VST 3 Interfaces  VST 3.7
SDK for developing VST plug-in
[3.5.0] Key Switch Support

Table of Contents

Allows information exchange between the plug-in and host about which key switches are currently used.

Introduction

Some instrument plug-ins support key switching functionality, which allows the user to switch between different layered sounds while playing notes. To achieve this, the user has to press a specific key associated to the wanted layer before playing new notes.

A typical example is a sample-based-player plug-in with a violin sound, which can include different layers or articulations: pizzicato, legato, tremolo, etc. By pressing or by keeping pressed a specific key, for example C-1, before playing new notes, the plug-in will choose to play pizzicato, by using D-1, it will play legato, and so on.

With VST Expression Map (feature of Cubase since 4.0), these key switches can be used in the Score Editor, to associated a symbol (articulation) to a note. Each time this note is played the corresponding key switch will be used (sent to the plug-in as noteOn event).

In order to help the creation of such a map, VST 3.5 defines a new interface Steinberg::Vst::IKeyswitchController. If an instrument plug-in supports such an interface, the host can get the current set of used key switches from the plug-in (megatrigg / articulation: Steinberg::Vst::KeyswitchInfo) for a given channel of a event bus, and then automatically use them (like in Cubase 6) to create a VST Expression Map.

How does it work?

Here the step by step example.
We want a plug-in with 1 event bus, which is mono-timbral (1 channel), and which supports 2 key switches:

  1. The instrument plug-in should have one input event bus (could be more):
    //------------------------------------------------------------------------
    tresult PLUGIN_API MyExampleProcessor::initialize (FUnknown* context)
    {
    //---always initialize the parent-------
    tresult result = AudioEffect::initialize (context);
    if (result == kResultTrue)
    {
    // we want a Stereo Output
    addAudioOutput (STR16 ("Stereo Output"), SpeakerArr::kStereo);
    // create Event In bus (1 bus with only 1 channel)
    addEventInput (STR16 ("Event Input"), 1);
    }
    return result;
    }
    //------------------------------------------------------------------------

  2. The controller should have the interface Steinberg::Vst::IKeyswitchController, here in the class declaration:
    //-----------------------------------------------------------------------------
    class MyExampleController: public EditController, public IKeyswitchController
    {
    public:
    ...
    //---from IKeyswitchController
    virtual int32 PLUGIN_API getKeyswitchCount (int32 busIndex, int16 channel);
    virtual tresult PLUGIN_API getKeyswitchInfo (int32 busIndex, int16 channel, int32 keySwitchIndex, KeyswitchInfo& info);
    ...
    OBJ_METHODS (MyExampleController, EditController)
    DEFINE_INTERFACES
    DEF_INTERFACE (IKeyswitchController)
    END_DEFINE_INTERFACES (EditController)
    REFCOUNT_METHODS(EditController)
    ...
    };
    //------------------------------------------------------------------------

  3. Now we have to implement the interface Steinberg::Vst::IKeyswitchController (only 2 functions), in our example Steinberg::Vst::IKeyswitchController::getKeyswitchCount should return 2 (2 key switches):
    //------------------------------------------------------------------------
    int32 MyExampleController::getKeyswitchCount (int32 busIndex, int16 channel)
    {
    // we accept only the first bus and 1 channel
    if (busIndex == 0 && channel == 0)
    return 2;
    return 0;
    }
    //------------------------------------------------------------------------

  4. Then, we have to implement Steinberg::Vst::IKeyswitchController::getKeyswitchInfo, which allows to inform the host about what the plug-in supports:
    //------------------------------------------------------------------------
    tresult PLUGIN_API MyExampleController::getKeyswitchInfo (int32 busIndex, int16 channel, int32 keySwitchIndex, KeyswitchInfo& info)
    {
    // we accept only the first bus and 1 channel and only 2 keyswitches
    if (busIndex == 0 && channel == 0 && (keySwitchIndex == 0 || keySwitchIndex == 1)
    {
    memset (&info, 0, sizeof (KeyswitchInfo));
    info.typeId = kKeyRangeTypeID; // in this case we want that Keyswitch should be maintained pressed for playĆ­ng
    // we could use keyRemapped to make easier the use of the keyswitch (near the available instrument key range)
    // take care that there are no overlap between keyswitches and real key (playing sound)
    if (keySwitchIndex == 0)
    {
    USTRING ("Accentuation").copyTo (info.title, 128);
    USTRING ("Acc").copyTo (info.shortTitle, 128);
    // if the user keeps pressed C-1 or C#-1 or C-0 then the Accentuation sound should be played
    info.keyswitchMin = 12; // C-1
    info.keyswitchMax = 13; // C#-1
    info.keyRemapped = 24; // C-0
    }
    else
    {
    USTRING ("Softly").copyTo (info.title, 128);
    USTRING ("Soft").copyTo (info.shortTitle, 128);
    // if the user keeps pressed D-1 or D#-1 or D-0 then the Softly sound should be played
    info.keyswitchMin = 14; // D-1
    info.keyswitchMax = 15; // D#-1
    info.keyRemapped = 26; // D-0
    }
    info.unitID = -1; // not used
    info.flags = 0; // not used
    return kResultTrue;
    }
    return kResultFalse;
    }
    //------------------------------------------------------------------------

  5. Last step, in the processor component, we have to adapt the process call to switch to the wanted articulation:
    //------------------------------------------------------------------------
    tresult MyExampleProcessor::process (ProcessData& data)
    {
    ....
    // get the input event queue
    IEventList* inputEvents = data.inputEvents;
    if (inputEvents)
    {
    Event e;
    int32 numEvents = inputEvents->getEventCount ();
    // for each events check it..
    for (int32 i = 0; i < numEvents; i++)
    {
    if (inputEvents->getEvent (i, e) == kResultTrue)
    {
    switch (e.type)
    {
    //-----------------------
    case Event::kNoteOnEvent:
    {
    // here a note On
    // check if this is a Keyswitch
    switch (e.noteOn.pitch)
    {
    // Accentuation Keyswitch
    case 12:
    case 13:
    case 24:
    currentLayer = kAccentuationLayer;
    break;
    // Softly Keyswitch
    case 14:
    case 15:
    case 26:
    currentLayer = kSoftlyLayer;
    break;
    default:
    // play the note in the currentLayer
    ...
    }
    } break;
    //-----------------------
    case Event::kNoteOffEvent:
    {
    // check if keyswitch is released
    switch (e.noteOff.pitch)
    {
    // Accentuation Keyswitch
    case 12:
    case 13:
    case 24:
    case 14:
    case 15:
    case 26:
    currentLayer = kDefaultLayer;
    break;
    default:
    // released note...
    ...
    }
    } break;
    ....
    }
    }
    }
    }
    ...
    }
    //------------------------------------------------------------------------


    That is it!


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