Most home studio owners now rely heavily on digital audio recording and processing, but you could be sacrificing the quality of your recordings if your system isn't clocked correctly.
It is probably fair to say that the vast majority of audio is recorded, manipulated, post-produced and replayed using digital audio equipment of one form or another. And, whereas a decade and a half ago such equipment was the costly preserve of professional studios and mastering rooms, today digital mixers and signal processors are available at almost bargain-basement prices. In the analogue world, you pretty much got what you paid for: the highest-quality products were extremely expensive — the old law of 'diminishing returns' applied in a very audible way — and cheap products usually sounded, well, not too good usually!
The very nature of digital audio has changed all that, though. Once an analogue signal has been digitised, there is no fundamental reason why a £1000 digital mixer should sound any different to a £250000 digital mixer, or a top-flight Sony 3348 multitrack should sound different to an Alesis hard disk recorder. Sure, in all these cases there will be very significant operational differences, and there may be reliability and maintenance issues, but basically the audio quality is locked into a digital signal, and you can't accidentally mess that up... or can you?
The most important part of the paragraph above is the phrase 'once an analogue signal has been digitised'. It is the A-D conversion process which defines the quality of a digital signal. Mess that conversion up and there is absolutely nothing you can do to recover the original quality. So, that means choosing an appropriate sample rate and bit resolution, and setting the analogue signal level so that true signal peaks don't exceed the maximum level of the converter.
It is reasonable to assume that the more expensive converters will do a better job of this critical A-D process than budget converters, and all other things being equal this is generally true — although the differential is small and getting smaller all the time. High-end converters will certainly have better analogue circuitry, though, and the sampling and quantising sections of the converter are likely to be more accurate and consistent. Having said that, the almost universal use of oversampling and delta-sigma topologies has reduced the performance differential in that regard enormously in recent years.
Without doubt, the major difference between high-end and budget converters is the quality, stability, and consistency of the internal clock circuitry — the part that determines when a sample is taken. If this reference clock is not particularly stable then the interval between samples — which should be absolutely precise — will vary. This problem is known as jitter, and it affects many different aspects of digital audio systems, but is of particularly critical importance in A-D and, arguably to a less critical extent, D-A conversion.
If the clock is jittery in an A-D converter, for example, then the analogue audio waveform is likely to be measured at the wrong moments in time — either slightly too early or too late, relative to the correct sampling instants. It may not seem immediately obvious, but measuring the waveform at the wrong time produces exactly the same kind of sample amplitude error as measuring inaccurately at the right time. In both cases, the quantised sample will be described with an incorrect amplitude value and, since the size of the error will vary more or less randomly, this will sound like added noise — the bigger the error, the greater the noise. Moreover, once these amplitude errors have been introduced, there is nothing that can be done to remove them, so having a really accurate and totally stable clock is absolutely essential when converting between the analogue and digital domains.
It is also important to have a stable clock when converting from digital back to analogue as well, since, although the size of the sample is provided by the digital data, if the regenerated samples are output at the wrong moments, the resulting analogue waveform will differ from the original. However, as I hinted at above, poor performance here is arguably less critical, since an inaccurate converter (or reference word-clock source) can always be replaced with more accurate equipment, if necessary, and the full audio quality represented by the digital stream can subsequently be retrieved.
At this stage you may be thinking that you can't hear any unwanted noise from your converters, so what's the problem? In fact, if you're using 24-bit converters, the noise floor of the recorded analogue audio signal is almost certainly higher than any jitter noise created by the level of converter clock instability even in a typical budget converter. So even if there were jitter noise present, I doubt you'd hear it — but that doesn't mean there isn't still a problem! The random sample amplitude errors caused by jitter also cause more audible problems, although they are often not immediately obvious.
Rather than thinking about a single analogue signal being converted to the digital domain, consider a stereo signal instead. The apparent spatial position of a signal source in a stereo sound stage is determined by the relative amplitude and timing of that signal between the two channels. As we have seen, an unstable A-D clock causes samples to be measured at the wrong time and therefore with the wrong amplitude. These amplitude inaccuracies are very small in relation to the foreground audio (although still significant), but are large compared with the very small ambient signals — things like sound reflections which help to define an acoustic space.
So, the effect of jitter errors is to impose a slight vagueness or loss of focus in the stereo image, as well as making the recorded acoustic environment appear less real. This ease with which this effect can be heard will depend very heavily on the ability of the monitor loudspeakers to recreate an accurate, three-dimensional sound stage in the first place — which is not something to be taken for granted with budget monitors. Where it is possible, switching between the A-D analogue input signal and the output of the complete A-D-A chain will reveal immediately the loss of spatial resolution caused by jitter (monitors permitting). However, I should say that this loss of spatial resolution is usually fairly subtle, and is easily overlooked amongst the other more obvious problems encountered when recording musicians! Subconsciously, though, we are usually aware that there is something wrong, and I suspect that many of those who complain about the 'unnatural' and 'lifeless' sound of digital systems are often referring, unknowingly, to the effects of jitter.
