[How to do instruction] 128x64 OLED with Song Status and Spectrum Display on Raspberry Pi

[adrii]

Hi Tim

I know very little about ALSA, what I know is just from trying to duplicate a stream!

The loopback is handled by the snd_aloop driver.

There are three configuration files in the mpd_oled issue, but it is worth reviewing all of them

My Moode config: https://github.com/antiprism/mpd_oled/is...-567146826

In my Moode /etc/mpd.conf file, mpd is outputing to "hw:0,0". I don't know how to overload "hw:0,0" (I am not sure if it can be done) but I can overload "hw", and this is what the configuration file does. It means that whenever Moode/mpd writes to hw:X or hw:X,Y the overload will be used and a copy sent to the loopback device. It may have consequences, because it is used whenever "hw" is used.

Sergey's Volumio config: https://github.com/antiprism/mpd_oled/is...-536098398

I think when Volumio uses hardware volume it outputs to the "default" ALSA device. This can be overriden. However, the original output device is not known, and so needs to be specified, in this case "hw:IQaudIODAC,0"

My Volumio config: https://github.com/antiprism/mpd_oled/is...-423255641

Using Volomio software volume the system set up an alsa config file with pcm.softvolume that called pcm.softvol. I changed pcm.softvolume to call  my duplication config plugin, which then called the origional pcm.softvol.


I think the last approach is the way to go. When Moode wants to play something it would always play it to the same named ALSA plugin, e.g. "playerout". When Moode configures a device, it wouldn't refer to a "hw" name directly, but configure it in a plugin, say "deviceout". Then, in a global configuration file "deviceout" is a slave of "playerout", so anything that plays to "playerout" is heard on the "deviceout" device.

Now, if anybody wants to do anything with the audio before it is played they can override "playerout" and have "deviceout" as a slave, and the processed audio will go to the configured device.

I'll give an example. On Ubuntu I have created a global ALSA config /etc/asoundrc

Code:

pcm.playerout {
    type plug
    slave.pcm "deviceout"
}

pcm.deviceout {
    type plug
    slave.pcm "hw:1"
}

I can now play some audio to the "playerout" device and it comes out of my speakers

Code:

aplay -D playerout tmp.wav

If I want to duplicate the audio, I can simply write an .asoundrc file to override "playerout", and send the audio to be played on to "deviceout" and I don't need to know the hardware device name

Code:

pcm.!playerout {
    type plug
    slave.pcm {
        type multi
        slaves {
            a { channels 2 pcm "deviceout" }  # the real device
            b { channels 2 pcm "plughw:Loopback,0" }  # the loopback driver
        }
        bindings {
            0 { slave a channel 0 }
            1 { slave a channel 1 }
            2 { slave b channel 0 }
            3 { slave b channel 1 }
        }
    }
    ttable [
        [ 1 0 1 0 ]   # left  -> a.left,  b.left
        [ 0 1 0 1 ]   # right -> a.right, b.right
    ]
}

I can now play audio as before, with the sound coming out the speakers

Code:

aplay -D playerout tmp.wav

In a separate terminal I can now show a spectrum analyser for the audio with cava by creating a config file conf.txt

Code:

[input]
method = alsa
source = hw:Loopback,1

And running cava with this configuration

Code:

cava -p conf.txt

   

For Moode you could probably specify a Moode-only config file in /usr/share/alsa/alsa.conf, then the user overrides could be put in /etc/asound.conf as now. Moode could create some duplicate streams in the UI with the same kind of configuration override, playerout -> moode processing/duplicates -> moodeout -> user processing/duplicates -> deviceout (the user would override moodeout in this chain). But even if Moode doesn't provide anything in the UI, the ALSA config file for creating duplicates is now much easier to write and should be robust over different audio configurations.

Adrian.