When you launch MIDI Monitor, it will automatically open a new document. The window shows you the MIDI messages that MIDI Monitor receives. There are also two sections, Sources and Filter, which start out hidden. Click the gray arrows to expand them.

MIDI Monitor uses the standard Mac interface for dealing with documents. Just open, close, and save as usual. A MIDI Monitor document saves its settings and the displayed MIDI events.

This section lets you choose the sources of MIDI events that this window listens to.

MIDI Monitor also provides a filter, so you can see only the events you are interested in.

New windows start with all of the checkboxes checked, meaning that all messages are displayed. The large checkboxes control the filters for a whole category of messages; the small checkboxes each control the filter for one specific type of message. For "voice" messages, which include a channel number, you can also choose whether to listen to all channels or just one.

Display

Time format

Controls how events' timestamps are displayed.

Clock time

Shows the timestamp in terms of normal human time, to the nearest 0.001 second. This is the most useful setting.

Host time

Shows the timestamp in terms of the computer's internal timebase. Most people won't find this very useful, but programmers might.

Integer

The time as returned by AudioGetCurrentHostTime() or UpTime(). This is an unsigned, 64-bit integer, which starts at 0 when the computer is started. This number will always increase at the same rate while the machine is running; however, one unit may represent different amounts of time on different machines, or different revisions of the operating system.

Seconds

The integer time divided by the number of ticks per second, as reported by AudioGetHostClockFrequency(). The value is shown to three decimal places.

Nanoseconds

The same as above, but with more digits displayed.

Note format

Controls how MIDI Monitor displays the musical note in note-on and note-off events.

Note (Middle C = C3)

The name of the note in musical notation. Middle C (note 60) is displayed as C3, and the full range is from C-2 to G8. Yamaha uses this convention for naming notes, as do some (most?) other manufacturers.

Note (Middle C = C4)

As above, but displayed an octave higher. Middle C (note 60) is displayed as C4, and the full range is from C-1 to G9. Roland uses this convention.

Decimal number

The note number in base 10, from 0 to 127.

Hexadecimal number

The note number in base 16, from $00 to $7F.

Controller format

The method of displaying the name or number of control events.

Standard name

The name of this controller as given in the MIDI standard. For example, controller 72 means "Release Time". Controllers without standard names are displayed as decimal numbers. Some controllers come in pairs; one controller conveys the "coarse" value (or high byte) and the other conveys the "fine" value (or low byte).

Decimal number

The controller number in base 10, from 0 to 127.

Hexadecimal number

The controller number in base 16, from $00 to $7F.

Data format

The method of displaying any other data in events.

Decimal number

The value in base 10. One byte values range from 0 to 127; signed two byte values range from -8192 to 8191.

Hexadecimal number

The value in base 16. One byte values range from $00 to $7F; two byte values range from $0000 to $3FFF, with $2000 as the "center" value.

Program number (Decimal)

How to display program numbers in program change messages.

1 - 128 (Standard)

Most MIDI devices number their programs starting with 1.

0 - 127 (Less common)

Some MIDI devices number their programs starting with 0.

Expert mode

Changes the display of all events. Intended for experts only.

Data formatting

Data is displayed as raw hexadecimal bytes, without any interpretation.

Note off events

According to the MIDI specification, a Note On event with velocity 0 is to be interpreted as the same as a Note Off event. Normally, MIDI Monitor shows "Note Off", to avoid confusion. In expert mode, it will show "Note On"

Zero timestamp

When a program sends MIDI, it can provide a timestamp to schedule when the MIDI data should be sent, or zero which means "send it immediately". Normally, when MIDI Monitor sees a zero timestamp, it will substitute a timestamp of when it received the data, to avoid confusion. ("Why do the timestamps say 0? Is something broken?") In expert mode, it will show "0".

Sources

New windows will observe:

When new windows are created, all of the sources in each of the checked categories will be selected.

When new MIDI sources or destinations are added:

Selects what to do when a new MIDI source or destination is added. (For example, when you plug in a MIDI interface, or start an app that acts as a virtual MIDI source or destination.)

Other

Save SysEx

This option lets you control how SysEx messages are saved to files. SysEx messages should always end with an EOX byte (hexadecimal F7). However, some devices do not follow this rule.

always with EOX byte ($F7)

The file will end with an EOX byte, even if the message didn't have one when it was received. This is the default, and the most useful setting (especially if you want to use the SysEx file in another application).

as it was received

Saves the message exactly the same way as it was received, with or without the EOX byte.

Ask to save when a changed window is closed

When checked, MIDI Monitor will ask you to save a modified window when you close it, which is the standard Mac behavior. If you don't tend to save MIDI Monitor documents, this can get annoying, so uncheck this preference to turn off the warning.

This preference depends on "Ask to keep changes when closing documents" in the General section of System Preferences. If that is OFF (which it is, by default) then documents are automatically saved for you, and the MIDI Monitor preference only affects untitled documents. If the system preference is ON, then the MIDI Monitor preference affects all documents.

Automatically check for updates

When checked, MIDI Monitor will periodically check if a newer version is available.

Detailed Event Data

To see the contents of an event, double-click it, or select it and use the menu command Events > Show Detailed Data.

The window shows information about the event, as well as a dump of the event's data in both hexadecimal and ASCII formats, which you can select and copy.

System Exclusive events also display a "Save As" button which saves the data to a file.

Rescan MIDI

You can use the menu command Special > Rescan MIDI to reinitialize the entire MIDI system and look for available devices. Generally you shouldn't need to do this, but I've included it as a convenience. The Audio MIDI Setup application has a "Rescan MIDI" button that does exactly the same thing.
(Developers: the menu command just calls CoreMIDI's MIDIRestart() function.)

How to uninstall MIDI Monitor

1. Drag the MIDI Monitor application to the Trash
2. In the Finder, select the menu item Go > Go to Folder…
3. Type the following, then press Go:
   ~/Library/Audio/MIDI Drivers
4. In the window that appears, you should see MIDI Monitor.plugin.
   Drag it to the Trash too.
5. Go to this folder:
   ~/Library/Preferences
6. Select com.snoize.MIDIMonitor.plist and drag it to the Trash.
7. Restart, just in case.