We’ve been getting a lot of questions about word clock lately. We plan to tackle many of them individually, but before we do perhaps it is time to review a few of the important points pertaining to word clock and its uses. Sweetwater Sales Engineer David Klausner has written the following primer for us.
One of the more confusing elements in today’s digital studios seems to be the function of word clocks. In any digital audio system, word clock is the pulse that lets units know when the beginning of a sample occurs. This means that in a system with a 48 kHz sampling rate, there is a pulse 48,000 times a second that lets all the devices know when those samples happen. This has great importance in two areas – digital transfers and A/D and D/A conversions.
One of the more common problems when hooking up a lot of digital gear, is the occurrence pops or clicks. This is usually an issue of the various word clocks in the system not being in sync. In a 24 bit system, the first, or most significant bit represents the loudest volume the system is capable of (full scale), and the last bit represents the lowest volume the system can represent (hence more bits = greater resolution at low levels). Each successive bit represents half the volume of the bit before it.
An analogy I use is one of a pitcher throwing balls to a catcher. Think of each of the bits as balls, with the first being the largest, and each successive ball being half the size of the one before it. If the first ball is a volleyball, the second is a softball, the third a tennis ball, then a golf ball, and so on, until we are throwing balls the size of a grain of sand. In a 48 kHz system, the whole series of balls, from largest to smallest, is thrown 48,000 times a second. Word clock is what tells the catcher “here comes the volleyball” or most significant bit. If the catcher is expecting a grain of sand, but ends up with the volleyball, there will be a momentary (1/48,000th of a second) burst of erroneous data, at the loudest volume the system can produce (full scale). This is what many of those pops and clicks are.
A stable master clock should prevent any of this from happening, by keeping the pitcher and catcher in sync. Some basic rules for word clocks are that whenever a clock is regenerated (run into the “clock in” of a unit and back out the “clock out” to the next unit) the clock signal is degraded, and that if a clock signal is split (or “Tee’ed off”), it is also degraded. The ideal situation is for each digital device to receive its own, first generation clock. Your Sweetwater sales engineer can help you configure your system that way.
The other big issue with clocks is the effect they have on converters. Remember, the clock is what tells us where those 48,000 samples per second (in a 48 kHz system) are taken. Here we are really specifying points to define a waveform, and we need to have at least two points to figure out what the frequency is (hence the Nyquist law, that the highest frequency a digital system can reproduce is half the sampling rate).
If we are experiencing jitter, the timing of those points can be suspect. With low frequency sounds, which have very long wavelengths, we sample a lot of points between peak and trough of the waveform, so this may not be too much of an issue, and we can interpolate around minor fluctuations. At higher frequencies however, we may get only a few (or two!) points to work with, and a slight instability of the timing of where these points are marked can become significant. Since high frequency information often consists of transients, and transients help to define signals as well as provide aural cues as to location, clock jitter in a converter can smear signals and cloud imaging and soundstage perception. This is why a good master clock can often make a system sound better, even if there are no obvious clock problems such as pops or clicks. As always, your Sweetwater sales engineer can help you get the most out of your system.