Digital data is comprised of bits that are organized into words to represent sample values. Word clock is a set of timed pulses used to synchronize signals at the clock rate, which is called the sampling frequency or sample rate (at a value typically in the kilohertz range; e.g., 44.1kHz, 48kHz, 88.2kHz, 96kHz, etc.). Word clock is used by the receiving digital device to lock to the incoming digital signal.
Every digital device, from a simple, portable CD player to a Pro Tools HD rig, has a word clock. Not to be confused with sync clock (like MTC or SMPTE), word clock is what tells an A/D or D/A converter when to begin each sample. It fires an impulse to the converter at 44.1k (or 48k, 96k, etc.) times per second. The stability of this clock (how evenly spaced those pulses are) determines the accuracy of the conversion process.
If you have interconnections in your studio between different digital gear (S/PDIF, AES, Lightpipe, etc.), you must designate a master clock source. If multiple devices are each trying to be the master clock, you will likely experience pops or clicks in your audio. The solution is to make sure that you designate one unit to be the master clock and that all slave units are receiving the identical clock signal. But even without obvious problems like pops and clicks, you may still have more subtle word clock problems.
Think of it like this: Ever seen clips from the very early days of film? With the hand cranked cameras of the day, it was extremely difficult to get the shutter speed to be perfectly even. The result was a moving image that, when played back at a constant shutter speed, seemed “jittery.” The motion was often jerky and unnatural. This was due to the imperfections in timing of the original film speed.
The same thing happens in digital audio. The more “jitter” present in the clock source, the less accurate the resulting conversion will be. Jitter is manifested in audio as phase distortions and discrepancies, particularly in the higher frequencies. Tracks can lose some of their sparkle, transients can lose some attack, and stereo images aren’t as vivid.
Improving the quality of your clock is an easy way to stabilize and interface every A/D and D/A converter in your studio. Adding a high-quality master clock will definitely solve any clocking issues you may be experiencing and may result in an improvement in the overall quality and clarity of your finished product.