Personal computers entered the audio scene about two decades ago but their role was mostly limited for professional environments. However, their overall growing use resulted in them being adopted home audio scene use as well. This move was partially made possible also by the increasing use of USB, a user friendly external PC interface with sufficient bandwidth for audio purposes. As usual, there are some initial misunderstandings associated with this issue, and we can easily find both too optimistic claims, as well as unfounded denials: such extreme views in fact still dominate discussions on related topics.

One of the most important points in this regard was and probably still is the question of jitter and this article will also try to explain the current state in this domain. Assuming the PC is, so long as it doesn’t perform some kind of signal processing, easily able to preserve data integrity (a.k.a. bit prefect information) we will concentrate on this topic. It still doesn't imply that the jitter is actually the complete answer for digital audio performance. In fact this article points out at least one reason why it is not.


S/PDIF prehistory

Indeed, things may get very confusing these days. Is it the old vs. new, or the mature against immature, or what is it about at all? Does the USB bring lower or higher jitter than classic home digital audio devices do? What is the current state of things in this domain, and can anyone predict the future?

Historically, things are apparently once again going the same old known route. At the time it was born, S/PDIF was, just as the whole digital audio concept, usually considered technically perfect. Since common high end audio wisdom has been always suggesting dividing the units in to functional blocks, people were satisfied with an interface that made possible external connection between the transport and D/A part of player. The interface was able to pass the bits, and preserve the data integrity, and to preserve it even if the line was hundreds of meters long, and even under quite critical conditions (related to the noise). It was simply one unquestionable part of the chain. Enquiring minds of our days will be probably surprised to look back and find no trace of concern about any associated problem that could be discussed further at all.

The nineties however brought much better understanding of the problems associated with S/PDIF. And exactly because these problems became visible, people started paying attention to, and correcting them. This consequently brought a progress and, as a result, today's S/PDIF hardware is incomparably better then it was 20 years ago. Today's Cirrus receivers' performance is 50ps of intrinsic jitter,¹ and they lock on preamble, thus being relatively free from data induced jitter. There are other manufacturers claiming similar performance. Funnily enough, today S/PDIF is exactly more then ever considered a big must-avoid bottleneck. Is it thus only because people now much better understand the seriousness of associated problems? Or is it a plain need of selling new technologies, and announcing existing ones bad and obsolete? And do we have to be afraid of audio history once again ignoring the actual performance and moving into new direction forced by “external reasons”? Indeed, once the main stream goes one way, accepting a particular attitude, it is hard to stop it and turn things back. Remember, when the digital audio knocked analogue records out, there was no way back!

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¹ - Jitter performance of Cirrus S/PDIF receivers is actually better than that claimed by the manufacturer itself.