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#post13345100
[2]:
https://www.reddit.com/r/linux/comments/3idb1h/clicky_noise_before_and_after_every_sound/
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