Developer Guide Engine Player

Introduction to Mixxx's Deck/Sampler Processing

Perhaps the most complicated part of Mixxx lies in its player (decks, samplers, etc.) processing code. As the result of years worth of work by multiple, unconnected people there has been significant rot and just plain buggyness introduced over the years here. In recent years we have been steadily rewriting it to try and improve the situation but it is still quite complicated. This section of the developer guide aims to demystify this monolith a bit.

EngineBuffer

The lion's share of complexity lies in EngineBuffer. This class has the following responsibilities:

Read decoded audio from a file loaded in the player.
Keep track of the state of the player (whether it's playing, how fast it's playing, what direction it's playing)
Respond to requests from elsewhere in Mixxx
For example: seek to location, start/stop playing, seek to hotcue, start a beatloop, change the loop start/end position, eject track, etc.
Re-sample audio from the native file sample-rate to the engine sample-rate.

CachingReader

ReadAheadManager

EngineBufferScale

EngineBufferScale is an interface for representing modules that are capable of re-sampling audio. Re-sampling is one of the key tasks that EngineBuffer must accomplish since if a track is recorded at a sample rate of 44.1kHz and the engine is outputting at a sample rate of 48kHz then without re-sampling, the track will sound 'chipmunky' since the track's samples will be played much faster than they would normally be played.

To prevent this effect, the track must be resampled to Mixxx's output sample rate. This is accomplished with an EngineBufferScale derivative.