Single Deck Vinyl Control

Single-Deck Vinyl Control

Summary and Rationale

Status: This specification is in drafting. Feel free to add ideas to this page.

Allow both players to be controlled via a single turntable with vinyl control. At least one commercial DJ application offers this feature, and Mixxx would be more competitive with it.

Use Cases

DJ Fred only has one turntable but wants to use Mixxx's vinyl control feature. He needs a way to control both players with a single timecoded vinyl.
DJ Jill wants to switch to Mixxx from djDecks, but can't do so because she needs single-deck vinyl control.
DJ Laura is thinking about buying turntables and wants to experiment with vinyl control first. However, her parents' have an old record player in her basement and she wants to see what it's like with Mixxx.

Design

Toggling Between Players

* How does Mixxx know which player should be controlled by the timecoded vinyl?

Via a button on the interface and/or hotkeys. When it's depressed, that player receives timecode signals.

Preferences Dialog Considerations

There should probably be some "single-deck mode" checkbox in the vinyl control preferences that greys out the second input combo boxes and enables this feature. Whatever the prefs dialog modifications end up being, they're probably contingent on the "Toggling Between Players" design above.

Internal Changes

In soundmanager.cpp, specifically in SoundManager::setupDevices(), there is the following snippet:

    //Initialize vinyl control
    m_VinylControl[0] = new VinylControlProxy(m_pConfig, "[Channel1]");
    m_VinylControl[1] = new VinylControlProxy(m_pConfig, "[Channel2]");

Each physical turntable's audio gets passed off to a VinylControlProxy object to be processed. The VinylControlProxy/xwax/scratchlib object then manipulates Mixxx's internal pitch and playback position. The channel string parameter tells each VinylControlProxy object which player it should be connected to. From that string, the VinylControl class constructor grabs "ControlObjects" which allow it to hook into Mixxx's various internal controls. For example, inside the VinylControl::VinylControl(...) constructor, you will see:

playPos             = new ControlObjectThread(ControlObject::getControl(ConfigKey(group, "playposition")));    //Range: 0.0 to 1.0

That "playPos" object then allows the VinylControl object change the playback position (eg. when needle dropping) for the player specified by "group" (which would be [Channel1] or [Channel2]).

In order to allow a single deck to control either player/channel, one could modify VinylControl/Proxy and allow it to get the ControlObjects for the other channel (this is the hard approach). An easier approach is to keep all the VinylControl objects the way they are, and to simply route the audio to the correct VinylControlProxy object depending on whichever player is supposed to be controlled.

The starting point for this way would be to look at SoundManager again:

CSAMPLE * SoundManager::pushBuffer(QList<AudioReceiver> recvs, short * inputBuffer,
                                    unsigned long iFramesPerBuffer, unsigned int iFrameSize)

This SoundManager::pushBuffer() function gets called when a soundcard has given Mixxx (input) audio to be processed, like from a timecoded vinyl. Near the end of the function, you'll see the code:

        QListIterator<AudioReceiver> devItr(recvs);
        while(devItr.hasNext())
        {
            AudioReceiver recv = devItr.next();
            if (recv.type == RECEIVER_VINYLCONTROL_ONE)
            {
                //recv.channelBase
                Q_ASSERT(recv.channels == 2); //Stereo data is needed for vinyl control
                if (m_VinylControl[0])
                    m_VinylControl[0]->AnalyseSamples(vinylControlBuffer1, iFramesPerBuffer);
            }
            if (recv.type == RECEIVER_VINYLCONTROL_TWO)
            {
                Q_ASSERT(recv.channels == 2); //Stereo data is needed for vinyl control
                if (m_VinylControl[1])
                    m_VinylControl[1]->AnalyseSamples(vinylControlBuffer2, iFramesPerBuffer);
            }
        }

This code demonstrates of how the "routing" of the input audio works. Each SoundDevice object has a list of AudioReceivers in it, which you can think of as the things that it's going to pass its input audio to. In this code snippet, you can see that if the SoundDevice has an AudioReceiver of type RECEIVER_VINYLCONTROL_ONE, then that audio will be passed off to the first VinylControlProxy object and it will control the first player. If we want single-deck vinyl control, we need to modify the list of AudioReceivers inside a SoundDevice at runtime so the soundcard controls the desired player.

If you look at SoundDevice.cpp, it looks like you can do this with SoundDevice::addReceiver() and SoundDevice::clearReceivers(). This is roughly the end of the string here. As a starting point, look at SoundManager::setupDevices() and check out the addReceiver() calls in there.

Work Breakdown

This work breakdown structure (WBS) outlines the work that needs to be done to complete this feature:

1. Single-deck vinyl control
  1.1 Assess existing vinyl control code
    <del>1.1.1 Write internal changes section for wiki</del>
    1.1.2. Figure out how exactly we want to make control switch from one player to another.
  1.2 Modify existing vinyl control code
    1.2.1 Read from crossfader (or whatever) ControlObject to determine which player to control.
    1.2.2 Change the AudioReceiver for the soundcard that's opened for input, depending on which player should be controlled. 
  1.3 Modify preferences dialog
    1.3.1 Add single-deck vinyl control checkbox
    1.3.2 Make the checkbox grey out the second input comboboxes.