Friday, May 01, 2009

Simplest Circuit for converting s-video to Composite video coaxial TV video

If you have a laptop with an RGB out and s-video out, you would have thought it was impossible to get a TV out to connect it to a normal Large TV. It would be very cool to get the video on a TV screen instead of watching movies on your laptop's tiny 14 inch screen.

S-video to Composite RCA video cable on Amazon

How do I get my laptop's video on a normal TV (which has only a composite video in) which does not have any fancy S-video or RGB input ?

Very, very simple.

You could buy the cable - Buy S-Video to Composite Video Adapter 6-Inch cable

Or you could make it yourself.
This circuit I am showing here has only one Capacitor - but the circuit is absolutely useful.

Materials required : wires, capacitor, s-video pin and coaxial composite video pin (most of this can be salvaged from old wires and cables lying around the house)

1 Y ground
2 C ground
3 Y (luminance+sync)
4 C (crominance)

Or, you can buy this from an electronics shop for less than a dollar.

This simple adapter can be used to convert Y/C video (S-video) to a composite video. This adapter is useful in cases where your video output device has only S-video output but your signal source accepts only composite video input. This circuit works with both PAL and NTSC video standards.
+———- RCA/composite ground
+——— RCA/composite video
This circuit can be quite easily i inside a the S-video connector case if a physically small size 470 pF (ceramic) capacitor is used. Larger capacitor values will also work, but cause picture to become “softer”. The voltage rating of capacitor can be 10V or more.
This circuit works in practice quite well even though the circuit operation is not ideal. This means that impedances and signal levels not matched exactly right, but near enough to work acceptably. The picture quality you get from this circuit is is good, but not as good as with best possible composite video output circuitry.

Can I use different capacitor values ?

The circuit diagram lists the capacitor value 470 pF. Somewhat different capacitor values would also work. Practically all capacitor values form around 400 pF to around 10 nF should work somehow acceptably.

Can I buy ready made adapters like this ?

I don't have ready made circuits for sale. Ready made circuits are nowadays avaialble at video/hifi shops and form computer shops quite cheaply nowadays (3-10 Euros/Dollars). For example has this kind of devices for sale on-line.

Are there ways to improve S-video to composite video conversion ?

Some ICs that do S-video to composite conversion internally use luma trap to eliminate cross color artifacts. With a properly designed luma trap, the conversion works somewhat vetter than with my simple circuit. One IC that implements luma trap is AD725 from Analog Devices. The IC data sheet has information on luma trap design. When luma trap is used, the luma trap needs to match the video standard (PAL and NTSC) being used. For a filter that will work for both PAL and NTSC a means is required to switch the tuning of the filter between the two subcarrier frequencies.
My simple circuit described above works for all video standards without modifications. I have not designed any circuit that used this luma trap method myself and I have no plans to add this to my circuit.

What if my PC graphics card has a 7-pin S-video connector instead of 4-pin ?

The four pin S-video connector as shown above is the standard connector for carrying S-video. Those seven pin connectors seen on some PC graphics cards are non-standard connectors for carrying S-video. The use of the pins on those seven pin connectors is not standardized and can vary from manufacturer to manufacturer. (For some strange reasons some manufacturers in PC industry just keep constanly breaking the industry standars and create lots of confuzion to users when doing so).
Generally the four pins on those 7-pin connectors on the same places as the standard four pin S-video connector have practcly always the same fuctionality as those pins in S-video connector. The other three pins can have then some extra signals which are not part of S-video (usually some pins of those carry composite video and some control signals, but the use of those three extra pins vary quite much). So if you encounter 7 pin connector for S-video, then forget the three center pins... just use the four on the standard positions. The circuit shoudl work with those connectors as well.

I am using the adapter ans still do not get colors. What is wrong ?

This adapter is designed to convert S-video signals to composite video. And it has proven to work well on this. This circuit cannot solve other possible incompatibility issues that you might have. A typical problem that causes black and white video instead of color is incomatible video standards on the signal source and the receiver. If your video signal source (for example PC video card TV output) puts out NTSC video but your TV is designed to handle only PAL standard you get usually black and white image, and with some devices video signal does not sync at all. PAL signal to NTSC TV does not work either. One very common problem with computers that have TV putput is that the default output video standard is not right for your TV. If you get problems with computer TV output, configure your video output to right standard that your TV supports and try again. The exact details how o do this configuration varies between different computers and graphics cards. Consult your computer user's manual and other documentation for exact details.

Can the circuit used in other way ?

If you try to connect the circuit in opposite way so that you have a composite video signal which yoiu want to feed to S-video input then you might wonder would this circuit work also in that way. The answer is that the circuit sort of work also in this way. If you connect a comoisute video source to a S-video input of your TV using this circuit, you will get a full color picture on your TV screen. The downside of this is that the picture quality will be worse than if you were using the real composite vidoe input of your TV. The reason for this is that after the circuit the color information is still in the brighness signal, you you see some interference on the screen caused by color subcarrier which gets to the screen. This all means that you can use this circuit as last change in situations where you don't have a composite video input. The picture quality is degraded, but propably still usable for some not so demanding applications. A real well working solution would be much more complicated than this circuit (there are some commercial adapters based on two filters, band-pass and subcarrier frequency for C and color subcarrier band-stop on Y line). Please note that this kind of circuits are always video standard specific, meanign you need a different circuit for PAL and NTSC video standards. You can find one picture of the filter responsenses for NTSC system at (filters out colro signal around 3.58 MHz frequency from Y component). The idea is the same for PAL, but the frequency here must be 4.43 MHz. There are both passive and active devices available to do this conversion.
Here are some links to some products:
Elektro Electronics magazine has published an active Video to S-VHS Converter circuit at their September 2001 issue. Due copyright reasons I can't publish the circuit here. You can find some information on the circuit at the magazine web page and you can even buy a copy of that article. This circuit design is designed for PAL video. I have tested this circuit and it works if you want go go from composite video to S-video.

Credits to Tom Tomi Engdahl <> - thanks for the circuit man, saved me a bunch of money and besides, none of the electronics shops nearby even knew what s-video was !!!

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