MIDIplex is a utility for volume-aware distributing a polyphonic MIDI stream into multiple output streams. The output streams are guaranteed to be monophonic. Output streams are allocated to a note in proportion to that note's velocity, relative to the velocity of other notes being played.

Why MIDIplex?

Music is often polyphonic, i.e., at times, there are multiple notes playing simultaneously. However, some synthesizers are monophonic, i.e., they can only voice one sound at a time. MIDIplex consumes a MIDI stream and automagically distributes it across a set number of output channels in such a way that it is guaranteed that no output will be expected to play two notes or more notes simultaneously. You can use MIDIplex to render a polyphonic MIDI across a given number of monophonic synthesizers, be it a choir of musical floppy disks, or (as I am) across a lab of computers with beep installed.

For a visual depiction of how MIDIplex distributes notes, watch this demonstration video, in which events from an input keyboard (top) are distributed across three output channels.

System Requirements

MIDIplex integrates with the Advanced Linux Sound Architecture. MIDIplex therefore is only supported on systems running Linux.

Usage

Overview

USAGE:
    midiplex [OPTIONS] <OUTPUT MODE>

OPTIONS:
    -i, --input-pool-size <I>   sets the ALSA input pool size
    -m, --max-allocation <N>    sets the maximum number of outputs allocated to any note

OUTPUT MODES:
    alsa                        ALSA output mode
    udp                         UDP output mode

Input

MIDIplex consumes input as a virtual ALSA MIDI device named ‘midiplex’ with a writable port named ‘input’. You can verify that MIDIplex is running with aconnect:

$ aconnect -l
client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 128: 'midiplex' [type=user,pid=6496]
    0 'input 

Output

MIDIplex supports two output modes. For most users, the ALSA output mode offers the most flexibility.

ALSA Output

In the ALSA output mode, midiplex creates a specified list of readable ports to which it distributes notes. You can then patch those notes to other MIDI devices using the aconnect utility from alsa-utils, or a visual connection manager such as patchage.

Overview:

USAGE:
    midiplex alsa [OPTIONS] <NAMES>...

OPTIONS:
    -o, --output-pool-size <O>  sets output pool size

ARGS:
    <NAMES>...                  space-delimited names of output ports

Example:

$ midiplex alsa melete mneme aoide & aconnect -l
[1] 10337
client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 128: 'midiplex' [type=user,pid=10337]
    0 'input           '
    1 'melete          '
    2 'mneme           '
    3 'aoide           '

“Help! I’m getting ENOSPC”:

The ALSA output mode is resource-intensive. For 𝘯 outputs, each input note on and off event must be copied 𝘯 times. If those 𝘯 output ports are then patched to 𝘯 synthesizers, that entails additional copying of the event. These issues can be somewhat assuaged with --input-pool-size and --output-pool-size, but for large values of 𝘯, intensive pieces may nonetheless cause MIDIplex to terminate with ENOSPC.

UDP Output

For cases where ALSA output mode is unsuitable, MIDIplex can write its output directly to a UDP socket. Note on and off events are encoded as three-byte datagrams, as described by the MIDI specification. You can receive these messages using a utility such as MIDInet or qmidinet.

Overview:

USAGE:
    midiplex udp [HOSTS]...

ARGS:
    <HOSTS>...                  space-delimited socket addresses

Example:

Assuming three reachable hosts, melete, mneme and aoide:

$ midiplex udp melete:8336 mneme:8336 aoide:8336 & aconnect -l
[1] 13114
client 0: 'System' [type=kernel]
    0 'Timer           '
    1 'Announce        '
client 14: 'Midi Through' [type=kernel]
    0 'Midi Through Port-0'
client 128: 'midiplex' [type=user,pid=13114]
    0 'input           '