Article applies to: ISA One, 220, 430, 430 MkII, 428, 828. 

You can retro-fit an optional digital converter card into the ISA One, 220, 428, 430, 430 MkII and 828. If your ISA has one of these cards fitted, and you are connecting digitally to other equipment (via AES, S/PDIF, ADAT or Dante) the ISA unit must sync with the other digital audio devices in the signal chain. The way you sync the devices together depends on how many devices are in the signal chain and which device you select as the Clock leader.

Word Clock is the name of the signal which keeps devices in sync. Word clock can be embedded within a digital audio stream (e.g. within S/PDIF, AES or ADAT signal) or can be carried between devices via a 75 Ohm BNC connection. The digital cards for all the units listed above feature two BNC connectors - one is a word clock input and the other a word clock output.

There are two ways to configure the ISA in a digital audio system:

1. Word clock leader - in this configuration the ISA generates the clock signal so you do not need to connect a word clock cable to it. If the receiving device can sync to a word clock signal embedded in a digital audio stream (virtually all audio interfaces do) then you do not need to connect the word clock out of the ISA to anywhere. Instead, connect the digital audio output of the ISA to the digital input on the receiving device and set it to sync to its digital input.

For example, to use ADAT, connect the optical ADAT output of the ISA unit's digital card to the optical ADAT input of your audio interface, then select the clock source for your audio interface to ADAT. If the digital audio signal chain in your set-up is simple e.g. ISA → sound card → computer, then it is best to use the ISA as a word clock Leader.

Note that there's no 'Internal' or 'Leader' clock source indicator on the front panel of the ISAs. If all external clock source indicators are unlit, the ISA is clocking Internally.

2. Follow word clock - in this configuration you need to connect a BNC word clock cable to the word clock input of the ISA so that it can sync to an external clock leader. You will need to select 'Ext' as the sync setting on the front of the ISA, or 'Ext S/C' if the clock signal is 256x 'Super Clock' (NOTE: the ISA's will not generate Super Clock, only receive it). The signal seen at the ISA's 'word clock in' connector will be regenerated at its 'word clock out' connector.

This can be useful in more complex setups where you have multiple digital audio devices as the clock can be passed through (daisy-chained) to other devices from the ISA. If you have a very complex setup, it may be beneficial to utilize a third-party word clock generator to provide clock to all of your digital devices.

NOTE: it's important to set the sample frequency and bit-depth settings on the front of the ISA to the same settings for the rest of your system, even if the ISA is slaving to a word clock signal.