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http://jackaudio.org/multiple_devicesSeveral JACK backends, including the ALSA, FFADO and CoreAudio versions, support the -P and -C arguments that can be used to specify two different devices, one for playback and one for capture/recording. You cannot use this to merge multiple devices for playback or capture. This approach will not do any clock drift correction, so as the two devices drift over time, you may get glitches in the audio stream. Nevertheless, it can be an easy if unreliable way to set up JACK so that, for example, it records from a USB microphone and plays back via a builtin audio device.
When using -P or -C to specify different devices, do not use the -d argument (which specifies a single device) and do not use the -D argument (which tells JACK to configure a device for playback & capture).