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Question: Aggregate audio interfaces w. different samle rates

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I use Logic X on a Mac Mini 2018 with a RME UCX interface, working flawlessly! Furthermore, all of my Logic projects are at 44.1 kHz.


The other day I added a NDSP Quad Cortex (QC) that can operate as an 8-channel USB interface. My first thought was to run the UX and QC as an aggregate device (monitors etc connected to the UCX and guitar, bass etc to the QC). However, the USB interface option on the QC can only run at 48 kHz and as all my old and current projects are at 44 kHz I planned to run the QC via analog I/O - not very desirable but what to do...


OK, to the point: I still created an aggregate device - maybe I'll switch to 48 kHz for new stuff. To my pleasant surprise the aggregate works flawlessly at 44.1 KHz, using the UCX as clock master master :shock: Not a trace of any glitches or artefacts in the audio!!


I've been on the digital audio train since the early 90s (AudioMedia II etc) and knows how sample rate mismatch wreaks havoc with audio, but obviously not in this case. I've tried to google this but find no conclusive answer - does the audio aggregation function in Mac sample rate convert the 48kHz from the QC to match the 44.1 kHz of the UCX, set by the Logic project?? What is going on here?? :?: :?: :?:


In the audio device settings i can see that the QC does not secretly handle 44.1 kHz - only 48kHz is available with the QC as clock source while the UCX can be set to several different (44.1 up to 192kHz). Any clarification would be much appreciated!




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That the QC can do only 48k is not a real world limitation, but probably a (UI?) design decision, as it makes things far easier if you as the designer and you as the user don't need to deal with multiple sample rates. In the old days, 44.1 was deliberately omitted as a choice in some devices to prevent digital 1:1 copies of CDs...


It looks like the QC's clock can be varied enough to match the 44.1 of the clock master and if so, all is well.

Edited by fuzzfilth
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There is no SR conversion going on in an aggregate device, i.e., it's not that the ADA chain runs in 48 and the digital date gets converted to 44 and back. It's the slaves' clocks that get bent so much that they match the master's clock rate, so even the ADA runs that slow, which is perfectly fine if it's consistent.
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