Negative Video

Forum for discussion of narrow-bandwidth mechanical television

Moderators: Dave Moll, Andrew Davie, Steve Anderson

Postby Andrew Davie » Mon Jun 04, 2007 11:31 pm

I have decided not to continue testing/experiments with the "new" CD player, because I have an issue where I need to turn the 'volume' way up to get any sort of synch lock (I'd reported this before -- when I turn the contrast down I lose synch lock). But more to the point, even with the volume turned way down, doing a manual synch on the disk, and turning the brightness up -- I still get the way odd corruption on the image (the top of the black circle in the test pattern missing, for example).

So I'm going back to my "old" CD, and abandoning MP3.

I may still keep the "negative video" circuit, I think -- it could be useful at some later date.

However, there appears to be another problem. Originally I'd wired in the wire coming from the CD line out directly to the contrast potentiometer. To allow the "negative video" circuit to be easily plugged in, I wired in an RCA male/female plug sort of setup, so that I could plug the CD's line-out into the "negative video" circuit, or directly to the contrast potentiometer (via the RCA on each).

BUT, now that I have had a very close look with the "negative video" circuit NOT connected -- that is, connecting the line-out directly to the RCA of the contrast potentiometer, and turn the brightness down significantly, I can see some artefacts in the image. Artefacts that I'm pretty sure were NOT there originally.

I suspect a couple of things. One, the RCA connections might be a bit lossy, and the signal getting to the NBTV circuits is no longer "good". Or possibly there's even noise being introduced. More likely, I think, is that I've increased the length of the wire that the signal has to traverse. Or possibly, I have a very minor "leak" or short circuit in my wiring somewhere and I'm getting a bit of interference.

It will be an interesting one to track down. I did not spot this problem on a couple of other tracks that I looked at -- specifically, track 36 was at issue. I saw the problem mostly around the L of "32 LINE" when the brightness was quite low. It was a sort of cyclic 'fluttering' of the background image/colour. Now that I think about it, though, why should it be cyclic... that tends to suggest a problem in the original video/source.

Mmmh... well,more experimentation needed.
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Postby Steve Anderson » Tue Jun 05, 2007 12:34 pm

Andrew Davie wrote:It will be an interesting one to track down. I did not spot this problem on a couple of other tracks that I looked at -- specifically, track 36 was at issue. I saw the problem mostly around the L of "32 LINE" when the brightness was quite low. It was a sort of cyclic 'fluttering' of the background image/colour. Now that I think about it, though, why should it be cyclic... that tends to suggest a problem in the original video/source.

Mmmh... well,more experimentation needed.


Because of it's cyclic nature I suspect you're getting some hum into the signal, this will beat with the 12.5Hz frame rate as they are not locked together. The 50Hz/240V frequency is very well controlled in Oz, but will be just slightly different to the source of the frame rate when and where the recording was made.

Another source could be ripple from power supplies which will amount to almost the same as above. So check them with the scope. Also beware of magnetic fields from nearby transformers, have you moved any recently?

Check all your ground/earth/0V connections, make sure you're using screened cable where possible and give any connectors a brush-up. RCA/phono connectors are usually quite reliable and are used for FSTV on domestic gear, so it's doubtful they'll be 'lossy' at audio.

What are far worse are the 3.5mm stereo jacks used on portable CD players and PCs, they're simply crud.

Not being 'funny' here, check your soldering, look for dry joints.

The length of the cable in a domestic situation shouldn't be an issue.

Steve A.

I just looked up the word 'crud' in the Oxford English dictionary..."A substance which is considered unpleasent or disgusting, typically because of its dirtiness."......Yea....that's about right!
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Postby Hampus1982 » Sat Aug 15, 2009 1:22 pm

Talking about inverted signals...

Just bought a new pc since windows 98 started to really show signs of getting old... no new program to use, no updates, no new drivers and so on... but instead I had to buy a new scanner, and the printer only works in a very basic mode without all but the most necessary settings. And now I've discovered that the soundcard invert the sound... I miss my old pc!

Maybe I shold do my faithful old servant a dedicated NBTV machine? I feel a bit bad about putting it away...
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