MIDI MIXER

MIDI synthesisers nowadays are often multi-timbral allowing the playing of multiple instruments or voices at the same time. While the level of these can be selected and controlled by a computer controlled sequencer I wanted to be able to instantly fade up/down any MIDI channel or number of channels in real time. This enables instant control of MIDI file playback sequencers so that any instrument can be adjusted to suit a particular choice, for example turn off all instruments except bass and drums. 

Also it allows setting of the volume of different instruments to different levels when working in a particular live-environment. The circuit has dual operation and can be selected  to operate on incoming velocity bytes, allowing operation with synthesisers that may not respond to volume byte control MIDI data, or to produce directly outgoing volume MIDI data bytes by operating the potentiometers. This allows operation with most types of multi-timbral synthesisers.

The MIDI Mixer Unit utilizes:

• A Printed Circuit Board (PCB), 
• A pre-programmed 16F877 PIC,
• A Volume/Velocity switch and associated Velocity LED,
• A 1-8/9-16 channel select switch and associated 9-16 channels LED,
• A 9 volt battery connector
• A MIDI 5-pin DIN output socket
• A MIDI 5-pin DIN input socket
• A screw terminal connector for an external 9-12 volts DC supply and associated power-on LED,
• Screw terminal connectors for connecting the external switches 
• The circuit schematic diagram is available

The  MIDI Mixer Unit requires:

• A 9v battery or equivalent DC power source
• A set of eight 10k rotary potentiometers/linear faders

SOFTWARE OPERATION:

When the circuit is switched on the software polls switch SW2 and decides to operate in Velocity or Volume mode. In velocity mode the software waits for a MIDI input and tests to see if it is a Note-on or Note-off status byte and if it is then the software outputs this byte to the serial transmit MIDI port, then reads the next data byte and outputs it also, then it receives the velocity byte and branches to a subroutine to read the potentiometer associated with the incoming MIDI channel and outputs this new data byte. 

The second choice on switch on is that SW2 is set to the Volume position. In this mode of operation the software first fills a table with the converted values of the 8 potentiometers and then continues to scan them, whenever a value changes up/down by a value of 1 or greater then the corresponding volume MIDI data is transmitted to the MIDI out socket.

The switch SW3 decides if the eight potentiometers (VR1-8) correspond to MIDI channels 1-8 or 9-16. There are two LEDS corresponding to operating in Velocity mode and on channels 9-16, see table 1