Raspberry Pi HDMI audio vs USB CPU use

Quick note after some experiments last night. Not completely scientific, but enough to show a trend. I set out to compare CPU usage of the Pi running Volumio, upsampling lossless 44.1KHz 16bit stereo with ‘fastest sinc’ to 192KHz 32-bit stereo.

Streaming to the USB uses between 70 and 90% CPU. Streaming to the HDMI output uses 95% and more! Audio gets choppy in the latter case even without other processes getting in the way, whereas the former only gets choppy when the Pi happens to try and update the MPD database at the same time.

Wonder if anyone knows why onboard streaming should use so much extra CPU time to do the same work, and whether I2C suffers the same fate? Not sure I want to spend on a custom DAC if the current EMU 0202USB is more efficient?

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