wanderings/content/posts/midi-on-rpi.md

+++
title = "Arch on a Raspberry Pi - sound and MIDI"
date = 2020-09-26
tags = ["admin"]
toc = true
+++
This post follows a first one on [basics of Arch onto a Raspberry
Pi][arch-basics]. Now we will see how to have high quality, low latency sound
and a MIDI keyboard on this beautiful but still useless system.

## MIDI keyboard with JACK

To reliably output sound using a MIDI keyboard and have a reasonable latency,
consider dropping pulseaudio, installing and setting up JACK, and setting up
fluidsynth to use JACK.

### Disable pulseaudio's autostart with systemd
Run as the relevant user :

  $ systemctl --user mask pulseaudio.socket

### Setup JACK

First install the `jack2` package, and enable it at login with :

    $ systemctl --user enable jack@lowlatency.service

The `@` argument sets a JACK configuration file that will be searched in the
following places, first match wins :

    ~/.config/jack/lowlatency.conf
    /etc/jack/lowlatency.conf

If you use a DAC, drop a second conf file in `~/.config/jack/MYDAC.conf`.  For
instance HifiBerry-style DACs are identified as device `hw:sndrpihifiberry`.
In this case create `~/.config/jack/hifiberry.conf` as follows, and instead of
the former, enable `jack@hifiberry.service` :

    # The name of the JACK server
    JACK_DEFAULT_SERVER="default"
    # Options to JACK (e.g. -m, -n, -p, -r, -P, -t, -C, -u, -v)
    JACK_OPTIONS=""

    # Audio backend (e.g. alsa, dummy, firewire, netone, oss, portaudio)
    DRIVER="alsa"
    # Device name (used by the audio backend) - defaults to "hw:0"
    DEVICE="hw:sndrpihifiberry"

    # Specific settings for the audio backend in use
    DRIVER_SETTINGS="-n2 -p128 -r48000"

Note that `JACK_OPTIONS` is empty ; all the useful options go in
`DEVICE_SETTINGS`. I suppose it's a bug of the JACK service file.

More about DACs at [the end of the post](#use-an-external-sound-card).

### Setup fluidsynth

Install the `fluidsynth` package ; it comes with a default user service that
will connect to the JACK service if you tell it to in its conf file
`~/.config/fluidsynth`. First do :
```
$ systemctl --user enable fluidsynth
```
You will need a soundfont in `/usr/share/soundfonts/default.sf2`. I actually
made it a symbolical link to a soundfont file in my home dir. If you're
desperate about soundfonts install the `soundfont-fluid` package.

JACK2 is said to need package `a2jmidid` to detect MIDI keyboards. I've noticed
that in fact `fluidsynth` was very capable of detecting them on its own with the
option `-o midi.autoconnect=1`, be it at boot or hotplugged.

A useful feature is that it can read a command file with the -f option ; an
arguably preferable strategy is to create and load one in

    /usr/share/soundfonts/fluidsynth.command

Finally my `~/.config/fluidsynth` reads as follows :

    # Mandatory parameters (uncomment and edit)
    SOUND_FONT=/usr/share/soundfonts/default.sf2

    # Additional optional parameters (may be useful, see 'man fluidsynth' for further info)
    # -C0 disables chorus
    # -R0 disables reverb
    # -g0.2 would be the default gain
    # -m can be jack, alsa, alsa_seq or raw ? jack should be more efficient
    # -K number of MIDI channels (minimum 16)
    # -f command file input
    OTHER_OPTS="-a jack -j -f /usr/share/soundfonts/fluidsynth.command -o midi.autoconnect=1 -o synth.polyphony=64"

Its systemd service's default target used to be "multi-user" whereas it should
have been "default" (bug resolved as of version 2.1.5-2). One can now enable it
with
On older versions of the program, you had to do instead : `systemctl --user
add-wants default fluidsynth`

## Autostart

If you want the systemd user services started at boot do _as root_ :
```
# loginctl enable-linger YOURUSERNAME
```

## Deal with latency

To optimise latency, you will want to use a realtime process for jackd.

You can follow [instructions for Arch][aw-instr] to have the right to make your
process prioritary on system tasks. Eventually, you should belong to the
`realtime` group. After that jackd should detect it's able to run a realtime
process and proceed. You can check that it does with
```
journalctl --user -u jack@hifiberry -e
```
(replace `hifiberry` with the config name you gave at the beginning of this
post). The `-e` option jumps at the end of the logs.

I was only able to use the alsa-seq MIDI plugin with JACK, but some people
report more effectiveness with jack's own plugin. You can check [Archwiki's
JACK page][aw-jack-midi] for detailed instructions.

Another [Archwiki page on professional audio][aw-pro-audio] mentions a
`-Xalsarawmidi` option in JACK used with a2jmidid to enhance MIDI jitter.

## Use an external sound card

For Phat DACs, follow these [instructions][dac-instr]. What's a Phat ? It's a
small cardboard that fits above a Raspberry Pi, plugged into its IO pins. And a
DAC ? Digital to Analog Converter - in other words, an audio card.


[arch-basics]: {{< relref arch-on-rpi >}}
[aw-instr]: https://wiki.archlinux.org/index.php/Realtime_process_management#PAM-enabled_login
[aw-jack-midi]: https://wiki.archlinux.org/index.php/JACK_Audio_Connection_Kit#MIDI
[aw-pro-audio]: https://wiki.archlinux.org/index.php/Professional_audio#MIDI
[dac-instr]: https://www.hifiberry.com/docs/software/configuring-linux-3-18-x/