Ricard Wanderlof wrote:
bEndpointAddress 0x0d EP 13 OUT bmAttributes 5 Transfer Type Isochronous Synch Type Asynchronous Usage Type Data wMaxPacketSize 0x0070 1x 112 bytes bEndpointAddress 0x8e EP 14 IN bmAttributes 37 Transfer Type Isochronous Synch Type Asynchronous Usage Type Implicit feedback Data wMaxPacketSize 0x0070 1x 112 bytesWhen the Isochronous Synch Type is Asynchronous, there must be a feedback channel where the USB device reports back information so that it can adjust the output sampling rate.
I might be barking up the wrong tree here as I'm new to this, but it looks as if endpoint 0x8e is providing the feedback data, and there doesn't seem to be any other isochronous endpoint defined in the lsusb dump which could be used for the actual capture cdata.
The first endpoint in an audio interface is for the actual samples.
I'm not sure what 'Implicit feedback Data' means exactly though - is the feedback data multiplexed with the capture data perhaps?
The USB 2.0 specification says: | 5.12.4.3 Implicit Feedback | | In some cases, implementing a separate explicit feedback endpoint can | be avoided. If a device implements a group of isochronous data | endpoints that are closely related and if: | ⢠All the endpoints in the group are synchronized (i.e. use sample | clocks that are derived from a common master clock) | ⢠The group contains one or more isochronous data endpoints in one | direction that normally would need explicit feedback | ⢠The group contains at least one isochronous data endpoint in the | opposite direction | Under these circumstances, the device may elect not to implement | a separate isochronous explicit feedback endpoint. Instead, feedback | information can be derived from the data endpoint in the opposite | direction by observing its data rate.
In theory, the driver supports this, but there are known to be some bugs. And in any case there should be no problem getting the capture stream to start (it's the playback stream that would have a slightly wrong sample clock); it's possible that this device needs some vendor- specific commands, or a different order of commands. (The interesting USB transactions would be everything except the isochronous ones.)
Regards, Clemens