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UK Broadcast television standard ?

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mechie

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Does anybody know of any recent changes to the UK Broadcast video format (terrestrial 625 line)?

I have just been doing some experiments with a video sync seperator to strip the 15.625 kHz line sync pulses (suitably filtered for flyback 1/2 lines) but I seem to be getting 15.600 kHz.
This is true for all four proper terrestrial channels (forget Channel 5 - they are all over the place - Fisher-Price video mixer desk ?).

I wondered if something was happening on the quiet to 'ease us into the digital age' or something, if the good old 625 standard was being sneaked out? A direct email to Auntie was just ignored, so no help there then!

I would be surprised if anything has happened but I can't find a better explanation for THREE frequency meters all being wrong to exactly the same degree.
 

Nigel Goodwin

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Most Helpful Member
mechie said:
Does anybody know of any recent changes to the UK Broadcast video format (terrestrial 625 line)?

I have just been doing some experiments with a video sync seperator to strip the 15.625 kHz line sync pulses (suitably filtered for flyback 1/2 lines) but I seem to be getting 15.600 kHz.
This is true for all four proper terrestrial channels (forget Channel 5 - they are all over the place - Fisher-Price video mixer desk ?).

I wondered if something was happening on the quiet to 'ease us into the digital age' or something, if the good old 625 standard was being sneaked out? A direct email to Auntie was just ignored, so no help there then!

I would be surprised if anything has happened but I can't find a better explanation for THREE frequency meters all being wrong to exactly the same degree.
I've heard nothing about any changes, and I can't see any reason to change at all!.

However the difference on your meters is only 625Hz out of over 15.5KHz, so it's well within 1% of the specified frequency - what accuracy are your frequency counters?, and have they recently been calibrated?.
 

mechie

New Member
Sussed it !

I figured out what was wrong ... an over-enthusiastic monostable I used for suppressing the half-line sync pulses during frame-sync; this was totally killing that part of the pulse chain. Difficult to see on my oscilloscope as this event is almost 20mS after a frame sync pulse and only a few pulses long (200uS in total?) - pushing my 'scope a bit (15MHz dual channel, single timebase)

I couldn't see it being my frequency counters - what's the chances of ...
THREE frequency meters all being wrong to exactly the same degree
I now have a pretty good frequency standard that only cost me about 30 quid - this will form the basis of a workshop cal source and has prompted further research into video waveforms - for which the next project needs to be a trigger delay add-on for my oscilloscope.

[imagine an icon for a rolling snowball]
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Re: Sussed it !

mechie said:
I now have a pretty good frequency standard that only cost me about 30 quid - this will form the basis of a workshop cal source and has prompted further research into video waveforms - for which the next project needs to be a trigger delay add-on for my oscilloscope.
I'm sure I remember EPE doing a project for a PIC based video triggering unit, it allowed you to trigger on any line you wanted in the video signal.
 

mechie

New Member
Oscilloscope Trigger Delay

Thanks for the lead Nigel, I have seen something like that at ...
http://www.g3rfl2004.ukhome.net/linesync.htm but I thought that it would be more versatile (universal?) if I came up with something like a box containing a 'scope trigger circuit followed by some form of programmable delay (probably a 10MHz PIC as a uS counter).

I really haven't thought it through yet - maybe it would require either a 'TRIG-BREAKOUT' socket adding to the 'scope to feed into a delay or a seperate VERT amp and TRIG circuit:...
The first idea seems easier and cheaper but then works only on the one 'scope, the second idea would be more generic but harder to control (more knobs to fiddle with!).

Either way -- -- -- think how much fun you could have with a 'scope that starts its trace 230uS after every second TRIG event -- -- -- or multiple - input TRIG conditions ? -- -- -- or single-sweep with camera trigger output ? (yes, I know, PC 'scopes do this and that but I like real instruments :wink: ).
 
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