use to compete with Baird. 243 = 3 X 3 X 3 X 3 X 3 Change the last figure to 5 and we get 405.

DFWB
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Old 30th May 2006, 7:32 pm   #11
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Default Re: Baird 240 line standard details wanted

OK then, here's the Modeline:
Quote:
Identifier "CRT-240p25"
VendorName "Unknown"
ModelName "Unknown"
Option "ConnectedMonitor" "CRT"
HorizSync 5-7
VertRefresh 45-55
DisplaySize 75 56
Modeline "Baird 240" 3.75 560 560 576 624 220 220 232 240 -hsync -vsync
Which takes advantage of the sub-12MHz dot clock feature of the new nVidia driver.

Below are a few screen shots taken of the 240 line standard in opertion. I used my trusty Tektronix 535A as a display, with a type "O" dual op-amp plugin. Timebase B supplies the line timebase, timebase A ouput is connected to the Y input to provide the frame timebase, and one op-amp is used to turn the 0.7V of video from the RGB combiner into 50V of video to drive the Z input (CRT cathode in this case).

The picture is cut about 2/3rds of the way across, as I couldn't quite get the line timebase frequency right (timebase B doesn't have variable speed - a small tweak is needed to the scope). Flyback blanking is also poor.

Jim.
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Old 26th Jul 2006, 2:59 pm   #12
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Default Re: Baird 240 line standard details wanted

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.
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Old 26th Jul 2006, 3:25 pm   #13
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Default Re: Baird 240 line standard details wanted

David, welcome to the forum. It's a very long time (about 15 years?) since you brought your 405 line converter to my place (then in Harrow). I still have the circuits in my files.

Now you mention it, I also remember seeing those remarks about the Baird standard. But where. I suppose we ought to look in Burns history of TV in the UK as that's one of the best secondary sources.
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Old 26th Jul 2006, 4:59 pm   #14
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Default Re: Baird 240 line standard details wanted

David,

thanks for your comments. I was surprised that it was so easy to create a picture at 240 lines, and to display it. I really must get around to posting the details somewhere.

Jim.
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Old 26th Jul 2006, 7:55 pm   #15
David Looser
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Default Re: Baird 240 line standard details wanted

Thanks for your welcome Jeffrey, yes it must have been at least 15 years - how time flies! That converter is still in use and has never given me any trouble at all. A few weekends ago I used it at a "Bygones" exhibition in the local church feeding a Pye B16T and a Bush TV22 which attracted a lot of attention. The Bush worked fine for the two days of the show, but the Pye failed during the second day. A .001 paper capacitor in the RF strip had failed and shorted out the HT!. I've since replaced every single one of them.

Returning to the question of 240-lines, some years ago I designed a 625-240 line converter to feed my dual-standard HMV 901. But with one thing and another it never got built. Now with Kat's PC-based approach I might re-visit this.

David.
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Old 27th Jul 2006, 8:26 am   #16
jim_beacon
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Default Re: Baird 240 line standard details wanted

David, if you could get some pictures from a real 240 line set, I would be interested to see them - I think my modeline is correct, but using a scope as a display could be hiding a multitude of sins......

Jim.
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Old 27th Jul 2006, 6:44 pm   #17
David Looser
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Default Re: Baird 240 line standard details wanted

Hi Jim,

Yes, so would I! and I'd appreciate your help if available. First, though, the set needs a bit of TLC. It's been out of use since the EHT supply self-destructed a while back. And I'll have to do something about the systems switch, which was removed at some point in the first 44 years of the set's life. All the other 240-line components are present and correct (I think)

David.
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Old 27th Jul 2006, 7:10 pm   #18
Kat Manton
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Default Re: Baird 240 line standard details wanted

Hi,

"Interested" is an understatement; I would be positively excited to see 240-line pictures on a real set. Fantastic! I was wondering if anyone had a 240/405-line set with the 240-line bits still present in some way. Hopefully by the time you have the set sorted out and working, I might have a working car... I'd really like to be present; it's been a long time since that set saw a 240-line signal...

One interesting thing; when the PC is generating 240-line to Baird's standard; it's in RGB colour... Wonder what Baird would make of that?

Regards, Kat
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Old 27th Jul 2006, 7:12 pm   #19
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Default Re: Baird 240 line standard details wanted

David,

your welcome to any help I can give, though Kat is the expert on the system - I'm the calibrated idiot that is required to prove repeatability!

Jim.
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Old 28th Jul 2006, 2:28 pm   #20
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Default Re: Baird 240 line standard details wanted

The 240 line system waveform.

DFWB
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