Need to store a PAL signal...

[Chris P]

I need some help again please Guys...

I have ordered the chips I'm gunna need, but so I can fully understand the theory behind this I looked a bit deeper at the PAL signal. Below is a photo of my 'scope connected to a PAL signal, along with the theoretical durations I have found of various sources (which seem fairly accurate). The horizontal sync signal is clearly visable here and is well documented as a negative pulse, lasting 4.7 micro seconds.
However I cant find any information about the level or duration of the vertical pulse. I'm assuming its the same negative level of a longer duration? I can't examine it on the 'scope due to the triggering. Anyone have any ideas? Is there an official specification for the PAL standard as this would be useful, I cant find one or anything referring to one however?

Cheers
Chris

 

[J0HN1]

Hi,

i havent done any of this stuff for years,
but from what i recall,
this is pretty much what the video signal looks like,
i had always thought that the absence of signal
was detected as a frame pulse ...
but i could be wrong, its been a while.

this pic was just drawn up as an illustration,
its not a real scope image.
Just to show how i remember the frame pulses ...

Regards, John

 

[Nigel Goodwin]

i havent done any of this stuff for years,
but from what i recall,
this is pretty much what the video signal looks like,
i had always thought that the absence of signal
was detected as a frame pulse ...
but i could be wrong, its been a while.

No, the frame sync pulse is a negative going pulse, just like the line sync pulse - but longer. You should be able to trigger any scope from the frame sync pulses (most have a 'TV sync' setting which helps), set the timebase to 5ms/cm or 2ms/cm.

 

[Chris P]

Unfortunately my scope does not have a TV Sync setting (Its very old >30years!) It always seems to trigger from the same place, which seems to be just a line sync pulse., If I set it to '-' edge triggering you can see the duration of the negative period, which measures at around 4.7 micro seconds (a line sync pulse) and if you set it to '+' edge triggering, the first thing you can see is the signal rising up from a negative, not the duration of the negative period. I have also played around with manual triggering, but as the frame sync is the same level as the line sync, i think I'd be lucky to find it. 313 line sync pulses to every 1 frame sync pulse.
Viewing it on a 'scope is not essential, I just cannot find and information on its actual duration, which would be useful!

Cheers again

Chris

 

[Nigel Goodwin]

This may be of help, it's a couple of scans from 'Television Servicing Volume 1, by G. N. Patchett (c) 1958, 3rd edition 1966' the price on the front is 'eight shillings and sixpence'.

Not suprisingly, it doesn't mention colour :lol: but it does describe the 625 line signal.

Damn! - it won't accept the attachment, if you PM me your email address I'll send it you (it's a ZIP about 2.8MB).

 

[bmcculla]

It might be worth your while making a trigger for the PAL signal. If you have a sync separator IC You can use your PIC to count the sync pulses and generate a trigger pulse on the start of a specified frame. You can connect the trigger line to your scope's external trigger. This would be a useful tool and would be an easy place to begin working with the PAL signal. I remember seeing simple circuits online for a PAL/NTSC trigger too if you want something thats already designed.

Brent