Applies to: 

• Scarlett 18i20
• Scarlett 18i16
• Scarlett 16i16

To expand the number of inputs and outputs you can use in your recording/mixing sessions, you can connect a digital ADAT or S/PDIF device to your Scarlett. When you're using the digital ports on your Scarlett, the Scarlett and the other digital device(s) you've connected to the Scarlett need to be synchronised. Digital devices use clocking signal to synchronise and ensure the audio is recorded in time without glitches or pops.

There are a few principles when you're trying to synchronise multiple digital audio devices:

• The clock signal can be embedded into the audio signal, down the same cables (e.g. S/PDIF, or ADAT).
• Clock signals are always one-way, you can't send and receive clock signals using one ADAT or S/PDIF cable.
• There are clock leaders and clock followers.
• Devices 'follow' other devices' clock signals. One device in your setup must be the clock leader, the other devices must be followers and receive the clock signal from the clock leader.
• Every device with digital I/O will have an internal clock and should have the option to be a clock leader or a clock follower.

In the Preferences within Focusrite Control 2, you can select the Clock source for your Scarlett from 3 options; Internal, ADAT, and S/PDIF. The below outlines when each option should be used.

Internal (Clock Leader)

If you are using your Scarlett by itself without any digital expansion equipment, select “Internal” as the Clock Source. This means the device will use its own internal clock signal. If you are using digital expansion equipment with your interface, but, you are sending a clock signal to that device (or multiple devices), your interface still needs to be Internal (functioning as the Clock Leader for the rest of the devices).

ADAT (Clock Follower)

If you have an ADAT expansion device you want to use to get up to 8 further inputs and/or outputs, you should connect it to the Scarlett's Optical ports. You can use one or two ADAT cables with the Scarlett interfaces listed above.

One ADAT cable

When using one ADAT cable, the clock signal will follow the direction of the audio signal.

• If you are connecting the Optical output of your expansion device to the Optical input of your Scarlett, you should set the Clock source of your Scarlett to ADAT so that the Scarlett synchronises to the incoming clock signal.
• If you are connecting the Optical output of your Scarlett to the Optical input of your expansion device, you should set the Clock source of the Scarlett to Internal so that the expansion device synchronises to the clock signal from the Scarlett

Two ADAT cables

You can use It doesn't matter which device is the Clock Leader, as long as one device is set to Internal and is sending clock and the other unit is set to ADAT to synchronise.

S/PDIF (Clock Follower)

If you have an S/PDIF expansion device you want to use to get 2 more inputs and/or outputs, you should connect it to the Scarlett's SPDIF ports. You can use one or two S/PDIF cables with the Scarlett interfaces listed above.

One S/PDIF cable

When using one S/PDIF cable, the clock signal will follow the direction of the audio signal.

• If you are connecting the output of your expansion device to the input of your Scarlett, you should set the Clock source of your Scarlett to S/PDIF so that the Scarlett synchronises to the incoming clock signal.
• If you are connecting the output of your Scarlett to the input of your expansion device, you should set the Clock source of the Scarlett to Internal so that the expansion device synchronises to the clock signal from the Scarlett

Two S/PDIF cables

It doesn't matter which device is the Clock Leader, as long as one device is set to Internal and is sending clock and the other unit is set to S/PDIF to synchronise.

For more information and real-world examples of connecting ADAT and S/PDIF devices to your Scarlett, please see the User Guides here.