Dear ALSA developers, I seem to have immense trouble getting stereo input from a microphone to work. I tried many things, and all of my attempts showed one kind of trouble or another. I would like to find out whether all of this are separate problems or whether I am just spatially challenged. Here is what I observed (alsa-driver version is hg20060907): 0) I triple-checked that my speakers are not swapped. Everything seems to be ok there: the left/right channels of the volume control correspond to the correct speaker and "speaker-test -c 2" works ok. Thus the following problems cannot be explained by swapped speakers. 1) I tried to connect a Sony ECM MS907 microphone [1] via a Belkin iMic USB adapter [2] (supported by the snd_usb_audio module). This was my most successful attempt: I could record a stereo signal from the microphone. Problem: left and right input is exchanged. If I blow at the microphone from the side labelled "L" the volume at the right speaker is loudest, if I blow at the microphone from the side labelled "R" the volume at the left speaker is loudest. Why are the channels swapped? [1] http://www.sony.co.uk/view/ShowProduct.action?product=ECM-MS907&site=odw_en_GB&pageType=Overview&category=ACC+Microphones [2] http://seehuhn.de/comp/hardware/imic 2) I tried to connect the microphone to the microphone input of my ASUS A8V mainboard [3]. The board has a VT8327 chipset and is supported by the snd_via82xx module. Recording sound works, but the signal seems to be mono (the two channels are the same or almost the same). Does the VT8327 chipset support stereo recording? [3] http://seehuhn.de/comp/hardware/asus 3) I tried to connect the microphone to my Revolution 5.1 card [4], supported by the snd_ice1724 module. I can record, but the recording seems to be mono again: the signals on the two channels are the same or almost the same. The Revolution is, I think, supposed to be able to record in stereo. Is this true? [4] http://seehuhn.de/comp/hardware/revolution 4) The card a stereo capture volume control. I seem to be unable to control the capture volume separately from the output volume using amixer. Example: tarte:~# amixer -c 0 set "PCM" 0%,100% Simple mixer control 'PCM',0 Capabilities: pvolume cvolume cswitch cswitch-joined Playback channels: Front Left - Front Right Capture channels: Front Left - Front Right Limits: Playback 0 - 127 Capture 0 - 151 Front Left: Playback 0 [0%] Capture 0 [0%] [on] Front Right: Playback 127 [100%] Capture 151 [100%] [on] The above command sets the volume for capture and output simultaneously. I did not find a way to get output 100% on both channels and input 100% left, 0% right with amixer. How can I do this? Is it a good idea the 'PCM' combines the front speakers with the captuer channel anyway? 5) I can control the capture volume with alsamixer and display the result with amixer. But here left and right channel are swapped. When I have tarte:~# amixer -c 0 get "PCM" Simple mixer control 'PCM',0 Capabilities: pvolume cvolume cswitch cswitch-joined Playback channels: Front Left - Front Right Capture channels: Front Left - Front Right Limits: Playback 0 - 127 Capture 0 - 151 Front Left: Playback 127 [100%] Capture 0 [0%] [on] Front Right: Playback 127 [100%] Capture 151 [100%] [on] the recording has only sound in the left channel. And when I have tarte:~# amixer -c 0 get "PCM" Simple mixer control 'PCM',0 Capabilities: pvolume cvolume cswitch cswitch-joined Playback channels: Front Left - Front Right Capture channels: Front Left - Front Right Limits: Playback 0 - 127 Capture 0 - 151 Front Left: Playback 127 [100%] Capture 151 [100%] [on] Front Right: Playback 127 [100%] Capture 0 [0%] [on] the recording has sound only in the right channel. Why is this swapped? 6) For comparison: same setup as in step 2 above, i.e. microphone plugged into the mainboard. amixer shows a stereo control for the capture volume alone. I can use this control to mute either channel of the input: tarte:~# amixer -c 1 set Capture 100%,0% Simple mixer control 'Capture',0 Capabilities: cvolume cswitch cswitch-joined Capture channels: Front Left - Front Right Limits: Capture 0 - 15 Front Left: Capture 15 [100%] [on] Front Right: Capture 0 [0%] [on] The above mutes the right input channel. tarte:~# amixer -c 1 set Capture 0%,100% Simple mixer control 'Capture',0 Capabilities: cvolume cswitch cswitch-joined Capture channels: Front Left - Front Right Limits: Capture 0 - 15 Front Left: Capture 0 [0%] [on] Front Right: Capture 15 [100%] [on] The above mutes the right left channel. Thus, this works. Any help in further diagnosing and fixing these problems would be very welcome. Many thanks, Jochen -- http://seehuhn.de/
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