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Pro and Consumer S/PDIF Standards Explained.

Q: “I’m hoping for an explanation on something. I have a Prism A/D Converter that I’m trying to run in via S/PDIF to a Sony consumer CD recorder for a remote recording project. The Sony unit will not recognize the S/PDIF signal from the Prism, and I’ve recently been told it’s an incompatibility between the two S/PDIF formats (pro and consumer???). I thought S/PDIF was a standard so this is confusing to me. Can you shed some light on it?”

A: Of course! You know that old saying…”Standards are great. That’s why we have so many.” Why have one S/PDIF standard when we can have two, right? Well, there are two data formats, and it sounds like that may be the problem.

But before we dig in to that we have to clear up some language. The term S/PDIF is still very widely used in consumer and pro audio circles, and to many people it is represented by a coaxial RCA type connector. What many people don’t yet understand is that the S/PDIF standard and the AES/EBU standard for digital transfer are now parts of an overriding standard known as IEC-958, specifically they are known as IEC958 Part 3 (S/PDIF) and IEC958 Part 4 (AES/EBU). These are also known as “consumer” and “pro” standards respectively.

So, to be clear, there really is only one S/PDIF standard. It is technically obsolete and is now embodied in IEC958 Part 3. The AES/EBU standard (which is more properly becoming referred to as AES3) is described in IEC958 Part 4. These standards define slightly different data structures for digital audio, but there are also parts that define the hardware interconnect standards. For example, the AES3 “standard” calls for XLR balanced wiring at 110 ohms, whereas S/PDIF is an unbalanced 75 ohm interconnect. There is also a subset of AES-3 called AES-3id, which specifically allows for AES style data on an unbalanced 75 ohm coaxial connector, though it’s usually a BNC connector rather than an RCA style connector.

Further complicating matters is the fact that the plain old IEC958 “standard” encompasses all of this. Therefore it is legally possible for AES3 (pro) data OR S/PDIF (consumer) type data to appear at a connector that looks like a S/PDIF connector. In fact, many modern digital devices have such an RCA connector on the back cryptically labeled “IEC-958.”

Anyone who’s been messing with digital audio for very long has learned that in practice it is often possible to do transfers between devices with AES/EBU connections and devices with S/PDIF connections. The data is nearly (not exactly) identical, and assuming the cabling and impedance issues are reasonably well dealt with – which is not always very easy, and older gear tends to be much more temperamental in this regard – it will usually work.

Sometimes it doesn’t? Why?

If both devices happen to have the 75 ohm coaxial connection (whether RCA or BNC) then all of the electrical interconnect issues are a non-issue. Yet it sometimes still will not work. The only remaining problem is whether the data structure itself is compatible. This is no doubt the problem being experienced by the inSync reader with the question. He has two units with what appear to be S/PDIF connectors, when at least one of them is really a more generic IEC958 connector. Most likely the Prism is outputting an IEC958 Part 4 signal (basically an AES3 type signal (really AES3id)- just transmitted over a 75 ohm connection) and in this case and the Sony consumer CD recorder is expecting an IEC958 Part 3 signal. The Sony is reacting to the “incorrect” signal by not recognizing it.

That’s all we can cover today. Tomorrow, in part 2 of this Tech Tip, we will dig further into the differences between the “pro” and “consumer” data formats and begin to discuss some possible solutions to the problem.

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