My Sylvania TV's comb filter lives on

[wa2ise]

This was the comb filter board that was in the Sylvania set I had to part out a couple weeks ago. Now it takes composite NTSC video and makes S-video output. Modified it by adding a chroma buffer circuit to drive a 75 ohm load on an S-video jack. And to mop up residual chroma and hanging dots, added a 3.58 notch filter to the luma output. I think the Sylvania set had a notch filter in its chroma demod and color matrix board. Saw no hanging dots on this set. This new added notch here in this box is fairly narrow, so it doesn't trash luma resolution too much (just like back in the early 80s, this was a tradeoff of avoiding annoying artifacts vs desired picture sharpness). The main benefit this newly built box does is to much reduce cross luma into the chroma. This gets rid of the false rainbow crawling patterns you would see on the ref's shirt during football games.

The set it used to live in:

[andy]

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[richms]

What is the composite source that you have that needs a decent comb filter now tho? The only one in the past was broadcast television, digital gives you component/svideo out, so other than security cameras, and old nintendos etc I cant think of a use I would have for one of these.

[wa2ise]

Originally Posted by richms

What is the composite source that you have that needs a decent comb filter now tho?

The digital cable box I have from Comcast only has composite video output. Most likely inside that box, inside a chip, exists YUV, but it's not accessible.

And many CECBs only have composite NTSC (though the Channel Master 7000 has S video out).

Oh, and I found that the notch filter I added to the luma introduced too much ringing, making ripples or echos of sharp edges to the right. So I removed it, and did some tweaking of the null circuits (the one that subtracts chroma from composite to make luma).

[richms]

Ah ok, Didnt realize that there was gear being sold that crippled. Some low cost sat receivers over here have only composite and stereo audio on the back of them, but the type of people that get them typically only have that on their TV so its no loss.

[andy]

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[wa2ise]

Originally Posted by andy

I guess a better question would be: what has an S-video input that doesn't have a built in comb filter?

Of course. But another thought would be to use the separate comb filter to preprocess composite video to get luma and chroma, then low pass the luma to around 2 1/2 MHz, then merge this new luma with the chroma to feed to notch filter equipped TV sets. The goal is to prevent the TV set from having cross luma contaminating the demodulated chroma (the false color rainbow stripes on the football ref's shirt). And as these TV sets roll off the higher luma frequencies anyway, you won't miss them not being there in the new composite video anyway. One does need to exercise care that the luma low pass filter has even group delay (say you have a white vertical line in a black background, you want the low passed version's frequency content to be evenly stacked on top of each other, easy to do in digital filters, harder in analog) to avoid ringing artifacts from messing up the picture. Asymmetrical left and right responses look terrible on a TV picture.