Midi timestamp resolution

Audio Plugins
1

Hi folks,
I have a question regarding timestamps in midi messages.
I want to avoid a pb with a Midi keyboard that sends from time to time an unexpected Note ON to 127 followed by its Note OFF. They is due to a faulty key contact.
I tried to make a small Midi filter VST plugin to try to avoid that pb.
Here is what I did :

void NewProjectAudioProcessor::processBlock (juce::AudioBuffer& buffer, juce::MidiBuffer& midiMessages)
{

buffer.clear();

juce::MidiBuffer processedMidi;
int time;
juce::MidiMessage m;

for (juce::MidiBuffer::Iterator i(midiMessages); i.getNextEvent(m, time);)
{
    // test only duplicate on channel 1 to simplify
    if (m.isNoteOn() && (m.getChannel()==1))
    {
        mNoteOnCounter[m.getNoteNumber()]++;

        if (mNoteOnCounter[m.getNoteNumber()] == 2)
        {
            // Delete this note On because a duplicate note has been detected
            mNoteOnCounter[m.getNoteNumber()]--;
            continue;
        }
        if ((m.getVelocity() == 127))
        {
            DBG("Note ON with velocity 127");
            
            if ((m.getTimeStamp() - mLastNoteOffTime[m.getNoteNumber()]) < 0.002)
            {
                DBG("Bad Note ON received -> Delete");
                
                // Remove bad Note ON
                mLastNoteOnDeleted[m.getNoteNumber()] = true;
                continue;
            }
        }
    }
    else if (m.isNoteOff() && (m.getChannel() == 1))
    {
        mNoteOnCounter[m.getNoteNumber()]--;
        if (mLastNoteOnDeleted[m.getNoteNumber()])
        {
            // Remove bad Note Off
            mLastNoteOnDeleted[m.getNoteNumber()] = false;
            continue;
        }
        else
        { 
            mLastNoteOffTime[m.getNoteNumber()] = m.getTimeStamp();
        }
    }

    processedMidi.addEvent(m, time);
}

midiMessages.swapWith(processedMidi);

}

This code is basically aimed to remove a Note ON of 127 that followed a Note OFF for less of 2 ms + its related Note OFF.
But it seems the Value returned by m.getTimeStamp() is not accurate enough.
Any ideas ?

Thanks in advance.

2

There are several problems in this code. The Note ON and Note OFF might be in different buffers. Also don’t use getTimeStamp() which is meant for MIDI messages in a MIDI file, compute their position in samples from their “int time” that you get from getNextEvent() and the host transport position:

AudioPlayHead::CurrentPositionInfo pos; 
if (getPlayHead() != 0 && getPlayHead()->getCurrentPosition(pos)) 
{ ...}
3

Hi Qfactor,
thanks for your answer but as a beginner in Juce I don’t understand what you are saying.
I used the tutorial example as a base to buld this project:
Ref: https://docs.juce.com/master/tutorial_code_basic_plugin.html paragraph: Modify MIDI notes.
So if this example is not good could you point me to a more convenient one ?
Thanks.

PS: Remind that my little project must be a VST plugin.

4

Hi levioter,

The argument “midiMessages” doesn’t contain all the notes of your MIDI track, only a small chunk of them of a certain block size at a certain sample rate. This is not specific to JUCE, this is how real time audio processing works (which is the basis for audio plugins of any sort). I don’t have a specific reference in mind (I learned it a long time ago!), maybe somebody else can suggest a good modern introduction to real time audio/MIDI processing?

5

Just for clarification I am using a Midi Keyboard live and not a track or sequencer.
That’s why I need precide timestamping in order to try to filter this bug.
I guess I need to intercept the midi input and timestamp accurately with a high resolution timer before any timestamp manipulation.
Thanks I will dig a liitle bit more then.
Have a good day.
Cheers.