What's with the SPDIF format?

[Thundergod]

Ive got some fx units, casete recorders, cd recorders and other stuff (music->computer interface related) and most of them come with SPDIF connections... some come with the SPDIF connector that looks like a RCA cable... some come with a optical SPDIF connector.

 

I've used the one that looks like a RCA cable connector. My GT-6 and later my gt-8 came with those... they could be conected directly to the computer interface, giving you 2 channels (L/R) with just one cable. It saved 2 input jacks that could be used for other stuff (really useful when working with bands that record demos playing everyone at the same time due to low budget).

 

I did some recordings with the gt-8 (with the gt-6 you had just 1 preamp, stereo was used for mod and delay fx) using 2 different preamps and different fx settings for each SPDIF channel, the results were just fantastic, like using 2 different amps or 2 really different mics (you can set different amp model, mic, cab, fx and just about everything for each channel).

 

 

But I've never used that optical connection... I see some fx consoles have it and some other pieces of equipment have it too, from home theatres to car stereos.

 

So, what's the deal with it? and is it obsolete technology now that lots of things come with USB2 and Firewire conections? And now that there is USB3 and will probably be present on most of next years products?

[Thundergod]

I'd also like to know if there's a way to convert one type of SPDIF signal to the other (optical out on a pedalboard to RCA type on a interface).

 

I ask this because that tonelab LE is really growing on me... and it comes with that SPDIF optical conection.

[DrJustice]

Yes, you can convert electrically between S/P-DIF (RCA), TOSLINK (optical) and AES/EBU (XLR) digital interfaces. They all convey the same protocol, only the signalling is different (although AES/EBU may carry more information in the 'user' bits). All you have to do is rig a few resistors and a logic gate or a couple of transistors to go from one to the other. Any extra information, such as carried by AES/EBU, will just be ignored by the other standards (it's just a few bits for management, not any extra audio data). S/P-DIF to AES/EBU can even be done with just a transformer (balanced 600ohm to single ended 75ohm). There's probably examples of suitable simple circuits on the net.

 

DJ

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[DrJustice]

Just to add that there are cheap converter boxes available, shouldn't cost much unless they come with samplerate converters and other bells & whistles. You'll find them for sure.

 

Also on the USB/Firewire vs S/P-DIF question: USB et al are highly complex multipoint protocols, while S/P-DIF is a simple serial interface, just like RS/232 serial ports. Thus, I imagine that S/P-DIF, TOSLINK and AES EBY are here to stay - they are often just direct pipes to and from AD/DA converters, DSPs and the like, whereas USB and Firewire has layers of software to make it work.

 

DJ

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[Thundergod]

Thanks DrJustice, your responses were very helpful!