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dev:units.txt [2018/02/07 17:07]
dev:units.txt [2018/02/07 17:07] (current)
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 +====== Basic Units ======
 +
 +===== Performance pitch =====
 +
 +Events store pitch values in simple integers. ​ These are
 +fixed-frequency pitches in the MIDI pitch scale, independent of clef
 +and key.  Adding 12 to a pitch increments it by one octave; pitch 60
 +is the treble-clef middle C.  (Previous rewrites have considered using
 +double the MIDI pitch so as to allow quarter-tones;​ this time let's go
 +for the simpler option as if we ever want quarter-tones we can always
 +code them using special Event properties.)
 +
 +
 +===== Display pitch =====
 +
 +For notation purposes we need a display pitch, which is a composite of
 +height on the staff plus accidental. ​ The correspondence between
 +display pitch and raw pitch depends on the clef and key.  For height
 +on the staff, we use the RG2.1 convention of 0 = bottom line, 1 = gap
 +between bottom two lines and so on up to 8 = top line.  Negative
 +heights are below the staff (i.e. on leger lines), heights over 8 are
 +above. ​ Display pitch is therefore independent of clef, so height 0 is
 +always the bottom line in any clef.
 +
 +If we ever see pitch as a plain integer, we assume it's a raw internal
 +pitch rather than a display pitch. ​ A plain integer used for display
 +pitch would probably be referred to as a "​height"​ rather than a pitch,
 +as it would be lacking the extra information (accidental,​ clef, key)
 +required to interpret it as a true pitch.
 +
 +===== Duration =====
 +
 +This part of the original documentation is completely obsolete.
 +
 +All I've ever needed to know is that a quarter note is 960 units, and everything else multiplies or divides from there.
 +
 +Tempo
 +
 +
 +Tempo specifications are usually informally described in beats per
 +minute (bpm). ​ However, as far as sequencers are concerned tempo is
 +generally measured in crotchets (quarter-notes) per minute, regardless
 +of the actual value of a beat in the current time-signature. ​ And the
 +MIDI file format inverts this, recording tempo in pulses per
 +quarter-note (ppq).
 +
 +For Rosegarden we choose to encode tempo in crotchets per minute,
 +but because we may want to have non-integral values and the Event
 +mechanism doesn'​t support floating-point properties, we actually store
 +crotchets per hour and convert to and from floating-point per-minute
 +values in the API.  We will also continue to use the name "​bpm"​ even
 +though our "​beat"​ is not always strictly a beat (it is always a
 +crotchet).
 +
 +
 +===== Velocity =====
 +
 +The range used by MIDI to represent velocity (initial relative volume
 +of a note) is 0-127, with maximum at 127 and silence at 0.  We use the
 +same range to store velocity values.
 +
 +(But note that some components such as the VU meters refer to levels
 +in a floating-point 0.0 -> 1.0 range.)
 +
 +
 +===== Bar numbers =====
 +
 +Bar numbers are counted from 1 on the GUI (i.e. the first non-count-in
 +bar is by default bar 1), but are counted from 0 internally (because
 +we're sad geeks). ​ That means we have to add 1 every time we display a
 +bar number, and subtract 1 from any the user supplies.
 +
 +Absolute times and bar numbers can both legitimately be negative, so
 +we need to use signed types, to avoid treating values like -1 as
 +special no-value cases, and to avoid making assumptions such as that
 +we can enumerate all bars by counting from zero.
 +
 +The "​bar"​ referred to here is currently an internal unit as well as an outward representation of barlines on a staff in a notation view, but this system is not adequate to address the needs of music notation. ​ Eventually we need some other way to represent barlines on a staff that's independent of our internal "​bar"​ unit.
  
 
 
dev/units.txt.txt ยท Last modified: 2018/02/07 17:07 (external edit)
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