I have a very high quality D-A converter that boasts particularly low clock-jitter figures, so I thought it would be an ideal candidate to serve as a master clock — yet it doesn’t have a word-clock output socket! Am I missing something, or is there a good reason for that?
Hugh Robjohns replies: The stability and accuracy of the digital clock is most critical in the A-D and D-A conversion stages where signals are transformed between the digital and analogue domains. When recording, the most critical stage as far as clocking is concerned is undoubtedly the A-D, since this determines the digitisation quality, but in a mixing or mastering scenario it could be argued that the D-A converter demands the most stable and jitter-free clock. Of course, D-A manufacturers go to great lengths to try to achieve that, and so for that kind of application it might appear to make sense to have the system’s master clock built into the D-A converter.
However, in the vast majority of cases, digital-to-analogue converters don’t contain a master clock circuit of their own. Instead, they almost always work as clock slaves, locking on to the sample rate of the incoming audio data stream. Familiar digital audio connection formats like AES3, S/PDIF, ADAT, and USB all incorporate an embedded clock signal which is extracted by the clock-recovery circuitry in the D-A, filtered to remove both intrinsic and cable-induced jitter, and then used to drive the D-A conversion process — and to be fair, that usually works pretty well!
Now, if this recovered clock was made available as a ‘master clock’ output and used to synchronise the rest of the digital system (computer interfaces, mixers, digital outboard, etc.), they would all inherently be slaved to whatever was connected to the D-A’s input. So in reality, there wouldn’t be a clock master at all — everything would be a slave and there would be a clock loop (the digital equivalent of an acoustic howl-round). The inevitable result would be that the sample rate of the whole system would chase its own tail, and that usually manifests as a gradual speeding up or slowing down to a crazy sample rate, or something in the system would realise the clock rate was daft and mute (which would be a rather more obvious, and therefore better, outcome). So, for that reason it’s usually not practical for D-A converters to serve as a system master clock!
However, there are always exceptions, and there is one type of professional D-A converter that does have its own internal master clock. A handful of manufacturers including Benchmark, Crane Song, Crookwood, and Drawmer offer D-A converters with a jitter-removing technology that relies on asynchronous sample rate conversion (ASRC) to isolate the D-A conversion clock from the input signal’s embedded clock (with the aim of minimising clock jitter).
In these designs, the input signal’s embedded clock is recovered in the usual way, but it is only used to clock the audio data into the ASRC at the source’s sample rate. The D-A converter itself is clocked from a very precise local clock which also controls the up-sampled data stream coming out of the ASRC. With this configuration there really is a master clock inside the D-A which is completely independent of the sample rate of the input signal... so couldn’t that be made available as a master clock output?
Well, in theory, yes it could, and as a completely independent master clock it would avoid the clock-loop problem described above. However, in most designs of this type, the local clock operates at a non-standard sample rate, chosen specifically to optimise the performance of the D-A converter chip. Typically, this local sample rate is higher than the maximum allowed input sample rate (211kHz seems quite a common choice). Clearly, there’d be no point in making that available directly as a master clock output, so to make it useful the manufacturer would have to add some complicated clock-rate conversion processing to provide all the required standard sample rate outputs. Unfortunately, that would potentially compromise the high quality of the local clock, which would defeat the original purpose of the design — but, more practically, it would also add significantly to the cost of the product and would probably still be unwanted by the majority of customers anyway!