(This is in reference to getting the Zoom R16 to work, but the main query in this post is of a more general nature).
In trying to get a Zoom R16 in audio interface mode to work in Linux I'm trying to understand how the USB-audio (snd-usb-audio module) subsystem works in Linux. I've looked in the USB audio version 1 and 2 documents, as well as the USB 2.0 standard, but there are some things that don't seem to be well defined there, like exactly how the audio data is packaged in the isochronous transfer packets.
I've been using Wireshark to visualize the USB communication, and also comparing with the corresponding transfers on a Windows box by dumping the traffic with USBPcap.
The only other USB audio interface I have access to here is an Edirol UA-5. I've noted that when plugging in the UA-5, there's period of about 15 seconds during which capture and playback are set up and audio data transferred, after which the interface is shut down again. When enabling the R16, during the corresponding initial period I see playback data but no capture data during this phase. I don't know if that is significant or not, for instance, if the lack of capture data during this phase is because the playback is not working as it should.
My question is: what governs this startup phase, can someone point to a specific part of the code or some documentation describing what is supposed to go on here? Is it some sort of device test period to determine that the device actually works?
The problem with the R16 is that although capture works fine, playback doesn't result in any audio. With the correct quirk set up (most importantly, using a 24 bit format), aplay appears to work fine with no errors, neither are there any complaints from dmesg, and looking at the communication I can see outgoing isochronous data from the host being transferred fine.
More work needs to be done, but I've looked at the startup of the Windows driver which is deceptively simple: it basically just sets the appropriate capture and playback interfaces, together with sending off some sample rate USB control commands, the isochronous link comes up and that seems to be it. I've tried to emulate the specific USB control commands in Linux, as well as add some delays that there appear to be in the Windows driver, but so far it hasn't made a difference.
/Ricard