Franc Zabkar wrote:

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.


If you over modulate the video, you compress, or clip the sync, and
destroy the DC restoration in NTSC video. The distorted signal affects
the AGC system, compounding the problems. That modulator may be
expecting a terminator, or pot t the input to set level, and provide
termination. It was no fun to find that some idiot had swiped the
terminators from the modulators in a CATV headend.


--
http://improve-usenet.org/index.html

aioe.org, Goggle Groups, and Web TV users must request to be white
listed, or I will not see your messages.

If you have broadband, your ISP may have a NNTP news server included in
your account: http://www.usenettools.net/ISP.htm


There are two kinds of people on this earth:
The crazy, and the insane.
The first sign of insanity is denying that you're crazy.