Preparing the track
Which formats load, what to convert to, and why an mp3 may one day shift your whole beat map
Formats
Convert the track to ogg. It weighs 8 to 10 times less than wav. And unlike mp3, it adds no silence to the start of the file
What else loads: mp3 and wav, nothing more. oga, mp2, aiff, flac and the tracker modules mod, xm, it, s3m are refused
flac does not load at all: the engine does not decode it. Convert it first
An mp3 encoder pads the start with a few milliseconds of silence, and different encoders pad differently. If the track is re-encoded by another encoder, the whole beat map shifts
Offset
Even in ogg a track almost never starts exactly on the first beat. That is what the beat segment's offset is for.
It is set in fractional frames, so it is finer than one frame
The order is: bpm first, then the offset, and only then content
Loudness
Normalise the track before it goes into the level. Peak normalisation to -1 dB is a reasonable default
A level noticeably quieter than the next one reads as the game being broken rather than the track
Length
The level's length and the track's length are different numbers and do not have to match. A level can end before the track does. That is one way to build a fade-out ending
Several tracks
There can be more than one track. Each has its own position on the timeline, its own speed, volume and chain of effects
Effects: chorus, compressor, distortion, echo, flange, filters, normalisation, EQ, pitch shifter, reverb
An effect on a track is heard everywhere that track plays, not only where you meant it