com.dixshtix.midi
Interface Chunk

All Known Implementing Classes:

Track

public interface Chunk

Base Class for self-consistent segment of a MIDI file.

What's a Chunk?

Data is always saved within a chunk. There can be many chunks inside of a MIDI file. Each chunk can be a different size (ie, where size refers to how many bytes are contained in the chunk). The data bytes in a chunk are related in some way. A chunk is simply a group of related bytes.

Each chunk begins with a 4 character (ie, 4 ASCII bytes) ID which tells what "type" of chunk this is. The next 4 bytes (all bytes are comprised of 8 bits) form a 32-bit length (ie, size) of the chunk. All chunks must begin with these two fields (ie, 8 bytes), which are referred to as the chunk header.

NOTE: The Length does not include the 8 byte chunk header. It simply tells you how many bytes of data are in the chunk following this header.

Here's an example header (with bytes expressed in hex):

4D 54 68 64 00 00 00 06

Note that the first 4 bytes make up the ASCII ID of MThd (ie, the first four bytes are the ASCII values for 'M', 'T', 'h', and 'd'). The next 4 bytes tell us that there should be 6 more data bytes in the chunk (and after that we should find the next chunk header or the end of the file).

In fact, all MIDI files begin with this MThd header (and that's how you know that it's a MIDI file).

NOTE: The 4 bytes that make up the Length are stored in Motorola 68000 byte order, not Intel reverse byte order (ie, the 06 is the fourth byte instead of the first of the four). All multiple byte fields in a MIDI file follow this standard, often called "Big Endian" form.

Version:

0.1

Author:

Richard C. Penner II

Field Summary

static byte[]	kMThd MThd Chunk Header bytes.
static byte[]	kMTrk MTrk Chunk Header bytes.

Field Detail

kMThd

public static final byte[] kMThd

MThd Chunk Header bytes.

MThd Chunk

The MThd header has an ID of MThd, and a Length of 6.

Let's examine the 6 data bytes (which follow the above, 8 byte header) in an MThd chunk.

The first two data bytes tell the Format (which I prefer to call Type). There are actually 3 different types (ie, formats) of MIDI files. A type of 0 means that the file contains one single track containing midi data on possibly all 16 midi channels. If your sequencer sorts/stores all of its midi data in one single block of memory with the data in the order that it's "played", then it should read/write this type. A type of 1 means that the file contains one or more simultaneous (ie, all start from an assumed time of 0) tracks, perhaps each on a single midi channel. Together, all of these tracks are considered one sequence or pattern. If your sequencer separates its midi data (i.e. tracks) into different blocks of memory but plays them back simultaneously (ie, as one "pattern"), it will read/write this type. A type of 2 means that the file contains one or more sequentially independent single-track patterns. f your sequencer separates its midi data into different blocks of memory, but plays only one block at a time (ie, each block is considered a different "excerpt" or "song"), then it will read/write this type.

The next 2 bytes tell how many tracks are stored in the file, NumTracks. Of course, for format type 0, this is always 1. For the other 2 types, there can be numerous tracks.

The last two bytes indicate how many Pulses (i.e. clocks) Per Quarter Note (abbreviated as PPQN) resolution the time-stamps are based upon, Division. For example, if your sequencer has 96 PPQN, this field would be (in hex):

00 60

Alternately, if the first byte of Division is negative, then this represents the division of a second that the time-stamps are based upon. The first byte will be -24, -25, -29, or -30, corresponding to the 4 SMPTE standards representing frames per second. The second byte (a positive number) is the resolution within a frame (ie, subframe). Typical values may be 4 (MIDI Time Code), 8, 10, 80 (SMPTE bit resolution), or 100.

You can specify millisecond-based timing by the data bytes of -25 and 40 subframes.

Here's an example of a complete MThd chunk (with header):

4D 54 68 64	MThd ID
00 00 00 06	Length of the MThd chunk is always 6.
00 01	The Format type is 1.
00 02	There are 2 MTrk chunks in this file.
E7 28	Each increment of delta-time represents a millisecond.

kMTrk

public static final byte[] kMTrk

MTrk Chunk Header bytes.

MTrk Chunk

After the MThd chunk, you should find an MTrk chunk, as this is the only other currently defined chunk. (If you find some other chunk ID, it must be proprietary to some other program, so skip it by ignoring the following data bytes indicated by the chunk's Length).

An MTrk chunk contains all of the midi data (with timing bytes), plus optional non-midi data for one track. Obviously, you should encounter as many MTrk chunks in the file as the MThd chunk's NumTracks field indicated.

The MTrk header begins with the ID of MTrk, followed by the Length (ie, number of data bytes to read for this track). The Length will likely be different for each track. (After all, a track containing the violin part for a Bach concerto will likely contain more data than a track containing a simple 2 bar drum beat).