A piece of music, as part of its basic rhythmic structure, possesses bars. A bar is divided into a number (usually 2, 3, 4 or 6 depending on the metre) of beats, and more often than not, the strongest beat occurs at the beginning of the bar. The duration of a bar can only be changed in conjunction with the tempo. In live music there are no two bars having absolutely identical duration, as the tempo of the performance always varies slightly, emphasizing one nuance or another. In musical notation the end of a bar is defined by a bar-line, so that designating note-length is dependent on the arrangement of bar-lines and the division of bars into beats. For example, if a note of one-quarter length begins at the very start of a 4/4 bar, it should be displayed as a quarter note. If it begins 1/8 before the end of a bar, then according to musical notation it is necessary to display it as two eighth notes, connected across the bar-line with the "legato" symbol, even though the sound is identical in both situations. Notes crossing a bar-line are not often met in music.
Upon opening a MIDI file with any note editor, a quantifying procedure is carried out first, which allows you to divide a piece into bars and arrange bar-lines. With this the position and duration for all notes are set to the nearest multiple of duration of a 1/32 or 1/16 value (this parameter is usually adjustable), after which the bar-lines can be positioned. In the majority of situations, the user is himself able to choose the metre and tempo, but this is not very convenient: with such an "automatic" arrangement of bar-lines many notes overlap bars and beats, and the notation becomes awkward for reading and performing. It is of course possible to attempt to edit such a recording directly in a note editor, shifting notes in such a way that they do not overlap bars, but in this case the sound of the musical piece is altered. Besides which it is actually the bar-lines that are arranged incorrectly, and not the notes. What is one to do in such a situation?
MIDI supports the accurate positioning of bar-lines up to a point. This is achieved by the multiple use of the Change Tempo command. Using this technique, one is able to write bar-lines to a MIDI file, with certain restrictions*.
The Able MIDI Editor lets you arrange bar-lines in two ways. Firstly, you can add, delete and reposition them in visual mode. This can be convenient when working with a musical piece possessing a complex rhythm. The other way is to use the Tapping Tool, which lets you arrange bar-lines by ear, guided by the sound of the MIDI. Bar-lines arranged by such a method may also be corrected visually. The Editor also lets you adjust the marking on the Time-Ruler, using it to display either the time since the start of the piece or the bar number.
*Note: Sections where bars are too long (more than 10 seconds in duration) are, upon export to MIDI, filled by bars of identical length consistent with the tempo value indicated in the MIDI file specifications. Bars that are too short (lasting less than 0.5 seconds) are automatically merged with a neighboring bar.