Apollo Hardware Manual
Digital Clocking Basics
27
Digital Clocking Basics
Digital clocking is a complicated issue, with a number of important aspects that are often not very well
understood.
First and foremost, a digital clock is used to maintain synchronization between different digital devices. There
are two primary purposes for clock synchronization:
•
Digital Conversion. Analog-to-digital (A/D) conversion and digital-to-analog (D/A) conversion need
extremely accurate clocking in order to correctly process the digital data. A low-quality clock can degrade
the signal in many ways, including loss of transparency, clarity, imaging and transient response, as well
as increased noise and distortion.
•
Digital Transmission. All digital devices need accurate clocking in order to properly transfer digital data
between interconnected devices. A low-quality clock can cause data reception errors, which add distortion
and noise, and if the clock isn’t synchronized correctly, samples may be dropped or repeated, resulting in
audible clicks or dropouts.
Clock quality is defined two ways: First, the sample rate must match the signal. This is referred to as “sample
rate synchronization.” Second, the clock signal must be stable over both short-term and long-term clocking
intervals. “Jitter“ refers to short-term clock accuracy, and “stability” or “drift” refers to long-term clock
accuracy. These terms are discussed in more detail below.
Sample rate synchronization is required for proper digital transmission, and is relatively easy to maintain.
Basically, there must be one and only one “clock master“ for all interconnected digital devices. This is done by
setting one device to “master” mode (where it synchronizes to its internal clock and transmits that clock
signal) and setting every other device to “slave“ mode (where it receives and synchronizes to external clock),
with the appropriate clock signal routed between the master and slave devices. Keep in mind that any device,
whether it’s the clock master or a slave, can send or receive data once everything is synchronized correctly.
When doing digital conversion, it’s best to have the converter serve as the clock master. For example, if you’re
recording, clock everything off the A/D converter. Likewise, if you’re mixing, clock everything off the D/A
converter. If you’re running multiple converters, use the device with the best quality clock as master.
For all-digital transfers, e.g., a digital transfer from one DAW or storage device to another, clock
synchronization is maintained by simply setting up the proper master-slave relationship between devices.
Digital transfers can be affected by clock jitter, but not in the same way clock jitter affects analog conversion.
This is a widely misunderstood concept we’ll discuss in detail below.
Clock jitter is short-term variations in the timing of edges of a clock signal, as opposed to clock drift, which is
long-term variation in the clock rate. A clock could be very stable over the long term, but still have jitter, and
vice versa. Timing variations are caused by noise and/or interference. If the noise/interference is a high-
frequency signal, the result is jitter, and if the noise/interference is a low-frequency signal, the result is drift.
As an analogy, a car with an out of balance wheel may drive straight, but you’ll get lots of vibration (jitter);
conversely, a car with a loose steering wheel might have a smooth ride, but it will drift all over the road.
Clock drift affects long-term synchronization, like sound to picture, and can introduce slight pitch variations in
the audio. Usually however, the drift is so slow that these pitch variations are only tiny fractions of a cent, and
thus unnoticeable.
