Support for ALSA PCM softvol plugin

[joerg-krause]

I am trying to setup shairport-sync with an ALSA PCM softvol plugin.

asound.conf:

pcm.!default {
      type plug
      slave.pcm "softvol"
  }

pcm.softvol {
     type softvol
      slave {
          pcm "hw:0"
      }
      control {
          name "Master"
          card 0
      }
     min_dB -57.2
     max_dB -6.2
}
ctl.!default {
        type hw
        card 0
}
ctl.softvol {
        type hw
        card 0
}

shairport-sync.conf:

alsa =
{
  output_device = "softvol";
  mixer_control_name = "Master";
};

Running shairport-sync -vvv:

Version: "2.8.3-openssl-Avahi-ALSA-stdout-pipe-metadata"
[...]
Output device name is "softvol".
Open Mixer
Mixer device name is "softvol".
Mixer Control name is "Master".
note: the hardware mixer specified -- "Master" -- does not have a dB volume scale, so it can't be used.
[...]

Looking into audio_alsa shows that snd_mixer_selem_get_playback_dB_range() returns an error code. This is all fine when using a hardware mixer.

However, with an ALSA software mixer, I can get the dB range using snd_ctl_get_dB_range(). e.g.:

min dB: -57.2
max dB: -6.2

What do you think about an option like mixer_type as the MPD audio output configuration has?

Is there any drawback using the softvol plugin for volume setting?

[mikebrady]

Hi Jörg-Krause. Thanks for the post. TBH, I have not considered the use of any plug-ins in Shairport Sync. The reason is that i don't know enough about them. In particular, I envisage them as standing between the output of Shair[port Sync and the DAC. Shairport Sync ask the output device for its latency every 8 milliseconds or so, and i don't know if a plug-in will report latency correctly or at all. I've been too concerned with improving the reliability and ease-of-use of Shairport Sync in other areas to look at the plug-ins world.

If you don't specify a mixer name, then Shairport Sync uses the software volume control in Shairport Sync itself. In any case, you could disable the volume control in the Shairport Sync setting and have its output directed to the plug-in at 100% level.

After that, it really depends on the plug-in. If it reports latency correctly, then we are in business; otherwise it's going to lose sync.

[joerg-krause]

Many thanks for the feedback! What do you mean by:

If you don't specify a mixer name, then shairport-sync assumes So, you could disable the volume control in the Shairport Sync setting...

[joerg-krause]

@mikebrady I managed to get shairport-sync using the softvol control. Unfortunately, I have a problem with vol2attn. For an airplay volume of -6.2dB to 0.0dB the scaled attenuation is fixed to -12.4dB and the amixer volume slider is not set above 88%. Note, that my max dB value is set to -6.2:

Volume min -5720dB, max -620dB, range 5100dB.
On line 0, end point of -2550.000000, input vol -5.625000 yields output vol -478.125000.
On line 1, end point of -3825.000000, input vol -5.625000 yields output vol -95.625000.
returning an attenuation of -1240.000000.

Volume min -5720dB, max -620dB, range 5100dB.
On line 0, end point of -2550.000000, input vol -3.750000 yields output vol -318.750000.
returning an attenuation of -1240.000000.

Volume min -5720dB, max -620dB, range 5100dB.
On line 0, end point of -2550.000000, input vol -1.875000 yields output vol -159.375000.
returning an attenuation of -1240.000000.

[mikebrady]

Very interesting. Let me check that out.

[joerg-krause]

I opened a pull request for testing: #293.

[mikebrady]

Yep, thanks for that!

[joerg-krause]

Many thanks for merging and making a release! Did you had the chance to look at the scaled attenuation problem with vol2attn? I am suspicious about this line, as it sets vol_setting = (-6.2) + (-6.2) = -14.2. However, you are the expert here 😄

[mikebrady]

You're welcome! I haven't had a look at the vol2attn thing – there were more pressing issues to examine, IMHO. Real Soon Now, promise!

[joerg-krause]

I think I have found the issue. The transfer functions start and end points does not take the max dB value as an offset into account. The y-axes ranges always from 0 to (max - min) dB, where it should range from max to max - (max - min) dB.

I make some tests now and propose a PR later.

[mikebrady]

That sounds plausible alright, Jörg-Krause.

[joerg-krause]

Yes, it does. However, I was wrong as the max_dB offset is added to the volume value later in https://github.com/mikebrady/shairport-sync/blob/development/common.c#L464. The only mistake is to initialize vol_setting with not zero. Now, I got a correct volume attenuation within -6.2 and -57.2 dB.

[joerg-krause]

Note, that the ALSA softvol plugin in version 1.1.1 and below is affected by a bug which may result in wrong dB values. This bug has been fixed: http://git.alsa-project.org/?p=alsa-lib.git;a=commit;h=85bf9915989e0a338632739684c75192c1753239.

