Poke around in the settings of your digital audio device, and you’ll find Word Clock options. Every digital audio device needs a Word Clock. Most have Word Clock generators built-in, some have to take their clock signal from another device. But what is it exactly?
Digital audio works by measuring and recording the frequency and amplitude values of analogue audio signals a certain amount of times per second at a set bit rate. The amount of times per second it measures the frequency data gives us the sample rate; 44.1kHz (44,100 times per second), 48kHz (48,000 time per second), 96kHz (96,000 times per second), and the bit depth (16, 24, 32) gives us the amount of detail it can measure and render amplitude. 16 bit gives us 65,536 possible values, for example, and 24 bit 16,777,216. Those bits are stored in each sample. Those two things together are our digital ‘Word’.
So when a digital audio device is converting from analogue to digital, or back again, it is doing so with a certain amount of information (bits) per second, a certain amount of times a second (sample rate). So it’s vitally important the device knows how long 1/96,000th of a second is when it’s running at 96Khz, and when each fraction of that second starts and stops. It needs to know what the time is at a granular level. So it looks at a Clock.
Word Clock is a continuous square wave pulse running at the sampling frequency. Word Clock signals are usually generated by crystal oscillators, which use quartz that resonates at a precise frequency. The digital audio device locks onto it and takes each maximum and minimum value of the wave as the ‘ticking’ of the Clock, marking where each slice of audio starts and ends. It’s vital that this Clock be as steady and free of variations, referred to as jitter, as possible. Irregularities in the Clock signal will result in less high frequency definition, poor stereo imaging, and ‘dullness’.
Word Clock becomes even more important when connecting two or more digital audio devices that are sending signals to each other. They all must synchronise to one ‘Master Clock’. Not setting up Word Clock settings correctly will either result in no signal passing, or signal errors including drop-outs, pops, clicks, or massive bursts of full-range white noise which can blow loudspeakers. Now that most live sound tools share signal digitally, you need to be across how to set-up and administer your Clock.
Simple digital set-ups like ‘audio desk and remote stage box’ usually take care of the Clock settings for you. Most digital stageboxes are built to only be capable of being a Clock slave, and take the Clock from the digital connection to their desk. Most desks have their Clock set to their internal generator by default. Changing the sample rate in your desk’s setting from 44.1kHz to 48kHz won’t cause any dramas in this scenario.
Some older devices, like rack mounted CD players and effects units that are connected to your system via AES/EBU or ADAT need to be Clocked too. Sometimes, this can run down the AES/EBU cable, but other times, a totally separate 75ohm coaxial cable with BNC connectors will need to be run from the master device to the unit.
Word Clock set-up gets a bit more involved when you’re running a network of devices (for example desk, multiple stageboxes, DSP amps) over Dante. Thank fully, Dante can generate and distribute its own clock, and even decides which device is best to be the Master. In these scenarios, it’s best to set each device to Clock off of its Dante port, and let the network handle the rest.
If you’re operating on a system that has invested in an external Clock like an Apogee or Rosendahl, then you’re going to have to set-up even your Dante devices to Clock from that. High-end Clocks like these are generally only used when multiple sample rates are being employed, or there is a heavy video or broadcast component to account for.
Some engineers swear by external clocking their PA systems, claiming clearer high end response, better summing, and a range of other improvements; it’s a great topic to bring up if you want to start an argument! In reality, most audio digital audio devices like Soundcraft digital mixers already have good clocks built into them, and adding an expensive component like an Apogee Big Ben or a Tascam CG-1000 isn’t really going to give you any noticeable improvement, especially if you’re only using one digital device!