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[ DESIGN ]

Project Idea

Initially, we wanted to build a drum machine, which synthesize its own drum-sounds. We then learned about the MIDI protocol and the General MIDI (GM) Drum Specification from a friend who owns a music keyboard. We thought it would be more fun to be able to play “real” percussion sounds by striking physical drum pads instead of playing from the keyboard. Thus, the idea for the “ MIDI drum machine” was born.

Project Structure

An overview of our project setup is as follows:

When a drum-pad is hit, the z-axis accelerometer on the underside of the drum-pad would send an analog voltage waveform to the microcontroller. Using the ADC to track the peak voltage recorded by the drum hit, and de-bounce the “tail”, the microcontroller will then output the appropriate MIDI message through USART to a drum synthesizer or sound module to play the drum sounds. As we are using a Roland RS-70 keyboard as our sound module, we have to write our MIDI messages according to the Roland MIDI Exclusive Format for the keyboard to recognize the MIDI messages to generate the right drum sounds.

Hardware/ software Tradeoffs

Our basic drum-pads provide an alternative way of playing percussion sounds instead of playing from the keyboard. They have limited range of expressive drum performance. The drum-pads have limited velocity and volume control compared to playing percussion sounds on the keyboard. Since our simple drum-pads are small, they would not withstand drum sticks or mallets, but they would provide hours of entertainment of drum playing with fingers or hands.

Standards

MIDI (Musical Instrument Digital Interface)

The MIDI messages that are relevant in our project are the note-on, note-off, and program change message. Most MIDI messages consist of three bytes. The first byte is a status byte which identifies the message type such as note on or note off. The second byte is the value of the MIDI note of the drum triggered as shown in the GM Drum Key Map. The third byte is the velocity byte. The velocity byte denotes the loudness of a sounding note. When the velocity byte is zero, it is equivalent to a note off message.

General MIDI

General MIDI (GM) is a standard that assigns 128 instrument patches in a specific order. GM enables musicians to put Program Change messages in their MIDI song files, where the correct or originally intended instruments will be selected to play the song file. The percussive instrument patch relevant to our project is shown in the GM Patch Map with the Program Change number of 113 to 120. These numbers are used in our Program Change MIDI message.

GM Patch Map

Usually a GM module will also have a “drum component ” with each MIDI note triggering a different drum sound. The assignments of drum sounds to specific MIDI notes are shown in the GM Drum Key Map. The drum module is usually set to receive MIDI message on Channel 10.

GM Drum Key Map

Patents/ Copyrights/ Trademarks

The Roland RS-70 owner’s manual is the copyright property of Roland Corporation. We had to refer to the manual for the MIDI Exclusive Format for our MIDI messages to be recognized by the RS-70 Sound Synthesizer. The MIDI Exclusive Format contains exclusive messages used for exchanging patches and data information between compatible devices from Roland Corporation.

Our drum controller can also use the following addtional drum patches specific to Roland RS-70.

Roland Rhythm Set
Prog#	Instrument	Prog#	Instrument
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15	Standard Kit 1 Standard Kit 2 Standard Kit 3 RS Rock Kit RS Funk Kit RS Jazz Kit RS Brush Kit RS Orch Kit R&B Kit 1 R&B Kit 2 Hip Hop Kit Techno Kit House Kit Break Bts Kit TR Kit	16 17 18 19 20 21 22 23 24 25 26 27 28 29 30	Perc Menu Vox & Clap Menu Fx Menu Menu 1 Menu 2 Menu 3 GM2 Standard GM2 Room GM2 Power GM2 Electric GM2 Analog GM2 Jazz GM2 Brush GM2 Orchestra GM2 SFX

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