[ntwallace]

@joerg-krause can you provide any advice on how you got softvol working? I'm still stuck with the dB vol scale error, despite setting a min_dB and max_dB

[joerg-krause]

Hi @ntwallace, basically I am using the configuration settings as said in the first post of this thread.

What is exactly the error message?

What are your settings for asound.conf and shairport-sync.conf?

What ALSA version does you're system provide?

[ntwallace]

Thanks for the quick response @joerg-krause. I'm getting the similar error about my mixer not having a dB volume scale.

FYI my hardware setup is a Rpi Zero with a Phat DAC attached (which doesn't do hardware volume), so I am using a combo of dmix and softvol to control volume via software.

The ALSA version is 1.1.3 -- I guess at some point they made some changes to the architecture so that softvol is implemented by the .asoundrc file in root instead of asound.conf in /etc/.

shairport-sync -v error:

...
alsa output device name is "hw:0".
note: the hardware mixer specified -- "Master" -- does not have a dB volume scale, so it can't be used. Trying software volume control.
Volume control "Master" has dB volume from -50.500000 to -7.150000.
...

shairport-sync.conf code:

general =
{
        name = "Pi Music Box";
        volume_range_db = 60 ;
};
...
alsa =
{
    output_device = "hw:0";
    mixer_control_name = "Master";
};
...

asound.conf code:

pcm.!default  {
 type hw card 0
}

ctl.!default {
 type hw card 0
}

.asoundrc code:

pcm.!default {
    type           hw card 0
    slave.pcm      "softvol"   #make use of softvol
}

pcm.softvol {
    type            softvol
    slave {
        pcm        "dmix"      #redirect the output to dmix (instead of "hw:0,0")
    }
    control {
        name       "Master"       #override the PCM slider to set the softvol volume level globally
        card        0
    }
    min_dB -50.5
    max_dB -7.5
}

[mikebrady]

Just wondering if the very latest fix on the development branch is relevant at all — perhaps @joerg-krause has a view?

[ntwallace]

@mikebrady Can you share the 2 missing lines of code for the v3.2d8 of audio_alsa.c and I can test? Or should I compile the dev branch and update to test?

[joerg-krause]

@ntwallace You're configuration is a bit messy. First, shairport-sync is not configured to use the softvol plugin, but the hardware device. Second, your softvol plugin uses a dmix plugin as pcm slave, but no dmix is configured in your ALSA config.

For the beginning, please try this:

Replace the alsa stanza in shairport-sync.conf:

alsa =
{
    output_device = "softvol";
    mixer_control_name = "Master";
};

Replace the .asoundrc code:

pcm.softvol {
    type            softvol
    slave {
        pcm        "hw:0"
    }
    control {
        name       "Master"
        card        0
    }
}
ctl.softvol {
    type hw
    card 0
}

If it works, you can add the min_dB and max_dB settings. Note, that ALSA disables dB settings for soft-float targets.

[ntwallace]

Thanks @joerg-krause, good point.

I actually tried that at first, but was getting an error. When I update both to those, shairport-sync -v gives me:

alsa output device name is "softvol".
ALSA lib control.c:1373:(snd_ctl_open_noupdate) Invalid CTL softvol
fatal error: Failed to attach mixer
Request to shut down all rtsp conversation threads
asking playing threads to stop

I was able to get shairport-sync to load by updating asound.conf to include a ctl.softvol:

pcm.!default  {
 type hw card 0
 slave.pcm "softvol"  #added softvol as slave 
}

ctl.!default {
 type hw card 0
}

ctl.softvol {  #added ctl for softvol
    type hw
    card 0
}

However, shairport-sync -v still gives me the same dB error:

alsa output device name is "softvol".
note: the hardware mixer specified -- "Master" -- does not have a dB volume scale, so it can't be used. Trying software volume control.
Volume control "Master" has dB volume from -50.500000 to -7.150000.

Adding code for pcm.softvol directly into to asound.conf doesn't change the above error:

pcm.softvol {
    type            softvol
    slave {
        pcm         "hw:0"      #redirect the output to dmix (instead of "hw:0,0")
    }
    control {
        name        "Master"       #override the PCM slider to set the softvol volume level globally
        card        0
    }
    min_dB -50.5
    max_dB -7.5
}

[mikebrady]

Nick, I'd say it's probably best to move to the development branch. For one thing, in the event that any changes are needed, they'll go into the development branch.

[joerg-krause]

@ntwallace Actually, it isn't an error message, but a note that softvol plugin is been used by shairport-sync. My guess is that your ALSA config is still wrong.

This stanza at leadt is still wrong:

pcm.!default  {
 type hw card 0
 slave.pcm "softvol"  #added softvol as slave 
}

Replace it by:

pcm.!default  {
 type plug
 slave.pcm "softvol"  #added softvol as slave 
}

Furthermore, maybe some config options in your system wide alsa config file /etc/asound.conf does collide with your user alsa config file .asoundrc. Please post the content of both and a full log of shairport-sync.

Does aplay -D softvol <somewavfile> work?