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ALSA: 2 input channels, only one is recording

Posts: 4
in Multimedia

So here is my pickle. I am very new to linux and to make matters worse i am working on an embedded system and not a PC. i have been searching for this topic but almost everything i find shows just 1 recording device.

we have a codec/sound card that has 2 input channels and 2 output channels and i need all of them at the same time. when i run my code to capture the audio samples i am only getting 1 channel and not the other. when i do arecord it is the same result.

my first series of questions are: Does ALSA only do 1 input at a time by default? if so is there a way to change that? if not does it mix the 2 inputs when doing an arecord or do i have to specify which channel i want?

my other thought about my issue is that it isn't being routed properly and it is taking the samples off an input that is not connected to anything. the thing is i can't figure out, in the OS, which port it thinks it is connected to. i think that is too deep into the hardware for linux to have an idea about that.

thanks for any help you can give

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Answers

  • Posts: 368

    By default ALSA has all channels muted. Those have to be unmuted manually.

    Source - https://wiki.archlinux.org/index.php/Advanced_Linux_Sound_Architecture

  • Posts: 4

    so here is more information:

    when i do "amixer" i get:
    ...
    Simple mixer control 'Left PGA Mixer Line1L',0
    Capabilities: pswitch pswitch-joined
    Playback channels: Mono
    Mono: Playback [on]
    ...

    which, if "Mono: Playback" means the samples are being collected is right

    and

    ...
    Simple mixer control 'Right PGA Mixer Line1L',0
    Capabilities: pswitch pswitch-joined
    Playback channels: Mono
    Mono: Playback [off]
    Simple mixer control 'Right PGA Mixer Line1R',0
    Capabilities: pswitch pswitch-joined
    Playback channels: Mono
    Mono: Playback [on]
    Simple mixer control 'Right PGA Mixer Line2R',0
    Capabilities: pswitch pswitch-joined
    Playback channels: Mono
    Mono: Playback [off]
    ...

    which looks like it is pulling from Line1R, which is wrong

    even though in my DTS file i have:

    simple-audio-card,widgets =
    "Speaker", "POTS Line Out",
    "Speaker", "Handset Speaker",
    "Line", "POTS Line In",
    "Microphone", "Handset Mic";
    simple-audio-card,routing =
    "POTS Line Out", "MONO_LOUT",
    "Handset Speaker", "LLOUT",
    "LINE2R", "POTS Line In",
    "LINE1L", "Handset Mic",
    "Handset Mic", "Mic Bias";

    it would appear to me it is pulling the samples in from the wrong port

    so when i do:
    amixer set 'Right PGA Mixer Line1R' off

    amixer set 'Right PGA Mixer Line2R' on

    and then do an arecord, i get the audio i was looking for. so it is routing the audio incorrectly

    hopefully somebody can help me with this: in code (c) is there a way to do the input switching like above? i have been using snd_mixer_* commands but i am not sure how to use something like that to achieve what i want. or am i stuck using "system" calls?

  • Posts: 4

    well i might have figured it out so nevermind:

    /****************************************************************************************
    * found @ https://alsa.opensrc.org/HowTo_access_a_mixer_control
    */
    void SetInputToLine2R(void)
    {

    1. const char *card = "default";
    2. const char *selem_name_line1r = "Right PGA Mixer Line1R Switch";
    3. const char *selem_name_line2r = "Right PGA Mixer Line2R Switch";
    4. int err;
    5.  
    6. snd_hctl_t *hctl;
    7. err = snd_hctl_open(&hctl, card, 0);
    8. err = snd_hctl_load(hctl);
    9.  
    10. snd_ctl_elem_id_t *id;
    11. snd_ctl_elem_id_alloca(&id);
    12. snd_ctl_elem_id_set_interface(id, SND_CTL_ELEM_IFACE_MIXER);
    13. //snd_ctl_elem_id_set_id(id, 1);
    14.  
    15. snd_ctl_elem_id_set_name(id, selem_name_line1r);
    16. snd_hctl_elem_t *elem = snd_hctl_find_elem(hctl, id);
    17.  
    18. if(elem != 0)
    19. {
    20.     snd_ctl_elem_value_t *control;
    21.     snd_ctl_elem_value_alloca(&control);
    22.     snd_ctl_elem_value_set_id(control, id);
    23.  
    24.     snd_ctl_elem_value_set_integer(control, 0, 0);
    25.     err = snd_hctl_elem_write(elem, control);
    26.  
    27.     //turn on line2r
    28.     snd_ctl_elem_id_set_name(id, selem_name_line2r);
    29.     elem = snd_hctl_find_elem(hctl, id);
    30.  
    31.     if(elem != 0)
    32.     {
    33.         snd_ctl_elem_value_set_integer(control, 0, 1);
    34.         err = snd_hctl_elem_write(elem, control);
    35.     }
    36. }
    37. snd_hctl_close(hctl);

    }

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