Wednesday, July 04, 2007

The PAL vertical interval

Our very own Kevin King asked me about the vertical interval in a PAL signal - the above diagram is excellent and has all you need to know. It's taken from a Tektronix training manual from the late eighties by Margaret Craig called Television Measurements - PAL Systems.
Essentially you should bear in mind the following;

  • Active video starts at line 23 - actually line 23 is a half line so the first 26 u-sec is blank - people who don't know about video often notice this!

  • The other half line is the first half of line 623 (end of field 2) - the reason for the two half-lines is to give the line-scan circuit the best chance to make it back to the same point when it starts the new field.

  • There is no half-line at the start of field-2 or the end of field-1

  • Field 1 and Field 2 both end with five equalising pulses and then start the next field with five broad pulses, to be followed by more eq pulses. These are the only lines not to have a colour burst - that's what Bruch Blanking does on older SPGs.
The reason for a lot of this is down to the stability of phase-locked loops constructed from valves rather than transistors - if you have (potentially) unstable oscillators for every waveform (line-drive, field-drive, sub-carrier etc.) then you have to take every effort to make sure nothings changes phase too quickly - hence the half lines, the broad and equalising pulses etc.

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