Applies to: Scarlett 18i20 3rd Gen
The sample rate of Scarlett interfaces can be changed from within your D.A.W, from Focusrite Control or, if running a Mac system, from Audio MIDI Setup. However, the sample rate for the Scarlett 18i20 3rd Gen can only be changed from Focusrite Control or Audio MIDI Setup - not your D.A.W.
The reason for this is as follows;
The Scarlett interfaces are class compliant and use Apple’s Core Audio USB driver which is built into the operating system. When switching between different sample rate bands (I.e 44.1/48Khz, 88.2/96kHz and 176.4/192kHz), the number of channels that the Scarlett 18i20 3rd Gen presents to the operating system changes due to it's extensive channel count. (4x channels of ADAT are lost at 88.2/96kHz and all ADAT channels are disabled at 176.4/192kHz).
We implement a mechanism in our firmware to tell Core Audio that the channel count has changed when this occurs. We have to do do this otherwise Core Audio will still think that there are, for example, 20 outputs available when the device is set to 88.2/96kHz at which point audio won’t play back at all.
Due to a change in Core Audio in macOS 10.13 and above, the mechanism for changing sample rate correctly between bands is now slightly different to before. We’ve had to make a change to Focusrite Control to facilitate this and whilst some D.A.W manufacturers have also implemented the change, many have not. This is the reason why the sample rate cannot be changed from within most D.A.Ws with the Scarlett 18i20 3rd Gen.