Article applies to: ISA One, 220, 430, 430 MkII, 428, 828.
The ISA One, 220, 428, 430, 430 MkII and 828 can be fitted with optional digital converter cards. If your unit has one of these cards fitted and you are connecting digitally to other equipment (via AES, SPDIF or ADAT) then the ISA unit must be in sync digitally with all other digital audio devices in the signal chain. The way you sync the devices together depend on how many devices are in the signal chain and which device you select as the Master clock.
Wordclock is the name of the signal which keeps devices in sync - this can be embedded within the digital audio stream (e.g. within a SPDIF, AES or ADAT signal) or can be carried between devices via a 75 Ohm BNC connection. The digital cards for all of the units listed above feature two BNC connectors - one is a wordclock input and the other a wordclock output.
There are two ways to configure the ISA in a digital audio system:
1. Wordclock master - in this configuration the ISA generates the clock signal so you do not need to connect a wordclock cable to it. If the receiving device can sync to a wordclock signal embedded in a digital audio stream (virtually all audio interfaces do) then you do not need to connect the wordclock 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 the ADAT format, 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 wordclock master.
Note that there's no "Internal" or "Master" clock source indicator on the front panel of the ISAs. If all external clock source indicators are unlit, the ISA is clocking Internally.
2. Slave to wordclock - in this configuration you need to connect a BNC wordclock cable to the wordclock input of the ISA so that it can sync to an external master clock. 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 'Superclock' (NOTE: the ISAs will not generate Superclock, only receive it). The signal seen at the ISA's 'wordclock in' connector will be regenerated at its 'wordclock 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 wordclock generator to provide clock to all of your digital devices.
NOTE: it is 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 wordclock signal.