Circuit needed to buffer/amplify Luma+Chroma signals to S-Video out levels? - Page 1
Forum section: electronics / beginners

author: Orihalcon
author_rank: Contributor
timestamp: December 18, 2024, 05:06:10 am
content: Hello all I am working on a project where I'd like to see if there's an improvement in adding S-Video output to a typically composite only consumer VCR. It has been fairly easy to locate valid chroma and luma signals on video boards near where they exit combined into composite, however, they are not at standard S-Video levels prior to being combined - say luma and chroma might be half of what their peak to peak values would need to be for standard S-Video and my guess is that they’ll drop even lower without some type of buffer or op amp once they are put under the usual 75 ohm load.My question is whether there’s a (hopefully) simple circuit that I can can try to boost these existing chroma and luma signals to levels that are usable for a standard S-Video out that will have a 75 ohm termination. Ideally it’d be some type of Op Amp that buffers the signal so that loading the output down doesn’t affect the existing circuitry or input signal. I would imagine it would contain a couple of variable resistors to fine tune to the desired output levels as well.I also understand that different op amps have different noise and bandwidth profiles and I do not know what would be ideal for these analog video formats, so I’d like specific part numbers suggested if possible and a crude drawing of how it might be wired up to try. It’d also be ideal if this can be accomplished with a single input voltage to the ICs as opposed to say BOTH plus 5V and minus 5V as some op amp applications require.Any help is greatly appreciated!

author: Tim S
author_rank: Contributor
timestamp: December 18, 2024, 09:48:35 am
content: Out of curiosity - yes indeed. Practically - not worth it.Unless it's a digital recorder, the video signal is so bad to start with all you're going to do is reveal more noise.

author: Rafiki
author_rank: Contributor
timestamp: December 18, 2024, 11:58:07 am
content: As Tim S said, not worth the effort. VHS is that bad that you probably wouldn't see any benefit. Except your TV is that extremely bad that it makes a very bad job in converting CVBS to RGB.If I would have to do that I would exhume some old S-VHS service manual. Philips VR813 comes to mind. I think there are some NJM... buffers you can look at. Mind, if you've only 0.5Vpp Y you usually want x4 to get 2Vpp. With the usual 75R output resistor placed directly a the output buffer stage you get 1Vpp at the receivers internal 75R then. Similar with C but I don't remember the level anymore. You've also care about the valid DC level at output. And most probably some clamping on sync would be needed.

author: jwet
author_rank: Frequent Contributor
timestamp: December 19, 2024, 03:43:36 am
content: I like to play these questions as they lie- it probably won't make much difference but I can help you try for your self. How much gain do you think you need and what BW. Standard Def TV has nominal BW of 6 MHz. What amplitude and impedance are the luma and chroma you have access to.A goto part for this app is the AD8041. Single (or dual) supply, well behaved, VFB, - can drive a back terminated 75 ohm load. Data sheet has good apps section. Use a non inverting gain stage with a termination on the + input to set the input Z to your source. Get gain with feedback, keep Rf < 3k or so and keep the lower R between 500 and 1000. Add 75 ohm back termination to output. Use HF bypassing techniques- tight.Choose a supply between single 3,5 or +-5v.

author: Orihalcon
author_rank: Contributor
timestamp: December 23, 2024, 04:09:50 pm
content: Thanks for the replies! I will have to look into these and give them a try. Should be an easy enough experiment to show whether there's a benefit or not. Since the levels start out lower prior to the usual composite output, using a different amplifier that may be lower noise than the original may also result in a benefit for a less-noisy output as well. Luma and chroma are stored on the tape separately, so avoiding the combination step to composite only to immediately (and imperfectly) separate them again on the display or capture device seems like there should be a theoretical advantage at least.I will admit that often it is difficult to tell the difference between S-Video and Composite captures from VHS, but this may be due to YouTube compression losing some of the details, though certain patterns will always have a sort of rainbow interference or sometimes "dot crawl", but a lot of that depends on the comb filter on the capture device and the content in question I think.

author: edavid
author_rank: Super Contributor
timestamp: December 23, 2024, 04:37:35 pm
content: You didn't mention what you are connecting your VCR to. If the receiving equipment has a comb filter, you're unlikely to see any difference.Also, why not just try an SVHS VCR? I think they all have S-Video outputs.

author: calzap
author_rank: Frequent Contributor
timestamp: December 23, 2024, 06:06:27 pm
content: I agree with edavid on obtaining a VCR with S-video output if it’s the end-result that you want. If you want the learning experience of making your own, that’s different. You might see more improvement with a TBC rather than trying to create S-vid output. I still have a Sony vcr, TV-One TBC, and Diamond digitizer. About every other year, someone has a VHS tape they want converted to digital.Mike

author: Orihalcon
author_rank: Contributor
timestamp: January 13, 2025, 01:05:18 pm
content: I will give that a try!The advantage of S-Video is to negate the need for a comb filter to re-separate Y and C which is always imperfect since there is no such thing as a perfect comb filter to my knowledge.As for why not just use a playback machine that already has S-Video - the idea would be to do this for machines that are older than S-Video became a thing or just rarely had S-Video output except on very late models, such as betamax. The classic flagship models of these machines never really had S-Video output, but luma and chroma is stored separately on the tape and processed in separate areas on the machine. So while almost certainly not recorded from an S-Video source, the information encoded on the tape still contains luma and chroma separately on "color-under" formats. The one format that this mod would not be potentially helpful for would be Laserdisc which actually contains a pre-mixed composite signal on the disc itself.In short, having composite output is a form of mild generation loss since you're mixing luma and chroma and then imperfectly un-mixing it immediately afterwards on the display/capture device.

author: Terry Bites
author_rank: Super Contributor
timestamp: January 18, 2025, 01:18:27 pm
content: A dual or tripple video amplifer with a gain of 2 is the way to go. That lets you employ the standard 75R termination scheme.eg THS7314 is happy with a 3-5V supply.