Jitter is caused by a varying clock frequency, whereas a clock that has drifted has a pretty stable frequency. You might also hear people referring to word clock as sync clock, signal clock or simply clock.Ī stable clock compared to a jittery one, compared to one whose frequency has drifted. You’re very unlikely to encounter a problem with a bit clock, and if you do there isn’t much you can do except send it back for repair. These days they are only used for signal transport within devices, for example from one PCB to another. There are also bit clocks, which produce one cycle per bit. Clocks used to trigger the capturing of the signal are often called sample clocks. This can also happen when an accurately-captured signal gets reconverted with an unreliable clock, like a film being played on a clunky projector (see figure 1). Thinking back to the video analogy from the last post, it’s like film taken on old hand-cranked cameras: uneven capturing of the signal leads to weird inconsistencies when it’s played back. If they aren’t, the waveform will end up deformed when reproduced by something that is in time. AD convertors might take a fixed number of samples per second, but they still need to make sure those samples are evenly spaced. What the clock?Ĭlocks are necessary for a few different stages in the signal path. Some clock generators even keep their crystals in tiny ‘ovens’ to keep the temperature constant. The clock runs alongside the audio signal, with 1 usually meaning “this is the start of the sample” and 0 meaning “this is the end.” Different shapes and sizes of crystal resonate at different frequencies, then more subtle changes are controlled by variations in the voltage running through the circuit and temperature. The rising edge of the resulting wave means 1 the falling edge means 0. This signal is produced by passing an electrical current through a small crystal inside a word clock generator. ![]() How does it know whether 0000011100001011 is two samples, reading 00000111 (= 7) and 00001011 (=11), or the second half of a sample, a full sample (01110000 = 112), and the first half of the next one? As you can see, the resulting values can be very different, so it’s essential to get it right to the exact bit.Ī word clock is a signal that is sent at a very accurate frequency of one square wave per sample (the bits in each sample make up a ‘word’). So… what is a word clock?Īny device receiving audio sees a string of 1s and 0s. They might seem strange and complicated, but they are of course not black magic. However, you can become a rare and very valuable member of your audio team by actually learning what word clocks are, how they work and how to fix the most common problems they can cause. ![]() If anything weird ever happens with a digital set up, like odd clicks or pops over the PA, you can seem wise beyond your years by nodding sagely, saying “Hmm, it sounds like a clocking issue”, then making your excuses and leaving before any further questions can be asked. This is where the fabled word clock comes in. ![]() ![]() The next step is keeping that signal in time when it’s being captured, processed and sent to different parts of the system. My last blog dealt with translating audio into a digital signal. Never mind the bit clocks… it’s a word clock primer
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