[alsa-devel] Wrote MIDIcat program, to connect ALSA MIDI to standard input/output

[Josh Lehan]

Hi!  A few months ago, I posted about a program to send untimed MIDI
data to the ALSA sequencer.  Using amidi(1) wasn't acceptable, since it
only talks to "RawMIDI" devices.  Using aplaymidi(1) wasn't acceptable
either, because it required .MID files, and I simply wanted to send
untimed MIDI data, similar to what would be produced if you hooked up a
hardware MIDI keyboard and just started playing.

I needed something more general, that would simply connect to the ALSA
sequencer, and let me send arbitrary MIDI bytes to a sequencer
destination of my choice.  Unfortunately, no program existed that would
simply do this.

I'm pleased to announce that I've written such a program, It's called
MIDIcat, as it works on the same principle as "cat" and other similar
programs (Netcat, and so on).

It hooks up standard input, and standard output, to the ALSA sequencer.

This makes it easy to pipe data around.

It's an ALSA sequencer client.  Any data that is received on standard
input will be forwarded into ALSA.  You can choose another client as a
destination, or you can just start the program passively and use ALSA's
"subscription" mechanism to route the data later.

Similarly, any MIDI data that comes from ALSA, will be forwarded along,
and provided on standard output.

This makes it easy to make many small "one-liner" command lines that are
useful for testing and playing around with MIDI data in general.

As an example, this plays Middle C:

echo "90 3C 7F" | midicat --hex --port "TiMidity"

The --hex option makes things more human-readable: standard input and
output are hex digits, separated by spaces, instead of just raw binary data.
The --port option has the usual ALSA sequencer meaning.  Change it to
target your synthesizer (I'm using the TiMidity softsynth).

Here's another example for output (both of these commands block, so run
them in two separate terminal windows):

midicat --verbose --hex
vkeybd --addr 129:0

The --verbose option prints out some more useful information at startup,
such as the ALSA sequencer client:port number it got.  That can later be
passed into another program, such as vkeybd.  If you play notes with
vkeybd, you see the corresponding MIDI data appear on standard output.

Standard input and standard output can both run at the same time (it's
multithreaded).

Here's some more elaborate examples:

MIDI "panic button", sends "All Sound Off" to each channel:

perl -e 'use bytes;for($i=0;$i<16;$i++){print
chr(176+$i).chr(120).chr(0);}' | midicat --port "TiMidity"

Another MIDI "panic button", sends individual "Note Off" commands:

perl -e 'use bytes;for($i=0;$i<16;$i++){for($j=0;$j<128;$j++){print
chr(128+$i).chr($j).chr(127);}}' | midicat --delay 10 --port "TiMidity"

The --delay option inserts a slight delay between each MIDI command, to
avoid flooding ALSA with too much data at once.

Here's a fun one, sending random data into the MIDI subsystem and seeing
what happens.  DO NOT run this on a real synthesizer, there's the risk
of composing a random SysEx command that could mess up your settings!

It's fun to watch with aseqview(1):

aseqview &
cat /dev/urandom | midicat --delay 500 --port "MIDI Viewer"

If you sent random data to a device that plays audio, you'll probably
next want to use the "panic button" commands above...!

Hope this illustrates some of the examples and possibilities.  I'm
working on writing some more documentation.

The --help option is supported, there's a help output that shows you all
of the available options.

For now, the program is available here, on my Web server:

http://krellan.com/midicat

If you're interested, try it, and let me know what you think!

Josh Lehan