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The main goal of this project is to make AutoDJ sound more like a real DJ. The main feature that would allow this is beatmatching support in AutoDJ. Currently, AutoDJ only crossfades between two songs over a certain time interval, much like iTunes or other media players can do. I plan to use the power that Mixxx already has, such as BPM sync and beat detection, to improve the way that AutoDJ handles transitions.
Multiple transition styles will be added to AutoDJ. These different styles will be selectable through a combobox that is part of the AutoDJ controls. Users can select which specific transition they would like to use, or they will have the option to let AutoDJ decide which transition is used based on the two songs that are playing. If AutoDJ is deciding which transition to use, it will make this decision based on the BPMs of the two songs and the transition time that the user has set. For instance, AutoDJ will use a smooth crossfade for two songs with similar BPMs and a long transition time. For a short transition time and different BPMs, AutoDJ might use a spinback transition. A TrackTransition class will be used that contains a method for each type of transition. AutoDJ will create an instance of the TrackTransition, and will pass it ControlObjects for all of the buttons it needs, such as the play button and the crossfader. AutoDJ can then call a specific method of the TrackTransition object to use a specific transition. With this class, more transitions can be added later, and they are separate from the AutoDJ logic that decides which transition to use.
Some of the transition styles include:
More transitions can be added simply by creating a method for them in the TrackTransition class. There are many types of transitions, such as those using the EQ controls, that will not be added now, but could be added at a later time.
Cue in/out points that are used by AutoDJ are a feature that was mentioned by people in the Mixxx community, and had already been started, so it has been added to this project. It will be implemented differently than the version that was already started, however, which include fade in/out as well as cue in/out. The fade in/out option is being removed. The transition time option that already exists adds the same functionality that the fade in/out offered. The cue in would be used to mark the point where AutoDJ will start playing a track, while the cue out will mark the point where the next transition will start. Decks currently load tracks at the cue point, so this will be the cue in point for AutoDJ. This will help DJs who already have cue points defined in their library. The cue out point will be added to the cue database, and a button will be added to the GUI, so that users can set the AutoDJ cue out point.
To allow for all of these changes, the AutoDJ logic is going to be moved for dlgautodj.cpp to its own AutoDJ class. This allows for more organized expansion and fixes issues that are caused by the AutoDJ being linked to the GUI. The AutoDJ class will also be merged with the EngineXFader class, creating a Fader class that can be controlled through Control Objects. This also allows the AutoDJ access to the fader without repeatedly polling the EngineXFader class.
The AutoDJ class will interact with Mixxx the same way that dlgautodj does now. It will keep track of which songs are playing, the deck positions, which songs are going to be played next, and the ControlObjects that are needed to do the transitions. The main difference will be that the TrackTransition class will be used to handle the actual transitions. When AutoDJ transitions, it will call a method, which will be a specific transition, on the TrackTransition object. The TrackTransition will then handle the actual transition.
The AutoDJ class will also provide ControlObjects for the AutoDJ controls, including fade now, skip next, shuffle playlist, and enabling AutoDJ. This will allow users to control AutoDJ using a keyboard or controller and not just the buttons in Mixxx.
The Fade Now functionality has been expanded to be available even when AutoDJ is not on. Buttons have been placed on either side of the crossfader, and pressing either one will cause a transition to occur (according to the settings in AutoDJ). Pressing the Fade Now button during a transition will stop the transition. Using Fade Now outside of AutoDJ does not stop a track from playing.
Users will be able to take control of the crossfader while AutoDJ is transitioning if they choose to. If AutoDJ detects that the user is moving the crossfader, it will give up control to the user. AutoDJ will take back control for the next transition, or the user can use the Fade Now option to transition at a later point in the song.
The Fade Now option of AutoDJ is going to be expanded to be usable all of the time, not just when AutoDJ is enabled. This enables smooth crossfades for users do not have an external mixer, or quick transitions when needed. When users push the Fade Now button, AutoDJ will get control of the crossfader ControlObject, just like when AutoDJ is on. This fade can also be taken over by the user at any time.
Since AutoDJ will be using the crossfader ControlObject, the GUI will always be updated as AutoDJ transitions. This also means that a hardware controller can be used to take over the transition process as well.
The GUI changes are related to enabling the extra features that are going to be implemented.
Here is a diagram showing the slots, signals, and ControlObjects that will be involved with the AutoDJ class and DlgAutoDJ
These features have been discussed and could be added later
Being able to set cue in/out points from the preview deck would allow users to quickly manage their AutoDJ playlist by previewing a song, adding in/out points, and then placing the song in their AutoDJ playlist.
Silence at the beginning or end of a track would be detected and automatically not included in the song. AutoDJ would start the song after the silence (if it is at the beginning of the track) or transition before the silence (when the silence is at the end of the track). This would be beneficial when the user does not have any cue points set, so the entire track is played.
One of the new features that is going to be added is allowing the user to take over a transition while it is ocurring. The main topic of discussion is how to handle this best. Mixxx already has some options to deal with this, including soft-takeover. If the current options allow for the user to take over mid-transition, then there will not be a lot of change. But if anything needs to be added to allow for user takeover, then the wiki will be updated.
A transition interface has been suggested. This interface would have the ControlObjects that are needed for transitions, as well as actions to take. This would allow simple transitions such as crossfader.fade(8), which would fade the crossfader over 8 beats of the song.
AutoDJ should be instantiated outside of the Library, so that it will persist even when the skin is changed. It was originally planned to be instantiated by AutoDJFeature, but this does not solve the problem. Currently, AutoDJ is planned to be instantiated in mixxx.cpp along with the Library (instead of by the Library object).
Ideas that have been discussed but were decided against
An AutoDJ cue in button was discussed. This button would provide a cue point for AutoDJ to start playing a song at that was different from the default cue or the hot cues.
Use Hotcue 1 as the AutoDJ cue in
The Fade Now controls could be placed as part of each deck's controls. This control would be similar to the loop button, but instead of the loop controls, would contain cue in, cue out, and Fade Now.