MURPHY’S LAW SEEMS to apply to digital audio bitstream signals and inputs just as much as it does to any other aspect of electronics.

For example, let’s say that you have only one digital audio input left on your home-theatre amplifier and it’s an optical one. Now guess which kind of digital audio output you’ll find on your new DVD recorder or DTB set-top box when you bring it home?

That’s right, Murphy’s Law will ensure that it will be an coaxial output. It won’t be an optical one, because that would match the remaining input on the amplifier and make things easy for you. On the other hand, if your amplifier has only a coaxial digital input remaining, you can bet your last dollar that your new set-top box will have an optical digital output instead!

Either way, these are both situations where the easiest solution is to use a converter – one that can convert coaxial digital audio signals into optical, or vice-versa. And that’s exactly what this little gadget does. It uses only a handful of parts, yet can easily convert coaxial digital bitstream signals into optical form and/or the other way around.

It’s also easy to build and will set you back significantly less than a pair of commercial converters.

Digital audio signals

Basically, the digital audio signals found in domestic equipment are all in the form of SPDIF (Sony/Philips Digital Interface) bitstreams – either as 400mV electrical signals sent along 75-ohm coaxial cables or as optical signals (pulses of 660nm red light) sent along fibre-optic cables. The optical signal form is often called "Toslink".

Although domestic digital bitstream audio is split almost equally between the coaxial and optical forms, they’re both virtually identical in terms of the SPDIF/BMC encoding and serialisation used (see panel). So it’s relatively easy to convert between the two, in either direction. In fact, once you get hold of suitable Toslink optical transmitter and receiver modules, the rest is no problem at all.