Dark vertical stripes between color bars?

[Phil Nelson]

Here's something I hadn't been curious about until I viewed things on an oscilloscope: dark vertical stripes between bands in NTSC color bars.



In this scope pic, the lower trace shows the video output from my pattern generator. The top trace shows the messy signal at the input of my 1st video amplifier tube. Presumably the dark stripes are those "stalacites" dripping down between color bands, arising somewhere in the front end.



I hooked up a 1980s Panasonic and saw that it has stripes, too, although not so pronounced.



I'm not going to lose sleep over the stripes if they can occur in an OK-looking 1984 TV. Just curious about what causes them.

Phil Nelson

[old_tv_nut]

These are two different phenomena. The dark edges in the 1984 set are the result of video processing to improve sharpness, and are deliberate.

The effects in the earlier set indicate some sort of problem, like improper fine tuning or IF misalignment, or maybe AGC adjustment. Note that the dark edges are combined with greatly increased chroma level, which would indicate the chroma transients are also being enhanced. I think this indicates the set is tuned too high in frequency (or the IF is tilted) putting the video carrier at too low an amplitude.

[Phil Nelson]

The stripes do change intensity or move from here to there as you change the fine tuning. I don't think there's any setting where they're totally absent and the picture's otherwise acceptable, though.

Phil

[old_tv_nut]

Do you have a DVD with an Indian head and/or a video sweep? Would be interesting to see what they look like.

[Phil Nelson]

My pattern generator has the usual crosshatch, cross, dots, etc. Nothing like the Indian head, although years ago I pointed a camcorder at a printout and got a caveman version that way. Not sure what a video sweep is.

Phil

[old_tv_nut]

video sweep - sine waves with frequency increasing left to right:
http://www.horita.com/sweep.htm

[zenithfan1]

Hey! My pattern generator does that! Cool.

[Phil Nelson]

So, how do you imagine my old TV would display this pattern?

Phil

[John Marinello]

Test signal is called multi burst.

[bkharris]

Phil

Your Sencore VA-62 has this pattern.

[old_coot88]

Phil,
On several occasions, I've seen "ringyness" in the video caused by the last IF tube. You might try subbing it just as a shot in the dark. Have you checked the IF tubes for shorts/gas? Bill(oc)

[Phil Nelson]

Originally Posted by bkharris

Your Sencore VA-62

Oh no, you said the name that must not be spoken in this house

I got disgusted with my VA62A and put it away quite a while ago. Last I remembered, it made colorless color bars on new and old TVs. I hauled it out today and it hasn't reformed. Plus, it makes a wicked, slowly rolling horizontal fold on my tube TVs.





The same CTC-11 shows OK color bars with no fold when I hook up my humble Leader pattern generator.



VA62A makes the same rolling fold on the older TV.





It's a poor workman who blames the tool. Perhaps I'm doing something stupid with the VA62A, but I simply got tired of horsing around with it. Meanwhile, the Leader generator makes useful patterns without any fuss.

Phil

P.S. The final photo shows the pattern that VA62A calls "multiburst bar sweep."

[old_tv_nut]

Originally Posted by John Marinello

Test signal is called multi burst.

Strictly speaking, multiburst has groups (bursts) of vertical stripes, each burst having a particular constant frequency of stripes, while "sweep" changes frequency continuosly left to right - but they have similar use, testing the frequency response of the video.

[old_tv_nut]

looks like bad power supply ripple in that VA62A

[Phil Nelson]

Yah, the colorless bars were there before, but the ripple is new. Electrolytics dying like lemmings, I suppose. Working on new-fangled PC boards is just about my least favorite chore.

Phil