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Old 09-27-2018, 09:54 PM
wa2ise's Avatar
wa2ise wa2ise is offline
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NTSC vertical peaking circuit

I found a line delay chip in an old VCR I was parting out, ( can be a great resource) and decided to see if I could build a freestanding vertical peaking box. Vertical peaking is a video processing trick to make video images look sharper (in the up-down dimension). You take the input luma video, delay it with a 1H (horizontal) line delay element, then invert it, reduce its amplitude to around 10 to 20%, and then add it back to a buffered version of the input video. "buffered" because I didn't want delayed video passing thru the delay line again.

Above is a video screen grab of video without vertical peaking, and nearly the same video with vertical peaking. You can see the top of the H&I logo bug without and with the top edges of the logo with a brightened edge.

Yes, this peaking is asymmetrical, but shadows in the video image are almost always asymmetrical in the vertical dimension (the Sun is always above you).

RCA's CTC101 chassis had a vertical peaking circuit that was part of the comb filter. They low pass filtered the 1H delayed video to remove the chroma subcarrier to get the peaking video signal. Other outputs of the comb was a high passed chroma output, and luma where the chroma usually was cancelled out. I used to work on stuff like this in the 1980's at the RCA Snirnoff Labs in Princeton.

This is the circuit I threw together.

The LC8991 chip requires doubled chroma subcarrier frequency as its clock, so I used a TTL 14.318MHz oscillator module, and a 74LS74 flip flop to divide to down. At first I had these on my main analog board, but digital crud leaked into the video, so I moved them off the analog board.
Attached Images
File Type: jpg vert-peak.jpg (100.6 KB, 38 views)
File Type: jpg vertpeakoffon.jpg (77.8 KB, 22 views)

Last edited by wa2ise; 09-28-2018 at 11:13 PM.
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Old 09-27-2018, 11:33 PM
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old_tv_nut old_tv_nut is offline
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Neat little experiment - now you should keep this until you find another chip and can do the full symmetrical case.
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