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Q: I am trying to record into Pro Tools M-Powered at 88.2 or 96k sample rates using the ADAT ports on my ProFire Lightbridge or 2626, and I am not able to use the maximum of 18 inputs that Pro Tools allows for.
A: While some M-Audio interfaces support ADAT S/MUX for higher sample rates, Pro Tools does not.
An ADAT port is designed to transfer eight channels of audio at sample rates up to 48 kHz. When operating at higher sample rates, the port must operate in S/MUX II (up to 96 kHz) or S/MUX IV (up to 192 kHz*) mode.
S/MUX II mode combines pairs of ADAT channels (i.e. channels 1/2→1, 3/4→2, 5/6→3, 7/8→4) to create four high bandwidth channels capable of transferring high resolution audio up to 96 kHz.
Pro-Tools does not change its I/O configuration for S/MUX II, so it will still assign 8 channels for each of the optical ports of the M-Audio hardware. This means that optical inputs 1-4 will be able to carry audio to Pro-Tools, but inputs 5-8 will be “empty”.
Pro Tools M-Powered is limited to 18 simultaneous inputs and outputs. If your M-Audio hardware interface has more than 1 optical port (such as the M-Audio ProFire 2626 and ProFire Lightbridge), you will find that these ”empty” IO channels will count against your total IO limit at higher sample rates. This will reduce the total number of AUDIO channels that you can simultaneously record into Pro-Tools or route out of Pro-Tools. At 88.2/96k, the ProFire 2626 will be limited to 14x14 I/O, and the ProFire Lightbridge to 10x10 I/O.
* Note: Pro Tools M-Powered only supports up to 96kHz