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Soundcard Problems 13: Can I plug normal audio cables into a soundcard's S/PDIF digital audio sockets?

For anything relating to music-making on Windows computers, with lots of FAQs. Moderated by Martin Walker.

Soundcard Problems 13: Can I plug normal audio cables into a soundcard's S/PDIF digital audio sockets?

Postby Martin Walker » Tue Oct 05, 2004 2:03 pm

There are two types of S/PDIF (Sony Philips Digital InterFace) formats - coaxial and optical - and standard phono to phono audio cables can certainly be physically plugged into a coaxial S/PDIF socket.

However, this isn't recommended except in an emergency, since you can run into reliability problems, especially with longer cables over about half a metre. This is because the signal bandwidth extends to several megahertz, and can be seriously compromised when sent down cables that are only intended for audio up to perhaps several hundred kilohertz at the most.

If you're lucky the signal won't get through at all, making it immediately obvious that something's wrong. Far more frustrating is digital audio that mostly gets through, but occasionally causes a click or pop at the far end of the cable that normally proves very difficult to track down. I've alro run into problems where a digital audio signal wasn't recognised at all after being sent down an audio cable, but which 'reappeared' after being unplugged and plugged back in again - it can be this borderline. You may find a particular audio cable works with some S/PDIF gear, but not with others, or at one sample rate but not another. You may even get clock jitter problems in certain situations that compromise stereo imaging, due to reflections bouncing back from the far end of the cable. Overall, it really isn't worth running these risks - bits may be bits, but only if they all get through intact!

The 'consumer' S/PDIF standard specifies a cable with a 'characteristic impedance' of 75 ohms with a phono plug at each end (and the more 'professional' AES/EBU standard a 110 ohm cable with XLR plugs at both ends).

However, there's no need to buy exotic and expensive cables for digital reliability, as long as the characteristic impedance is 75 ohms. The easiest way to ensure this is to buy a cable intended for video use, such as those sold for connecting up VHS or DVD recorders - these are only a few pounds each.
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