Beware cheap composite to Y/C adapters!

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Martechnology
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Joined: Jul 22 2005

I came across an interesting circuit in Wikipedia for a Y/C to composite (and presumable vice-versa) adapter. The circuit is shown on:

http://upload.wikimedia.org/wikipedia/commons/2/24/S-video-composite-adapter.svg

The use of a 470pF capacitor to link the chrominance (C) signal to the luminance (Y) channel should be sufficient to adequately transfer the C signal (centred on about 4.43mHz) to the Y channel without either channel unduly loading each other in their respective bands of interest. 470pF looks like about 76 ohms at 4.43mHz.

This looks like a good idea, or so I thought, so I checked on eBay for a suitable adapter to experiment with. I bought one for less than £2 post free from China. The one I bought was phono female on the comp side, and 4 pin mini-DIN male on the Y/C side.I have just tried it, and the results are unusable.

I took a camera which provided both comp and Y/C outputs, and applied it's Y/C output directly to the Y/C input of channel A of a Panasonic WJ-AVE5 mixer, while the comp output was sent via the purchased adaptor into the Y/C input of channel B. The Y/C direct picture from the camera was fine, but the attempt to up-convert from comp to Y/C was a disaster. The picture brightness dropped like a stone.

A quick check with the meter showed why. The Y and C pins of the adapter were simply shorted together! There was either no capacitor, or if there was one in the adapter it was not doing anything. It meant that the comp output from the camera would have been looking into about 37.5 ohms rather than the 75 ohms expected. No wonder the picture was so dark.

Ever the glutton for punishment, I tried the adapter to down-convert from Y/C to comp, comparing via the same mixer against the direct comp output from the camera. The direct comp was fine, but the signal via the adapter looked as if the camera was completely out of focus.

All in all I would rate this adapter as useless regardless of which way round it was used, and I would advise any readers contemplating the purchase of a cheap adapter to research very carefully what they are buying, or cry all the way to the bank.

It would be instructive to try an adapter which has the correct capacitive coupling, but unfortunately I don't have one. Have any other readers tried such an adapter? If so, please let us hear about your results.