Alsa Equalizer HOWTO

[jonathanslenders]

This is a quick tutorial that explains how to equalize the 4 channels in software using an ALSA equalizer plugin.
It works by renaming the alsa outputs ch0..ch3 into ch0noeq..ch3noeq, then we add four equalizers which inputs are again ch0..ch3, so none of the Amplipi software has to be adjusted.

1. Connect to your Amplipi device over SSH.

2. Install the plugin:

sudo apt-get install -y libasound2-plugin-equal

3. Update /etc/asound.conf:

# AmpliPi ALSA config
# Place in /etc/asound.conf

# Default to playing to source 0
pcm.!default {
    type            plug
    slave.pcm       "ch0"
}

# Use softvol since the DAC doesn't have volume support
pcm.i2sdac {
    type            softvol
    slave.pcm       "hw:sndrpihifiberry,0"
    control.name    "Master Playback Volume"
    control.card     sndrpihifiberry

    # Set the max_dB to avoid clipping on the negative side of the preamp's
    # U2/U4/U6/U8 op-amps. The audio signal is centered at 2.5 V and the
    # op-amps cleanly pass down to ~1.4V, so max (2.5 - 1.4) / sqrt(2)
    # = 0.78 Vrms. Nominal (max_dB = 0) DAC output is ~1.7 Vrms.
    # 20*log10(0.78/1.7) ~= -6.8 dBV
    max_dB          -6.8

    # For now leaving at -6.8 dB to maximize SNR before the amplifiers,
    # and to leave some headroom for per-zone attenuation settings.
    # If clipping of the amplifiers is to be fully avoided, reduce further:
    # Max amp power output: 79 W @ 4 Ohms
    # Max amp Vrms = sqrt(79*4) = 17.8 V = 25 dBV
    # Amp gain = 32.8 dBV
    # Max amp input = 25 dBV - 32.8 dBV = -7.8 dBV = 0.41 Vrms
    # max_dB = 20*log10(0.41/1.7) = -12.4

}

# Create source 0 which streams to the softvol plugin.
# Use type plug to autonegotiate connection bitrates/mono/stereo.
pcm.ch0.noeq {
    type            plug
    slave.pcm       "i2sdac"
}

ctl.ch0 {
    type            hw
    card            sndrpihifiberry
}

# CM6206 USB audio DAC/ADC
pcm.usb71 {
    type            hw
    card            cmedia8chint
}

ctl.usb71 {
    type            hw
    card            cmedia8chint
}

# Only 1 client can use 'usb71' directly at a time.
# The dmix plugin allows multiple client connections at a time and mixes them.
pcm.dmixer {
    type            dmix
    ipc_key         1024
    ipc_perm        0666
    slave {
        pcm         "usb71"
        period_time 0
        period_size 1024
        buffer_size 4096
        channels    8
    }
    bindings {
        0 0
        1 1
        2 2
        3 3
        4 4
        5 5
        6 6
        7 7
    }
}

# Create sources 1-3 as individual stereo streams
# Attenuate to 64% before sending to mixer to avoid op-amp clipping
#   Attenuate to 28% to avoid amplifier clipping
#   For now leaving at 64% to maximize SNR before the amplifiers, and to leave
#   some headroom for per-zone attenuation settings
pcm.ch1noeq {
    type            plug
    slave.pcm       "dmixer"
    slave.channels  8
    ttable.0.6      0.64
    ttable.1.7      0.64
}
pcm.ch2noeq {
    type            plug
    slave.pcm       "dmixer"
    slave.channels  8
    ttable.0.0      0.64
    ttable.1.1      0.64
}
pcm.ch3noeq {
    type            plug
    slave.pcm       "dmixer"
    slave.channels  8
    ttable.0.4      0.64
    ttable.1.5      0.64
}

# Duplicate a mono stream to stereo for testing, since
# `speaker-test` only plays one mono output at a time.
pcm.ch0m2s {
    type            route
    slave.pcm       "ch0"
    slave.channels  2
    ttable.0.0      1
    ttable.0.1      1
}

# Equalizer 0.
ctl.ch0equal {
  type equal;
  controls /home/pi/.alsaequal-ch0.bin;
}

pcm.ch0plugequal {
  type equal;
  slave.pcm "ch0noeq";
}

pcm.ch0 {
  type plug;
  slave.pcm ch0plugequal;
  controls /home/pi/.alsaequal-ch0.bin;
}

