On Fri, 5 Dec 2008 21:28:22 -0800 (PST), put
finger to keyboard and composed:

On Dec 5, 5:25*am, (Samuel M. Goldwasser) wrote:
writes:

Are you _certain_ that the modulator requires sync negative? I've
never seen one but it's conceivable to build a modulator that
requires
sync positive. It's not like audio where the absolute phase
doesn't
matter. (OK some audio nuts swear they can tell the difference
but
most of us can't).

Composite video has negative sync.

Are you sure the channel tuning is correct?


Yeah I know composite video is negative sync - to the outside world -
but it _is_ conceivable that a manufacturer for some reason uses
inveted video internally. I know I've designed some gear that has
inverted video in some stages but of course is back to sync negative
when interfacing to other gear.

G²

But would inverted video produce the OP's symptom?

___ ___
| | | | ____ Black level A
__| |__ _______| |_ ____ Black level B
| |
| _| ---- Black level C
| _|
|_|

Assuming the TV's sync separator expects negative sync, then where
would the black level be?

AFAICS, if the black level is at B or C, then the TV wouldn't see the
sync pulses. If at A, then the TV wouldn't see any video information.

- Franc Zabkar
--
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