Filter out all MIDI events which are not sent on Channel #1

We want to create a MIDI filter, which only forwards MIDI events which are sent over MIDI Channel #1.

Copy the SDCC skeleton into a new directory, open the main.c file and enhance the hooks like described below. Thereafter type "make" in the command shell, and upload the new project.hex file to the core.

/////////////////////////////////////////////////////////////////////////////
//  This function is called by MIOS when a complete MIDI event has been received
/////////////////////////////////////////////////////////////////////////////
void MPROC_NotifyReceivedEvnt(
  unsigned char evnt0, unsigned char evnt1, unsigned char evnt2) __wparam
{
  // check MIDI channel, it should be 0 (-> channel #1)
  if( (evnt0 & 0x0f) == 0 ) {

    // before forwarding, we have to differ between 2-byte and 3-byte MIDI events!
    // for understanding the MIDI coding, please refer to the MIDI spec 
    // (-> http://www.borg.com/~jglatt/tech/midispec.htm)

    switch( evnt0 & 0xf0 ) {
      case 0x80: // Note-Off: 3 bytes
      case 0x90: // Note-On: 3 bytes
      case 0xa0: // Aftertouch: 3 bytes
      case 0xb0: // CC: 3 bytes
      case 0xe0: // Pitchbend: 3 bytes
        MIOS_MIDI_TxBufferPut(evnt0);
        MIOS_MIDI_TxBufferPut(evnt1);
        MIOS_MIDI_TxBufferPut(evnt2);
        break;
      case 0xc0: // Program Change: 2 bytes
      case 0xd0: // Poly Aftertouch: 2 bytes
        MIOS_MIDI_TxBufferPut(evnt0);
        MIOS_MIDI_TxBufferPut(evnt1);
        break;
      default:
        // note: status messages must be handled within MPROC_NotifyReceivedByte()
        break;
     }
  }
}

If SysEx or Realtime messages should be forwarded as well, then you need to enhance the MPROC_NotifyReceivedByte() function like shown below:

/////////////////////////////////////////////////////////////////////////////
// This function is called by MIOS when a MIDI byte has been received
/////////////////////////////////////////////////////////////////////////////
void MPROC_NotifyReceivedByte(unsigned char byte) __wparam
{
  static char fx_status;

  // normal MIDI events are forwarded in MPROC_NotifyReceivedEvnt
  // this function handles sysex and realtime messages
  if( byte & 0x80 ) { // Status message
    if( byte >= 0xf0 )
      MIOS_MIDI_TxBufferPut(byte); // transfer byte

    // determine status
    if( byte == 0xf0 ) {
      fx_status = 0xff; // forward until 0xf7
    } else if( byte == 0xf7 ) {
      fx_status = 0;    // f7 reached, no forward
    } else if( byte == 0xf1 || byte == 0xf3 ) {
      fx_status = 1;    // expecting one additional byte
    } else if( byte == 0xf2 ) {
      fx_status = 2;    // expecting two additional bytes
    } else {
      fx_status = 0;    // expecting no additional byte
    }
  } else {
    // check if fx status active
    if( fx_status ) {
      // forward data byte
      MIOS_MIDI_TxBufferPut(byte);

      // decrement counter if required
      if( fx_status != 0xff )
        --fx_status;
    }
  }
}

A list of available MIOS function can be found here.



Last update: 2018-05-10

Copyright 1998-2018, Thorsten Klose. All rights reserved.