Some Focusrite interfaces feature ADAT (optical) inputs and outputs. ADAT is a universal protocol, a digital format carried using optical cables (sometimes referred to as "TOSLink").
ADAT is a one-way connection that can carry a maximum of 8 channels at 44.1/48 kHz per connection. The number of channels transferred between two devices over a single ADAT cable depends on the sample rate used; each time the sample rate doubles, the channel count per connection halves. See the table below for reference:
| Sample Rate | ADAT Channel Count |
| 44.1/48 kHz (single-band) | 8 Channels |
| 88.2/96 kHz (dual-band) | 4 Channels |
| 176.4/192 kHz (quad-band) | 2 Channels |
Some interfaces disable ADAT I/O at quad-band sample rates (i.e. 176.4/192 kHz). You can verify your specific interface's user guide for reference.
An audio signal transmitted via ADAT can only travel in one direction over a single cable. If you wish to send audio signals in both directions using ADAT, you will require two cables, and both devices must have ADAT In and Out ports.
Single Optical I/O devices
ADAT devices with single optical input and output ports, like Scarlett 16i16 4th Gen, Scarlett 18i16 4th Gen, and Clarett+ 8Pre, allow you to expand up to a total of 8 additional inputs and outputs at 44.1/48 kHz over ADAT:
Scarlett 18i16 4th Gen
For information on configuring the Digital I/O Modes of your interface, see this article: Digital I/O Modes
Dual Optical I/O devices
ADAT devices with dual optical input and output ports, like Clarett+ Octopre, Scarlett 18i20 3rd Gen, and Scarlett Octopre Dynamic, as well as allowing you to expand up to a total of 8 additional inputs and outputs at 44.1/48 kHz over ADAT, also let you double the number of channels transmitted at sample rates higher than 44.1/48 kHz, using two optical cables:
Clarett+ Octopre
Scarlett 18i20 3rd Gen
Note that, on Scarlett 18i20 3rd Gen, the optical ports are disabled at quad-band sample rates (176.4/192 kHz).
Unlike the devices mentioned above, Scarlett 18i20 4th Gen, and legacy units like Liquid Saffire 56 and Clarett 8PreX, feature 16 channels of ADAT I/O, which allow you to expand up to a total of 16 additional inputs and outputs at 44.1/48 kHz over ADAT:
Scarlett 18i20 4th Gen
For more information on the 16 channel ADAT I/O for Scarlett 18i20 4th Gen, see this artcile: 16 Channel ADAT I/O - Scarlett 18i20 4th Gen
For information on configuring the Digital I/O Modes of your interface, see this article: Digital I/O Modes
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