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S-video to composite video converter

Some digital TV set-top boxes provide only S-video and component video outputs, which can pose a problem if your TV set only has a composite video input - or you've used up the S-video and component video inputs. The same can happen with video tuner cards for PCs. Here's an easy-to-build adapter to get you out of trouble.

By JIM ROWE

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Fig.1: adding a low-value capacitor in series with the chrominance signal at the S-video input can reduce cross-colour interference but also softens the picture.

You can buy cheap S-video to composite video adapters in bargain stores but the unit described here will do a much better job.

Although those really cheap bargain-store adapters do work, if you examine the pictures critically, you’ll find that their quality leaves quite a bit to be desired. In particular, you’ll find that wherever the image has large areas of fine detail – like a shirt with a fine striped or check pattern, or an exterior panning shot of a multi-storey building – then you’ll see a very obvious coloured Moire interference pattern, usually in shades of yellow and purple.

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Fig.2 (right): the unit described here uses an LC circuit to notch out a narrow band of frequencies centred on the 4.433MHz colour subcarrier frequency. This reduces cross-colour interference while leaving a sharp picture.

This effect is called "cross-colour interference" and it’s caused by heterodyne beats between the higher frequencies in the luminance (Y) signal and the chrominance (C) subcarrier in the receiver’s decoder. In effect, the higher luminance frequencies tend to behave as if they were part of the chrominance signal and as a result, produce fake colour patterns.

This happens when the two signals are simply mixed together in the video adaptor – which is what commonly happens in the bargain store units. This interference pattern can’t happen when the Y and C signals are kept separate, which is why S-video produces much better image quality.

Reducing the interference

Some of the better low-cost adaptors try to reduce this cross-colour interference by adding a small capacitor in series with the chrominance input signal, as shown in Fig.1. The capacitor’s value is chosen so that it passes most of the chrominance (C) information (it’s in a band about 2.5MHz wide, centred on 4.43361875MHz) while at the same time attenuating the higher frequency luminance signals – ie, by shunting the luminance output into the chrominance output of the S-video signal source.

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