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Old 20th Jun 2017, 2:30 pm   #21
peter_scott
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Default Re: Baird 240 line Operation

Perhaps a little unfair...

https://www.youtube.com/watch?v=xZ2EXYezE1c

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Old 31st Jul 2017, 3:51 pm   #22
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Default Re: Baird 240 line Operation

I've added another page to my website detailing the 240 line generation and showing both 405 and 240 line representations of Test Card C.

(Yes, I know Test Card C wasn't around in 1936 but it gives a better comparison than earlier test cards.)

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Old 31st Jul 2017, 4:16 pm   #23
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Default Re: Baird 240 line Operation

A valuable and unique resource on the web.

The results you have achieved are stunning, but there are still too many lines to see clearly - even at 240. Come and join us at the NBTVA, where even 120 is considered HD!



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Old 31st Jul 2017, 4:33 pm   #24
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Default Re: Baird 240 line Operation

Quote:
Originally Posted by Panrock View Post
there are still too many lines to see clearly - even at 240. Come and join us at the NBTVA, where even 120 is considered HD!
Thanks Steve.

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Old 31st Jul 2017, 4:44 pm   #25
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Default Re: Baird 240 line Operation

That's very informative on how to set it all up, maybe it should be made sticky on the standards converter page ?
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Old 1st Aug 2017, 1:11 pm   #26
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Default Re: Baird 240 line Operation

Fascinating Peter - well done !
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Old 9th Aug 2017, 1:44 pm   #27
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Default Re: Baird 240 line Operation

Peter,

In your experience, what video bandwidth would suffice for the Baird 240 line standard pictures? Clearly the 901 is providing about 3 Mc/s, but what did 240 line TV actually need?
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Old 9th Aug 2017, 5:14 pm   #28
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Default Re: Baird 240 line Operation

Hi Steve,

Assuming that we want to resolve the "3MHz" bars in Test Card C then I think you just need to scale it from the line rate. For 240 lines the rate is 6kHz and for 405 lines it's 10kHz so the bandwidth required for resolving the "3MHz" bars should be 1.8MHz.

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Old 10th Aug 2017, 11:08 am   #29
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Default Re: Baird 240 line Operation

Thank you for that Peter.

Thinking on it then the TV broadcasting 'experiment' had been set up in a way that suited the 405 line system. Unless there had been an idea to redefine the Sound and Vision Transmission frequencies in the circumstance that the Baird system won to narrow the bandwidth of the occupied radio spectrum (eg Sound 41.5 Mc/s, Vision 44 Mc/s) then retaining the 405 line 'choice' would have been an inefficient use of both spectrum and receiver design. This because receivers could have been made with narrower vision IF pass-bands so giving higher stage gain - and not so many 'deaf' sets !
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Old 10th Aug 2017, 11:58 am   #30
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Default Re: Baird 240 line Operation

To be fair, in order to make a comparison, both systems have to be catered for.
The system was set-up in a way that suited BOTH systems.
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Old 10th Aug 2017, 12:36 pm   #31
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Default Re: Baird 240 line Operation

Quote:
Originally Posted by SteveCG View Post
This because receivers could have been made with narrower vision IF pass-bands so giving higher stage gain - and not so many 'deaf' sets !
Looking at David's Baird T23 http://www.vintage-radio.net/forum/s...ird+T22&page=4

it quite possibly was tuned for a narrower RF bandwidth which would put it at some disadvantage when the broadcasts were all on 405 lines.

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Old 10th Aug 2017, 2:56 pm   #32
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Default Re: Baird 240 line Operation

Ah, Peter, you were kind enough not to point out my faulty thinking in talking about IF bandwidths on these TRF sets !

Brigham, understood. In an ideal world the comparison should have been between the two systems as they would have been finally used (RF bandwidth and carrier frequency choices as well as scanning considerations), but that would probably have been a complexity too far. Mind you, later we did have 405/625 dual standard sets...

With the benefit of hindsight the choice of scanning standards was a different issue to the choice of scanning methods (ie mechanical v. electronic) at the transmitter end; since by the mid 30s all the receivers were all electronic in design - unless, that is, I have missed any mention of a mechanically scanned 240 line receiver on the forum.
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Old 10th Aug 2017, 3:42 pm   #33
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Default Re: Baird 240 line Operation

There were mechanically scanned receivers made by the Scophony company in the mid 30s and they actually worked on 405 lines! However, they were afflicted by mechanical inertia and whereas electronic scanning circuitry was fairly tolerant of small timing variations the Scophony sets were not. Subsequently Scophony requested the BBC to synchronise their master oscillator to timing pulses generated by a rotating machine which they did.

http://www.tvhistory.tv/1938-Scophony-UK.htm

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Old 20th Aug 2017, 10:32 am   #34
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Default Re: Baird 240 line Operation

The graphics card syncs do differ in two respects from the EMI 405 line standard and this can cause rather critical line frequency adjustment or hooking at the top of the picture in older sets that don't have flywheel sync. Sorting this is work in progress. The existing composite sync is simply generated by exclusive OR of the line and frame pulses but the resulting inverted line pulses within the frame sync are slightly delayed relative to their correct positions. There is also no half line pulse stream. The latter is not essential and a simple monostable circuit can correct the pulse positions. See Here. To use this circuit the Modelines need to be changed such that the Composite Sync box should be unticked and the +hsync references should become -hsync.

Peter

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Old 22nd Aug 2017, 1:39 am   #35
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Default Re: Baird 240 line Operation

Thanks Peter,
I completed my board last night, just waiting for the TTL to arrive from China to plug into the DIL sockets, also the trimpots, so it will be a few weeks yet.

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Old 22nd Aug 2017, 1:58 am   #36
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Default Re: Baird 240 line Operation

Quote:
Originally Posted by Panrock View Post
The results you have achieved are stunning, but there are still too many lines to see clearly - even at 240. Come and join us at the NBTVA, where even 120 is considered HD!
I used to receive 120 line SSTV on a 5FP7 tube. I also transmitted SSTV using a 625 line videcon camera turned on its side with its frame (now line) scan frequency divided by 3 and samples taken from the video output using a ramp generator and FET switch.

Graham.
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Old 22nd Aug 2017, 12:36 pm   #37
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Default Re: Baird 240 line Operation

Quote:
Originally Posted by rambo1152 View Post
I completed my board last night, just waiting for the TTL to arrive from China to plug into the DIL sockets, also the trimpots, so it will be a few weeks yet.
Sorry Graham,

You can still use the circuit as previously drawn but I have now made a small change so that the monostables run as an injection locked oscillator to give the half line inverted pulses. The changes are that the delay of the first monostable has been halved and the output of the second monostable now also triggers the first. See here

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Old 22nd Aug 2017, 4:09 pm   #38
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Default Re: Baird 240 line Operation

Mods,
Could this thread be made sticky, in the Television Standards Converters, Modulators etc. section, with a title change to reflect the universality of this technique?
Something like, "Using a PC to generate any legacy analogue TV standard".

Kat Manton demonstrated the technique to us using KnoppMYTH under Linux over a decade ago, but it seems to have lain dormant ever since.

Peter and I have shown it is now very simple to implement this under Windows, and should be seen as an inexpensive alternative to a standards converter.

Last edited by Graham G3ZVT; 22nd Aug 2017 at 4:14 pm.
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