In the words of Joe Jackson and Anthrax, “Time, got the time, tick-tick-tickin’ in my head…”
In music, it’s all about time and synchronization. Think of a metronome or a click track — or the drummer — whatever it is that generates the pulse that keeps the music locked together, keeps the groove, and maintains the tempo. We can think of that as “macro-level” clocking.
But there’s a whole layer of clocking that operates on the sub-micro level if you’re working with digital audio. In fact, the whole concept of digital audio is based around a clock that runs in the background, keeping everything lined up on the sub-micro level and tying together different pieces of digital audio hardware. With a good clock, everything happily ticks along in sync, and the sound is pure, open, detailed, and accurate. With a poor clock, the sound can get cloudy and hazy, you lose detail and stereo imaging; things sound smeared. You might even hear pops, clicks, and other noises. In general, it just doesn’t sound as good as it could.
Tick, Tock, Tick, Tock
To understand why clocking is important, let’s take a step back: With digital audio, we’re taking a measurement of the amplitude (level) of an analog waveform at a particular instant — a snapshot, if you will — and representing it with ones and zeros. These snapshots are known as samples.
By taking a stream of these snapshots over time and storing them, we can record or capture a waveform. Later, we can run the samples back in sequence to more or less accurately recreate the analog waveform during playback.
The clock — also known as the word clock — does two things: First, it tells the system when it’s time to take the next sample. Second, it sets the rate at which samples are taken — the sample rate. This rate is important because it determines the frequency range that’s available for representing the analog waveform.
If You’re Not on Time, You’re Fired
All of this happens at an extremely high speed — the sample rate for CD-quality audio is 44,100 samples per second. High-resolution audio can reach 96,000; 192,000; 384,000; or even millions of samples per second with one-bit systems. You need a really stable clock to maintain accuracy and spacing between samples at those speeds! The amount of deviation in the stability of a clock is called jitter. The higher the jitter, the less accurate the digital conversion will be.
Why Do I Care?
So, given these clock rates, can you even hear jitter? I mean, samples are just flying by! As it turns out, you can. Jitter results in phase distortion, which typically shows up in the high frequencies, where detail and sparkle live. So, you’ll lose attack on transients, and that loss of detail can affect imaging in stereo or multichannel setups. Think of it as sort of “smearing” the audio.
Should I Worry?
If you’re after the best audio quality (and who isn’t?), then it’s worth considering the quality of the clock you’re using. With standalone audio interfaces, you’re relying on the internal clock. Many of these are pretty good, but the quality can vary. With a higher-end audio interface — something from Burl Audioor Apogee, or one of RME‘s new interfaces that use SteadyClock FS technology, for example — the internal, built-in clock can be reference quality with extremely low jitter.
To up your game, you can also consider adding an external, standalone clock to your system, such as those from Mutec, TASCAM, and Drawmer. An external clock connects to your audio interface (or other digital gear) using word clock connections, typically using BNC cables — the external clock becomes the “master” and sends the clock signal via a word clock output, while the audio interface (for example) becomes the “slave” and receives clock using a word clock input.
One Master to Rule Them All
Which brings up another point — there can only be one master clock in a digital system. If multiple devices are trying to be the word clock master or each is running off of its own internal clock, then the digital data streams won’t line up, and you’ll hear clicks, pops, and general badness in the audio. So, if you’re using two or more pieces of digital gear, and they’re connected using AES, S/PDIF, or ADAT digital formats, ensure that one piece (the one with the best clock, or an external, standalone clock) is the master and everything else is set to slave off of that.
It’s Time to Relax
Digital clocking is one of those things that you just don’t want to worry about in the heat of battle — if you’re wondering if the clock you’re currently using is good enough, it’s probably time to look at something better and remove that concern. Base your system around a great clock — whether an internal one or an external hardware clock — and set up the system properly to all lock together.
With a great clock at the core of your system, you can relax and be confident that one more variable has been removed in the quest for great-sounding audio.
Hebra Definition 