GUI button note generation NOT being output from MIDI plugin



This may be something nobody else wants to do, I’m not sure, but I have a need for buttons on my MIDI processing VST which will eventually trigger chords. Right now, to test this out, I have a simple plugin based on the Audio Demo Plugin. It transposes the MIDI passed through it by 5 semitones.

My aim at this stage is to trigger just one note, note number 80, G# in the 4th octave, which should then be transposed to 85, C# in the 5th octave. I have a button named ‘Note On Button’ which triggers:

[code]MidiBuffer chordOutput;
AudioSampleBuffer buffer = AudioSampleBuffer (512,2);

    getProcessor()->processBlock(buffer, chordOutput);[/code]

and a ‘Note Off Button’ that triggers:

[code]MidiBuffer chordOutput;
chordOutput.addEvent(MidiMessage::noteOff(1, 80),0);
AudioSampleBuffer buffer = AudioSampleBuffer (512,2);

    getProcessor()->processBlock(buffer, chordOutput);[/code]

My processBlock method is like this:

[code]void ZS1AudioProcessor::processBlock (AudioSampleBuffer& buffer, MidiBuffer& midiMessages)

const int numSamples = buffer.getNumSamples();
int channel, dp = 0;

// Go through the incoming data, and apply our gain to it...
for (channel = 0; channel < getNumInputChannels(); ++channel)
    buffer.applyGain (channel, 0, buffer.getNumSamples(), gain);

// Now pass any incoming midi messages to our keyboard state object, and let it
// add messages to the buffer if the user is clicking on the on-screen keys
//Any MIDI I play will be picked up and then transposed before passed out of the plugin.
keyboardState.processNextMidiBuffer (midiMessages, 0, numSamples, true);

// and now get the synth to process these midi events and generate its output.
synth.renderNextBlock (buffer, midiMessages, 0, numSamples);

// Apply our delay effect to the new output..
for (channel = 0; channel < getNumInputChannels(); ++channel)
    float* channelData = buffer.getSampleData (channel);
    float* delayData = delayBuffer.getSampleData (jmin (channel, delayBuffer.getNumChannels() - 1));
    dp = delayPosition;
    for (int i = 0; i < numSamples; ++i)
        const float in = channelData[i];
        channelData[i] += delayData[dp];
        delayData[dp] = (delayData[dp] + in) * delay;
        if (++dp > delayBuffer.getNumSamples())
            dp = 0;

delayPosition = dp;

// In case we have more outputs than inputs, we'll clear any output
// channels that didn't contain input data, (because these aren't
// guaranteed to be empty - they may contain garbage).
for (int i = getNumInputChannels(); i < getNumOutputChannels(); ++i)
    buffer.clear (i, 0, buffer.getNumSamples());

// ask the host for the current time so we can display it...
AudioPlayHead::CurrentPositionInfo newTime;

if (getPlayHead() != 0 && getPlayHead()->getCurrentPosition (newTime))
    // Successfully got the current time from the host..
    lastPosInfo = newTime;
    // If the host fails to fill-in the current time, we'll just clear it to a default..

MidiBuffer output;

MidiBuffer::Iterator mid_buffer_iter(midiMessages);
MidiMessage m(0xf0);
int sample;
    if (m.isNoteOn()) {
        const int ch = m.getChannel();
        const int tnote = m.getNoteNumber() + 5;
        const uint8 v = m.getVelocity();
    else if (m.isNoteOff()) {
        const int ch = m.getChannel();
        const int tnote = m.getNoteNumber() + 5;
        const uint8 v = m.getVelocity();

midiMessages = output;

Here’s the problem. I can see and hear the midi and audio buffers I made with the [color=#FF0000]Note On Button[/color] but [color=#FF0000]these messages are not transposed and are not output from the synth[/color]. The strange thing is that midi from pressing the Juce keyboard and MIDI passed through my VST to another channel (see image below) are both correctly transposed and output. I cannot think why my buffers should be treated any differently.

I tried also omitting the line keyboardState.processNextMidiBuffer (midiMessages, 0, numSamples, true);
Observations are summarised below:

[color=#FF0000]Without[/color] the line keyboardState.processNextMidiBuffer (midiMessages, 0, numSamples, true); the note from my button [color=#FF0000]is audible[/color] but [color=#FF0000]doesn’t[/color] get displayed on the MIDI Keyboard. Pressing the MIDI keyboard [color=#FF0000]displays the notes[/color] but [color=#FF0000]doesn’t trigger the synth[/color]. Neither my GUI button’s note or any notes I input on the keyboard are recordable by another channel in Ableton. Lastly, the midi I pass to another channel in Live is correctly transposed and recorded. I hope this info can help somebody help me out.

It seems the buffers I created with the buttons are somehow removed in processBlock.

Maybe the problem is that I should send the Note On and Note Off with the desired gap in the same buffer, adjusting the sampleNumber accordingly?

void MidiBuffer::addEvent (const MidiMessage& m, const int sampleNumber) If this is the problem, how do I equate desired notelength with sampleNumber?

I need the button pressed in my GUI to trigger a chord that the DAW can record. How can this be achieved? Is it even possible? Jules, if you are reading can you help? If not, is there a way to ‘press the MIDI keyboard’ programatically with my button or somehow achieve the desired result?

Hoping for a solution,

Dave :expressionless:


Unless you are writing a HOST you never call the process() method yourself. The only MIDI processing you do is inside the process() method, and you need to add/remove/clear the MidiBuffer that’s the paramter of the process() block. Anything you add to the MidiBuffer will be “sent out” to the host, anything in this buffer are the messages from the host, you can clear them or let them stay there (psuedo MIDI-THRU option). Don’t go outside that scope, you can write methods that do stuff but always remember that this MidiBuffer is your container for all MIDI/VST related data.


Thanks so much Atom,

The penny has finally dropped for me :mrgreen: