MIDI is a standard method for exchanging information between sound synthesizers and controlling computers. Humdrum provides a number of MIDI-related tools that allow MIDI data to be input and output. In this chapter we will discuss the output-related tools: midi, perform, smf and tacet. MIDI input tools are discussed in Chapter 30.

The **MIDI Representation

Humdrum provides a MIDI representation that closely parallels the commercial MIDI specification but conforms to the Humdrum data format. This representation provides an intermediate format; using Humdrum commands such as perform and [smf, true MIDI data can be generated. However, since the MIDI representation conforms to the Humdrum syntax, the data can be manipulated, modified and searched in the same way as other Humdrum data. For example, we can use grep to search MIDI data, etc.

MIDI is a type of “tablature” notation. It describes a set of performance actions rather than specifying either the sounded result or the analytic notation. MIDI represents note-related events for various “channels.” MIDI events include note-on, note-off, key-velocity, after-touch, control codes, and system-exclusive codes. The original commercial MIDI standard is unable to represent many other musically pertinent signifiers such as pitch spelling (e.g., F-sharp versus G-flat), stem directions, ties, slurs, phrasing, etc. In addition, MIDI does not represent rests.

A simple MIDI example is given below. It consists just a single note (middle C):

!! A single MIDI note.
**MIDI
*Ch1
54/60/64
80/-60/64
*-

Notice that there are two MIDI data tokens: one to specify note-on and one to specify note-off events. Each MIDI data token consists of three elements or components, delimited by a slash character (/). The first element in the data token represents the number of clock ticks before the current event is to occur. The absolute duration of a single clock tick is determined by the MIDI clock speed, which is variable. In our example, 54 clock ticks elapse before the note is turned on, and another 80 clock ticks elapse before the note is turned off. The initial pause before the first note begins is not necessary; however, many MIDI cards introduce a brief delay before beginning to send the first data in a stream. The leading pause prevents a “rushed” burst of initial notes.

The second element in a data token represents the MIDI key number. Successive numbers refer to semitone pitches where middle C is designated key 60. Key-off events are represented by negative integers, so -60 means turn off key 60. Key numbers range between 1 and 127.

The third element in a data token represents the MIDI key velocity. MIDI instruments normally interpret key velocity as dynamic or accent information. Higher key-velocity values are associated with accented notes. In the case of key-off events, the key-velocity component represents the key-up velocity.

Unlike most other Humdrum representations, the MIDI format requires two data tokens for each note. The following example shows a three note C-major triad. The kern data is shown in the left spine with the corresponding MIDI data in the right spine. Notice that each data token consists of three subtokens, one for each note:

!! A C-major triad.
**kern	**MIDI
*	*Ch1
2c 2e 2g	72/60/64 72/64/64 72/67/64
.	144/-60/64 144/-64/64 144/-67/64
*-	*-

Middle C corresponds to MIDI key 60; The pitches E4 and G4 correspond to MIDI keys 64 and 67 respectively. The half-note durations result in a delay of 144 clock ticks before the key-off commands are executed. Notice that all key-off events occur simultaneously.

Suppose we add a second chord consisting of the dyad D4 and F4. The dyad begins at the same time that the C major triad ends. This results in five subtokens in the corresponding MIDI data record. Three key-off events are synchronous with two key-on events:

!! Two chords.
**kern	**MIDI
*	*Ch1
2c 2e 2g	72/60/64 72/64/64 72/67/64
4d 4f	144/-60/64 144/-64/64 144/-67/64 144/62/64 144/65/64
.	72/-62/64 72/-65/64
*-	*-

Notice that the difference in duration between the half-notes and quarter-notes is reflected when the notes are turned off rather than when the notes are turned on.

Example 7.1 illustrates a slightly more complex excerpt from the beginning of Darius Milhaud’s Touches Blanches.

Example 7.1 Excerpt from Darius Milhaud’s Touches Blanches

!!!: Milhaud, D.
!!!OTL: Touches Blanches
**kern	**kern	**MIDI	**MIDI
*staff2	*staff1	*Ch1	*Ch1
*clefF4	*clefG2	*clefF4	*clefG2
*k[]	*k[]	*k[]	*k[]
*M3/4	*M3/4	*M3/4	*M3/4
=1-	=1-	=1-	=1-
4e	(4g	72/64/64	72/67/64
4c	2a)	72/-64/64 72/60/64	72/-67/64 72/69/64
4F	.	72/-60/64 72/53/64	.
=2	=2	=2	=2
4f	(8a	72/-53/64 72/65/64	72/-69/64 72/69/64
.	8b	.	36/-69/64 36/71/64
4d	2g)	36/-65/64 36/62/64	36/-71/64 36/67/64
4G	.	72/-62/64 72/55/64	.
=3	=3	=3	=3
4e	(4g	72/-55/64 72/64/64	72/-67/64 72/67/64
4c	2a)	72/-64/64 72/60/64	72/-67/64 72/69/64
4F	.	72/-60/64 72/53/64	.
=4	=4	=4	=4
4f	(8a	72/-53/64 72/65/64	72/-69/64 72/69/64
.	8b	.	36/-69/64 36/71/64
4d	2g)	36/-65/64 36/62/64	36/-71/64 36/67/64
4G	.	72/-62/64 72/55/64	.
.	.	72/-55/64	72/-67/64
*-	*-	*-	*-

The MIDI representation always expects a tandem interpretation indicating the MIDI channel assignment. In Example 7.1 both parts have been assigned to channel 1. Once again, simultaneous key-on and key-off events often appear as double-stops. Also notice that an additional data record is required at the end of the passage in order to turn off the final notes.

The midi Command

The midi command converts Humdrum kern data into Humdrum MIDI data. By way of example, the above MIDI data can be generated as follows:

midi inven05.krn > inven05.hmd

The .hmd filename extension is a common way of designating Humdrum MIDI data.

Since the kern representation does not encode key-velocity information, the midi command assumes a default key velocity of 64 (from a range of 1 to 127). If the input is monophonic, midi will also allow the user to set a fixed note duration using the d option. This is useful for auditing notes that do not have duration values. For example, a Gregorian chant might be represented without durations. The following command takes a file containing a 12-tone row (pitch information only) and produces a MIDI output where all notes assigned to a quarter duration:

midi -d 4 tonerow > tonerow.hmd

The most common use of MIDI data is to create a standard MIDI file using the smf command, or to listen to the output using the perform command. In some cases, it is useful to carry out processing of MIDI data itself.

The perform Command

The perform command allows the user to listen to synthesized performances of MIDI-format input. When invoked, perform provides a simple interactive environment suitable for proof-listening and other audition tasks.

The perform command accepts any Humdrum input; however, only MIDI spines present in the input stream are performed. Non-MIDI spines are simply ignored and do not affect the sound output. The perform command generates serial MIDI data which are sent directly to a MIDI controller card or on-board sound-card.

The perform command is typically the last command in a pipe preceded by the midi command. For example, a kern-format score can be heard using the following command:

midi clara.krn | perform

When invoked, the perform command reads in the entire input into memory. This allows the user to move freely both forward and backward through the MIDI score.

The perform command provides a set of interactive commands that allow the user to pause and resume playback, to change tempo, to move to any measure by absolute or relative reference, and to search forward or backward for commented markers. The perform command remains active until either the end of the score is reached or the user terminates performance by typing the letter q or the escape key (ESC).

Playback can be paused by typing the space-bar and resumed by typing any key. Typing the carriage return by itself will return to the beginning of the score and re-initiate playback. If a number is typed before pressing the carriage return then perform will search for a corresponding measure number and initiate playback from that measure. Other commands are provided that allow moving forward or backward a specified number of measures.

In the default operation, perform echoes all global comments on the screen as the comments are encountered in the input. For inputs containing appropriate annotations, the echoing of comments can provide useful visual markers or reminders of particular moments in the sound output. Whether or not global comments are echoed on the standard output, users can use the perform forward-search / or backward-search ? commands to move directly to a particular commented point in the score. For example, if an input contains a global comment containing the character string “Second theme,” then the user can move immediately to this position in the input by entering the following command:

/Second theme

Similarly, backward searches can be carried out by typing the question mark ? rather than the slash. If the search is successful, playback continues immediately from the new score position.

Data Scrolling During Playback

The midi command provides a useful c option that causes each data record to be repeated as a comment. For example, when the c option is used a sequence of data records such as the following:

4C	4E	4G	4c
4D	4F	4G	4B
4AA	4E	4A	4c

is transformed to:

4C	4E	4G	4c
!!4C	4E	4G	4c
4D	4F	4G	4B
!!4D	4F	4G	4B
4AA	4E	4A	4c
!!4AA	4E	4A	4c

Since, by default, the perform command echoes all global comments on the screen during playback, this means that the Humdrum data will also appear on the screen as it is being played. In addition, the commented data records are accessible to the forward- and backward-search commands. For example, in the kern representation, pauses are indicated by a semicolon; hence the user might search for the next pause symbol by typing:

/;

Similarly, the user could search for a particular pitch, e.g.

/gg#

Since the perform command accepts any Humdrum input, other Humdrum data may be used for searching. For example, the input data might contain melodic interval data (see Chapter 11), allowing the user to search for a particular interval such as a diminished octave:

/d8

If the string pattern is found in the input, perform will move immediately forward (or backward) to the next occurrence and begin playing from that point.

Changing Tempo

During playback, the tempo can be modified by typing the greater-than > and less-than < signs to increase or decrease the tempo respectively. In addition to modifying the tempo interactively, the performance tempo may be specified either in the command line or in the input Humdrum representation. The tempo may be specified on the command line by using the t option. For example, the following command causes the file Andean to be performed at half tempo:

midi Andean | perform -t 0.5

Performing at fast speeds can often be useful when scanning for a particular passage.

Tempo specifications may be present in the input data via the tandem interpretation for metronome marking (e.g. *MM96). If no tempo information is available, perform uses a default tempo of 66 quarter-notes per minute.

The tacet Command

In rare circumstances, ciphers (stuck notes) can occur during MIDI performances; for instance, an intermittently functioning MIDI cable may fail to convey a “note-off” instruction to an active synthesizer. The p command (“panic”) turns off all active notes. Should a cipher remain after terminating the perform command, the Humdrum tacet command can be used to send “all-notoff” commands on all MIDI channels.

In Chapter 12, we will see how perform can be used in conjunction with other commands (such as extract and yank) to listen selectively to specific parts or passages. In Chapter 21 we will use perform in conjunction with the patt command to listen to patterns (such as harmonic, rhythmic and melodic patterns) found in some repertory.

The smf Command

Another MIDI-related tool is the smf command. This command allows the user to create “standard MIDI files” from Humdrum MIDI-format files. Standard MIDI files are industry-standard binary files that can be imported by a variety of MIDI applications software packages on many different platforms, including sequencer programs and most music notation packages.

The smf command translates only MIDI input spines; all non-MIDI spines are simply ignored. Suppose we begin with a kern-format file named joplin. We can create a standard MIDI file as follows:

midi joplin | smf > joplin.smf

The smf command provides two options. The t option allows the user to set the tempo, whereas the v option allows the user to specify a default MIDI key velocity. See the Humdrum Reference Manual for details.

Reprise

In this chapter we have learned how Humdrum data can be output as MIDI data. Humdrum provides a MIDI representation that closely parallels MIDI but remains in conformity with the Humdrum syntax. This means that the data can still be processed with other Humdrum tools (as we will see in later chapters).

The midi command can translate kern data to MIDI and the perform and smf commands can be used to generate true MIDI data for listening. The perform command provides a simple interactive command-line sequencer for playing whatever input is provided. The smf command generates standard MIDI files that can be used to transport MIDI data to a vast array of commercial and non-commerical applications software. In Chapter 30 we will explore some of the Humdrum tools for inputting MIDI data into Humdrum.