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