Raspberry-Convolution

Recently i stumbled over this YouTube video which shows how to measure your listening room and make an impulse response file with "REW" to use it with Equilizer APO on a PC for real time room correction.

https://youtu.be/5YcH7j2-L1Y

REW (Room EQ Wizard) https://www.roomeqwizard.com

Equalizer APO https://sourceforge.net/projects/equalizerapo

It worked that great that i wanted the correction also for my TV which is connected to the amp via digital optical cable. The idea was to use a Raspberry Pi with the Hifiberry DIGI+ board to plug it between the TV and the amp and let it run the convolution filter in real time. After trying around a few days i came up with the following solution:

Preparing Rapsberry Pi 3B

With "Raspberry Pi Manager" i installed Raspberry Pi OS Lite 64 (kernel 5.15.84-v8+) including the WiFi settings on the SD card.

Activate HifiBerry DIGI+

sudo nano /boot/config.txt

add the follwing lines to load the DIGI+ driver:

# load HifiBerry DIGI+
force_eeprom_read=0
dtoverlay=hifiberry-digi

list sound cards:

cat /proc/asound/cards

Configure alsa:

sudo nano /etc/asound.conf

add the following lines with the number of your card

pcm.hifiberry {
  type hw card 1  
}
ctl.hifiberry {
  type hw card 1
}

list alsa devices:

aplay -L

list record devices:

arecord -l

play music:

flac -c -d hello.flac | aplay -D hifiberry

Install bmc0 dsp:

https://github.com/bmc0/dsp

download and unzip source:

wget https://github.com/bmc0/dsp/archive/refs/tags/v1.9.zip
unzip v1.9.zip
rm *.zip

install dependencies:

sudo apt-get install -y libtool libzita-convolver-dev libasound2-plugins libasound2-dev libfftw3-dev 
sudo apt-get install -y libsndfile-dev ladspa-sdk libzita-alsa-pcmi0 libzita-alsa-pcmi-dev alsa-tools zita-alsa-pcmi-utils

configure and make:

cd dsp*
./configure
make
sudo make install

test dsp:

dsp hello.flac -o hifiberry highpass 1.8k 0.5

Test convolver (ir44.wav is the impulse response file exported form REW):

dsp hello.flac -o hifiberry -e s24 zita_convolver ir44.wav

Loopback with dsp:

dsp -c 2 -e s16 -r 48000 -t alsa hifiberry -o hifiberry zita_convolver ir48.wav

Start at boot:

sudo nano /etc/systemd/system/dsp.service

content for new systemd config file:

[Unit]
Description=dsp for living room speakers

# don`t ever stop restarting this service
StartLimitIntervalSec=0

[Service]
WorkingDirectory=/home/henry

# check if there is some sound incoming, othersie don`t start dsp to prevent any unwanted output
ExecStartPre=arecord -f S24_LE -c 2 -r 48000 -D hifiberry -s 1

# start the dsp as a loop back device
ExecStart=sudo nice --19 dsp -c 2 -e s16 -r 48000 -t alsa hifiberry -o hifiberry zita_convolver ir48.wav

# restart this service in any case, even on exit 0 success
Restart=always

# restart this service after 3 seconds
RestartSec=3s

[Install]
WantedBy=multi-user.target

Install new service:

sudo systemctl enable dsp

Start new service:

sudo systemctl start dsp

after making changes to /etc/systemd/system/dsp.service:

sudo systemctl daemon-reload
sudo systemctl restart dsp

I had to shorten the impulse response wave file a lot otherwise the latency was too long. The zita_convolver should have a low latency even with longer impulse response files, but that didn`t work out that way. So i kept only about 100ms of the original wave file. (used Audacity)

Install dsp as alsa audio device

this makes it possible to play audio with other Raspberry Pi software through the dsp

create the dsp configuration file

sudo mkdir /etc/ladspa_dsp 
sudo nano /etc/ladspa_dsp/config_living_room_speakers

add these lines:

input_channels=2
output_channels=2
LC_NUMERIC=C
effects_chain=zita_convolver /etc/ladspa_dsp/ir48.wav

copy impulse response file to the configuration path:

sudo cp ir48.wav /etc/ladspa_dsp

setup the alsa configuration:

sudo nano /etc/asound.conf

add the following:

# output to "dsp" for convolved output to hifiberry
# first use type plug to convert to format FLOAT (ladspa can only process FLOAT)
pcm.dsp {
  type plug
  slave {
    format FLOAT
    pcm convolver
  }
  hint {
    show on
    description "Living Room Speakers Convolver."
  }
}

# the actual convolver
pcm.convolver {
  type ladspa
  channels 2
  path "/usr/lib/ladspa"
  playback_plugins [{
    label "ladspa_dsp:living_room_speakers"
  }]
  slave.pcm digital_out
}

# use plug to convert FLOAT back to S24_LE (HifiBerry DIGI+ can only output S16_LE or S24_LE)
pcm.digital_out {
  type plug
  slave {
    format S24_LE
    pcm hifiberry
  }
}

Test own audio device:

flac -c -d hello.flac | aplay -D dsp