# Equalizer 1.
ctl.ch1equal {
  type equal;
  controls /home/pi/.alsaequal-ch1.bin;
}

pcm.ch1plugequal {
  type equal;
  slave.pcm "ch1noeq";
  controls /home/pi/.alsaequal-ch1.bin;
}

pcm.ch1 {
  type plug;
  slave.pcm ch1plugequal;
}

# Equalizer 2.
ctl.ch2equal {
  type equal;
  controls /home/pi/.alsaequal-ch2.bin;
}

pcm.ch2plugequal {
  type equal;
  slave.pcm "ch2noeq";
  controls /home/pi/.alsaequal-ch2.bin;
}

pcm.ch2 {
  type plug;
  slave.pcm ch2plugequal;
}

# Equalizer 3.
ctl.ch3equal {
  type equal;
  controls /home/pi/.alsaequal-ch3.bin;
}

pcm.ch3plugequal {
  type equal;
  slave.pcm "ch3noeq";
  controls /home/pi/.alsaequal-ch3.bin;
}

pcm.ch3 {
  type plug;
  slave.pcm ch3plugequal;
}

4. Reload your alsa configuration:

alsactl restore   

5. Adjust the equalizers (type any of the following commands to adjust the corresponding equalizer):

alsamixer -D ch0equal
alsamixer -D ch1equal
alsamixer -D ch2equal
alsamixer -D ch3equal

I'm still looking in alternative ways to equalize. The above approach won't allow equalization of individual rooms if the same source is played in multiple rooms. A hardware equalizer for all of the output channels would be nice, but so far, with the current hardware, there is no easy way to inject an external equalizer.

edit: There is still something wrong with the above config. It doesn't work for ch0, but works fine for ch1..ch3.

[jonathanslenders]

I've been exploring using the dsp project as an equalizer: https://github.com/bmc0/dsp It's more powerful, but slightly more complex to configure.

Quick tutorial:

Install the required packages

sudo apt install libasound2-dev libmad0-dev libavcodec-dev ladspa-sdk  libavutil-dev libavformat-dev libfftw3-dev libao-dev          

Clone the dsp project:

git clone [email protected]:bmc0/dsp.git

Compile and install dsp: make && make install. See the README: https://github.com/bmc0/dsp

Edit /etc/asound.conf in the following way:

• Replace pcm.ch0 into pcm.ch0noeq
• Add the following:

pcm.ch0 {                                                                                                                                                                                                                                   
    type plug                                                                                                                                                                                                                               
    slave {                                                                                                                                                                                                                                 
        format FLOAT                                                                                                                                                                                                                        
        rate unchanged                                                                                                                                                                                                                      
        channels unchanged                                                                                                                                                                                                                  
        pcm {                                                                                                                                                                                                                               
            type ladspa                                                                                                                                                                                                                     
            path "/usr/lib/ladspa"                                                                                                                                                                                                          
            playback_plugins [{                                                                                                                                                                                                             
                label "ladspa_dsp:ch0"                                                                                                                                                                                                          
            }]                                                                                                                                                                                                                              
            slave.pcm {                                                                                                                                                                                                                     
                type plug                                                                                                                                                                                                                   
                slave {                                                                                                                                                                                                                     
                    pcm "ch0noeq"                                                                                                                                                                                                           
                    rate unchanged                                                                                                                                                                                                          
                    channels unchanged                                                                                                                                                                                                      
                }                                                                                                                                                                                                                           
            }                                                                                                                                                                                                                               
        }                                                                                                                                                                                                                                   
    }                                                                                                                                                                                                                                       
} 

Now create a config file for this EQ. If label in the above section corresponds to ladspa_dsp, then the EQ config goes in /etc/ladspa_dsp/config. If label is ladspa_dsp:ch0, then the config goes in /etc/ladspa_dsp/config_ch0. In this file, paste the following:

input_channels=1
output_channels=1
LC_NUMERIC=C
effects_chain=@/etc/ladspa_dsp/ch0_eqsettings

Now we can store the EQ settings in /etc/ladspa_dsp/ch0_eqsettings (according to the above config). In this file, we have to paste something like this (See github documentation for more information: https://github.com/bmc0/dsp ):

gain      -0.5
eq        400   1.0q  +4.0

The same can be done for ch1, ch2 and ch3

When done, run alsactl restore
If an Amplipi stream is playing, and the EQ settings were changed, you have to stop/restart the stream.

[linknum23]

This would be great to integrate into AmpliPi, something we will definitely consider in the future.