Tutorial: dealing with invalid measures

[guiv42]

TuxGuitar 2.0 added a new feature: detection of invalid measures. Such measures appear with red horizontal lines.

When coding this new feature, I realized that many of the tablature files I've been using for years were invalid! So I'm starting this discussion since I guess I'm probably not the only user facing this problem.

What is an invalid measure?

There are different types of measure errors, but the most frequent one is a too long voice: sum of all notes duration exceeds theoretical measure duration. To make it simple: more than 4 quarters in a 4/4 measure is invalid.

Other types or errors should be very rare, don't hesitate to ask for support here if you face this problem and need help.

Why is my tablature invalid?

There can be different causes. A non-exhaustive list:

• because of an old bug (now fixed), any GuitarPro tab containing grace notes and imported by a previous TuxGuitar version has a high probability to contain invalid measures
• GuitarPro tolerates invalid measures, so any gp tab author may have left some invalid measures in a tablature file (intentionally or not)
• another old issue in TuxGuitar (now fixed) led to invalid measures when using uncommon time divisions, like 11- or 13-tuplets
• in "free edition mode", TuxGuitar also accepts invalid measures. This mode gives you complete control over notes duration. That can ease dramatically the edition process, but you are then in charge of fixing your own errors
• ...

What should I do when a measure is invalid?

First, identify what and where the problem is thanks to dedicated window. It should open automatically when you open an invalid tablature file, or click menu item "Edit -> Show measure errors" to make it visible. A button is also available in "Edit" section of left toolbar.
This window displays a list of errors: click on one error to navigate to the invalid measure.

Window provides a "Fix" button that should be able to fix automatically most of common errors.
Warning: automatic fix may not be the best solution in many cases. I recommend you try to fix manually first.

Count manually the effective duration of the measure, and compare to the theoretical value, defined by the time signature. Evaluate the duration in excess.
If this excessive duration is very short (1/16 or less), the cause is probably an erroneous import of a grace note.
If the excessive duration is longer, most probably the author of the tablature is responsible for that. Don't be surprised, I've seen tabs with 5 quarters in a 4/4 measure...

A typical example: your measure is too long by a very short duration, like 1/32. Look for each 1/32 note in measure, most probably one of them should be replaced by a grace note. Find this note and hit 'Del' twice to delete the note and the beat. Your measure should be valid now. Select next note, and re-create the valid grace note.

A short video example on a real case:

fix.mp4

If you cannot fix a too long measure manually, you can try the "Fix" button. TuxGuitar will shorten the end of the measure to make it valid. Be prepared to lose some notes, and/or to see last note(s) shortened.

See screenshots below, from left to right:

• an invalid measure
• after a reasonable manual fix
• the sub-optimal result of an automatic fix. It's valid, but notes are not aligned on metronome beats
  

Note: invalid measures may be tricky to fix when multiple voices are used.