Well, the signal is AC. Anything below 0v is a sync pulse for NBTV anything above 0v is video. My only issue was that 0 volts were not sitting on the probe marker.
Sorry if I am confusing. I am not as skilled with electronics as many of you folks are I am sure.
Pete
Possibly your video source does not present blanking or black level as zero volts.
(NTSC has "black" as zero volts, PAL has blanking level, hence the use of "or" above)
In Broadcast analog TV, loss of the DC reference level was a common problem.(AC coupling in amplifiers, etc, tries to balance the signal around zero volts.)
To get around this, "DC restoration" of various types was used, principally, the "Keyed Clamp".
This took the line sync pulse, used it to create a narrower delayed pulse which was centred on the "back porch"(the bit of black or blanking level just after the sync pulse).
* This pulse would "turn on" a clamp circuit which would conduct, pulling the level desired to be clamped to zero volts.
*Usually-----sometimes
sync tip level was clamped to zero or even other voltages for special cases.
Classic circuits were combination of resistors capacitors & diodes but later designs used FETs or even analog switch ICs.
The capacitor was vital in all such circuits, as it would maintain the clamped voltage level, until the next clamp pulse came along around 64 microseconds later.
Such circuits were ubiquitous in analog TV.
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