Applies to: 

All interfaces featuring S/PDIF or ADAT (Optical) ports

Clocking is an important aspect when you're looking to sync and expand your setup. For reference, see these articles:

• Understanding Clocking
• What are the Clock Source and Sample Rate?

The control software for Focusrite interfaces that feature S/PDIF or ADAT (Optical) ports lets you select the Clock Source. By default, this is set to Internal.

Internal (default)

Select Internal when using your interface without any external gear connected over S/PDIF or ADAT. This means the interface synchronises (syncs) to its internal clock.

You should also select Internal when connecting external gear to your interface over S/PDIF and/or ADAT, and you need the audio interface to be the clock leader. In this case, the interface sends its clock signal (leader) to the external gear (follower).

S/PDIF

Select S/PDIF if you are receiving an S/PDIF signal from an external sound source connected to the S/PDIF Input of your interface.

This is convenient when the external device connected doesn't feature a selectable sync/clock setting of its own, for example, a TV, game console, or a DVD player. This lets your interface sync to the incoming clock from the external S/PDIF device.

For reference, see: Connecting TVs, Consoles, or other device to your interface

If using two RCA cables in a setup where both the external unit and the audio interface feature S/PDIF input and output ports, it doesn't matter which unit is the leader. As long as one unit is set to Internal (leader) and is sending clock. The secondary unit is set to S/PDIF to follow the sync information (follower).

ADAT

If you are using your interface with another multiple-input device and want all inputs to show individually, you would connect them via ADAT. Most ADAT devices have the option to choose Clock Source (e.g. the dedicated button 'Sync' on the front panel of an OctoPre).

• If using a single optical cable from the expansion unit's optical output to your interface's optical input, the interface should be set to ADAT (follower) and the expansion unit to Internal (leader).
• If using two optical cables in a setup where both the expansion unit and the audio interface feature optical input and output ports, it doesn't matter which unit is the leader. As long as one unit is set to Internal (leader) and is sending clock. The secondary unit is set to ADAT to follow the sync information (follower). 

Word Clock

Word Clock, or Word Clock, is the timing information embedded within S/PDIF and ADAT audio streams. However, some devices offer a dedicated Word Clock input or output, usually via a BNC connector. This carries only the timing signal, without audio, and can provide more stable and easier-to-manage synchronisation.

To change your Sync/Clock Source option, see the section below, depending on the interface you have.