I should state here that the problem of jitter is certainly not restricted to budget converters. For, while high-end professional A-D converters can usually afford to employ superbly stable and accurate internal clock circuits, they don't always achieve the necessary precision. A very well-known professional DAW used in recording studios around the world had a notorious jitter problem with its own converters, for example, which is why 'golden-eared' operators preferred to use third-party interfaces instead.
So, I hope it is clear that a stable, accurate, jitter-free clock is a prerequisite for proper A-D conversion. The next logical question is where do we get such a clock? In the case of very high-end converters — such as the likes of Prism, DCS, and so on — it is a pretty safe bet that their internal clocks are about as good as they come. All you have to do is select the required sample rate and bit depth, feed in the analogue audio, and pass the resulting digital output to the recorder, mixer or whatever. Job done.
But what if you have several converters all running at the same time? Whether feeding a digital multitrack or console, it is essential that all the digital outputs are synchronised to each other — which means referencing to the same word clock. This is the reason why quality A-D converters almost always have an external word-clock input facility. Typically, you would allocate one converter as the 'word-clock master' (switched to operate at the required sample rate), and connect its word-clock output to the word-clock input of the next unit, which would be switched to reference the external clock.
If there are more than two converters to link together, you have a number of choices, shown in Figure 2. The first is to 'daisy-chain' the units together, and the second is to connect them using BNC T-pieces. Which approach offers the best performance will depend on the design of the equipment, and you should refer to the handbook for the particular equipment to see what the manufacturer recommends.
The daisy-chain system connects the master word-clock output of the first A-D to the external clock input of the next converter, and then from the clock output of that machine to the external clock input of the next, and so on. The potential advantage of this method is that the clock signal passed between each pair of converters is regenerated in each unit. One disadvantage is the possibility of a propagation delay as the clock passes down the line, so the last A-D may not be correctly synchronised to the first.
Another issue to be aware of is how the jitter on the clock signal changes as it is passed from input to output. In a well-designed system, the regeneration circuitry will reduce the incoming jitter, so the output clock signal might be better than the source. However, if the PLLs (phase-locked loops) which are used to detect and regenerate the clock are poorly designed, they could just as easily make the jitter worse!
The alternative arrangement is to use BNC T-pieces to route the word-clock signal output from the master converter to each unit in turn. Word clock is normally output on a video-style 75Ω BNC connector, and BNC T-pieces are commonly available which allow the input and output cable to be connected to the arms of the 'T', while the base of the 'T' splits the signal out to the clock input of a converter.
The advantage of this connection strategy is that the master reference word-clock signal is routed more or less directly to each subsequent converter, so there's no possibility of propagation delays or increased jitter. The disadvantage is that, since word clocks require a constant-impedance interface, all but the converter at the end of the cable have to provide a high-impedance input, rather than the normal 75Ω loading. If the converters do not offer this facility, this form of interfacing cannot be used, as the clock signal will suffer reflections and loss of amplitude, resulting in an unreliable system.
It is worthwhile noting that the performance of even the best jitter-free converter can be seriously degraded by clocking it with a poor-quality external word clock — and conversely, the quality of a poor converter can often be improved by providing a better reference clock source. The latter is of particular relevance to home studios using budget equipment, of course. If you have a digital unit which is known to have a particularly accurate internal clock, you may be able to use this as the 'reference' for the rest of your equipment. Some people set their digital mixer up as the clock master, for example, and synchronise everything else to that.
Remember that you can synchronise clocks in digital equipment with more than just the classic BNC cable. Both AES-EBU and S/PDIF (coax and optical) signals carry clock information within the audio data stream, for example, and so recorders can often be synchronised using their input signals, rather than having to use dedicated word-clock cables. Professional digital studios generally employ a master clock generator to provide word-clock signals to each and every piece of equipment in the studio. There are several reasons for taking this approach. One is that everything is guaranteed to operate at the correct rate, and with perfect clock synchronisation. Another is that the entire studio can easily be synchronised to an external word-clock source — a facility which is important in applications involving video, for example. But the most important reason (as far as a budget studio is concerned) is that, by purchasing the highest-quality master clock generator, the performance of less accurate equipment can be enhanced to a high and uniform standard.
Master clock generators are made by companies such as Aardvark, Audio Design, C-Lab, Drawmer, Lucid, Probel, Rosendahl and others. Most are priced around £1000 and the facilities they provide can vary considerably. For example, some can be referenced to external video signals directly, while others only accept an external word-clock input, and some cannot be referenced to an external clock at all. Some provide a large number of both word-clock and AES-EBU or S/PDIF outputs to synchronise studio equipment, whereas others have very few outputs and require the use of digital distribution amplifiers. Some offer simultaneous standard and overclocked word-clock outputs, enabling use with Pro Tools Super Clock, for example. The bottom line here is that, before buying a master clock generator, you have to decide what facilities are required — both now and in the future — and how the equipment will be interfaced.
Another issue of very practical relevance, where budget digital equipment is being used, is that not all digital products can be synchronised to a digital word clock — although all digital recorders will happily synchronise to their digital input while recording. If you have equipment which cannot be locked to an external word clock, the offending product will have to be connected through a sample rate converter, which itself can be synchronised to the external master clock. The Drawmer M-Clock master clock generator — which is aimed squarely at the typical digital home studio — is unusual in that it incorporates four sample rate converters specifically for this purpose.