Hello,

I do have a Behringer U-PHORIA UMC22 USB soundcard which uses the PCM2902 soundchip. I am using that device with pulseaudio on top and the trouble occurs when I do not use the device.

Problem =======

In various intervals, but only when I don't play audio, I do hear a pop or click sound comparable to the sound a power amp does when connecting or disconnecting the power.

My Analysis ===========

I did watch the usb port using wireshark and the following frame has been the last frame before that sound:

Frame 3470: 64 bytes on wire (512 bits), 64 bytes captured (512 bits) on interface 0 USB URB [Source: host] [Destination: 2.4.0] URB id: 0xffff9522ee93f9c0 URB type: URB_SUBMIT ('S') URB transfer type: URB_CONTROL (0x02) Endpoint: 0x00, Direction: OUT Device: 4 URB bus id: 2 Device setup request: relevant (0) Data: present (0) URB sec: 1507602131 URB usec: 34054 URB status: Operation now in progress (-EINPROGRESS) (-115) URB length [bytes]: 0 Data length [bytes]: 0 [Response in: 3471] Interval: 0 Start frame: 0 Copy of Transfer Flags: 0x00000000 Number of ISO descriptors: 0 URB setup bmRequestType: 0x01 0... .... = Direction: Host-to-device .00. .... = Type: Standard (0x0) ...0 0001 = Recipient: Interface (0x01) bRequest: SET INTERFACE (11) bAlternateSetting: 0 wInterface: 1 wLength: 0

From what I understand from the PCM2902 Manual [0] on page 22 is that the streaming interface gets deactivated (zero bandwith setting) by this frame. If I read the documentation of the PCM2902 correctly that step is not nesessary, the chip suspends itself when no data comes in (p. 26).

Reading *linux/sound/usb/endpoint.c* I am convinced that the frame above and its counter part are send by this code and send everytime a first endpoint (pulseaudio, aplay, …) connects and the last endpoint disconnected. Due to that I do believe that this is a bug in the driver used for the pcm2902 chip.

References ==========

That bug is not unheard of as you can see in [1] and [2] but the workarounds provided in those links I cannot confirm to work.

I hope that my analysis is as conclusive for you as they sound to me. From this point I am unsure where to head next and was hoping for help here. So my followup questions are:

* What do you believe where and "why there" this should be fixed?

* Why is the frame above beeing send to the device? The soundcard does seem to have power after that frame.

* Isn't it possible to never set the pcm2902 into "zero bandwith mode"?

Greetings, Theodor

[0]: http://www.ti.com/product/pcm2902 [1]: https://ubuntuforums.org/showthread.php?t=2292143&p=13345100#post1334510... [2]: https://www.reddit.com/r/linux/comments/3idb1h/clicky_noise_before_and_after...