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Word Clock synchronization

In any digital audio system, care must be taken to properly select Word Clock master and slave devices in the system. Failing to do so can cause very audible problems. This is not an issue particular to AudioRail. If you are new to digital audio, you must read this carefully to understand what is involved.

AudioRail is very versatile and flexible, in that each ADAT Lightpipe output connection follows the timing of its source, as it appears at its input connector. Different end to end connections can be run at different speeds and use different clock synchronization schemes, even though they are time division multiplexed over a single CAT5 cable. Each connection appears as a virtual wire in a digital audio snake, as if it was a wire in a bundle of separate, individual digital audio cables. AudioRail has no common clock. AudioRail itself does not require a common clock source, and it does not require an additional parallel word clock cable to run alongside it. Users simply select which digital audio stream to extract the word clock from.

With this versatility, the user must consider each digital audio path and verify that exactly one clock source is selected to be master of each subsystem that requires synchronization. Consider the example pictured below:

In the example above, there are three independent digital audio subsystems:

  1. The two digital audio workstations (DAWs).
  2. The mating pair of A/D and D/A converters, below that.
  3. The digital mixer and multitrack recorder on the left, connecting to the A/D converter and D/A converter on the right (bottom).
In each subsystem above, exactly one device must be chosen to be the master (M) and the rest are designated as slaves (S).

Notice that there is an analog cable connecting subsystems 2 and 3 above. This does not matter. You do not have to synchronize components that have only analog connections between them. (If it had been a digital cable, the A/D converter would have had to be configured as a slave.)

The user's manual for each product describes how to configure it. Either a software or hardware switch will be available to select "master" or "slave", also often designated "internal" or "external" clock. Often a BNC connector is available to connect a 75 Ohm cable to externally slave one device's word clock to another. Configuration options vary considerably from product to product. Sometimes limitations in the available clocking schemes of one product will limit the user's choices, such as when a device can only operate as a slave, or defaults to slave mode when another device is connected to it.

Why does digital audio need a common clock?

The reason for the necessity of a common clock in any digital audio system is that no two independent clock sources can maintain perfect synchronization. If two digital audio sources running at a 48K sample rate were to each be driven by quartz crystal oscillators that had an accuracy of 100 ppm (parts per million), then one could actually be running at 48005, while the other was running at 47995. The difference between these two numbers means that the slower one would drop samples at the rate of 10 per second, or else the faster one would double-clock samples at the rate of 10 per second.

When a device is configured as a "slave" it extracts the clock from the digital audio stream and locks onto it, instead of its own quartz crystal oscillator.

What about sample rate conversion?

In some cases digital audio components have the capability to do "sample rate conversion." Wherever a specific digital audio port has this capability, there is no need for a common clock. Sample rate converters re-sample the digital audio, and can convert a few parts per million difference as easily as they can convert between entirely different sample rates. Again, each product's user's manual should be consulted.