Variable-length quantity

Variable-length quantity

A variable-length quantity (VLQ) is a universal code that uses an arbitrary number of binary octets (eight-bit bytes) to represent an infinitely large integer. It was defined for use in the standard MIDI file format [ [http://www.borg.com/~jglatt/tech/midifile/vari.htm MIDI File Format: Variable Quantities] ] to save additional space for a resource constrained system, and is also used in the later Extensible Music Format (XMF).It is also used in the WAP environment to represent unsigned integers of arbitrary length, where it is called variable length unsigned integer or uintvar.

General structure

The encoding assumes an octet where the most significant bit (MSB), also commonly known as the sign bit, is reserved to indicate whether another octet follows. If the MSB is 0, then this is the last octet of the integer, otherwise another binary octet follows.The octets within an integer are arranged most significant first.

Examples

Here we work an example with the number 137:

* Represent the value in binary notation (e.g. 137 as 10001001)
* Break it up in groups of 7 bits starting from the lowest significant bit (e.g. 137 as 0000001 0001001). This is equivalent to representing the number in base 128.
* Take the lowest 7 bits and that gives you the least significant byte (0000 1001). This byte comes last.
* For all the other groups of 7 bits (in the example, this is 000 0001), set the MSB to 1 (which gives 1000 0001 in our example). Thus 137 becomes 1000 0001 0000 1001 where the bits in boldface are something we added. These added bits denote if there is another byte to follow or not. Thus, by definition, the very last byte of a variable length integer will have 0 as its MSB.

The Standard MIDI File format specification gives more examples:Verify source|date=February 2008

References

External links

Official MIDI Standards Organizations

* [http://www.midi.org/ MIDI Manufacturers Association] (MMA) - Source for English-language MIDI specs
* [http://www.amei.or.jp/index_e.html Association of Musical Electronics Industry] (AMEI) -Source for Japanese-language MIDI specs


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