Applies to: Scarlett 18i20 3rd Gen
The Scarlett 18i20 has two ADAT input ports for connecting up to 8 additional channels via optical ADAT connection.
Whilst it has two ports this does not mean you can add an extra 16 channels via ADAT. The maximum number of ADAT channels you can use is dependant on the sample rate you're recording at, and is as follows:
44.1/48kHz - 8 channels
88.2/96kHz - 8 channels (four channels per port)
176.4/192kHz - 0 Channels (Optical inputs are disabled at quadband sample rates)
The reason for this is that ADAT, as a protocol, is a one-way connection that can carry a maximum of 8 channels at 44.1/48kHz. Each time the sample rate doubles, the channel count per connection is halved.
Some ADAT preamps, such as the 'Clarett Octopre' and 'Scarlett Octopre Dynamic' have dual ADAT output connections that allow you to get more channels at higher sample rates. Other preamps may only have one ADAT connection allowing you 8 channels maximum using the single connection.
ADAT is a universal protocol so any ADAT preamp will work with the Scarlett 18i20.
For information on configuring the Digital I/O modes of the 18i20 please see the section 'Using the ADAT connection' and 'Channel Listing Tables' on pages 22 and 26 of the Scarlett 18i20 User Guide:
Scarlett 18i20 User Guide