Steinberg Fw driver together with Motif XS on Macbook pro i7 with Sierra make the fans run high and make the processor to work 10 % in idle state. With usb cable just around 2 %. Why
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We moved your post here as you do not have a MOXF. The Motif XS is now a legacy product.
The Motif XS does not do audio via the Yamaha USB-MIDI driver - so the CPU load shown when connected by USB is MIDI only.
When you connect via Yamaha Steinberg FW driver, both MIDI and Audio are handled via the FW connection by the single driver. The CPU load is increased if you open more connections than you use. In other words at maximum, you could have 16 audio bus outputs connected to 16 inputs in your computer DAW, you can have 6 audio (3 stereo pairs) from the computer back to the Motif XS via FW.
You should activate the audio buses as you need them, reserving them naturally reserves CPU muscle.
The 16 audio buses can be configured as the Main L&R, plus fourteen assignable outputs... which can be in odd/even pairs, or mono buses. It is this additional data that adds to the processing usage. When not using a bus leave it "not connected"