How General MIDI 2 Expands General MIDI

Introduced in 1999 by the MIDI Manufacturers Association, General MIDI 2 (GM2) significantly upgraded the original 1991 General MIDI (GM1) specification to meet the demands of advancing music technology. While GM1 provided a baseline for consistent playback across different synthesizers, GM2 vastly expands polyphony, doubles the available drum channels, introduces a broader sound palette through bank switching, and standardizes advanced sound-shaping controllers—all while maintaining complete backward compatibility with the original standard.

Increased Polyphony and Channel Architecture

GM1 guaranteed a modest minimum polyphony of 24 voices, dynamically allocated between melodic instruments and percussion. GM2 increases this baseline requirement to a minimum of 32 simultaneous voices.

GM2 also enhances channel flexibility. In GM1, Channel 10 was exclusively reserved for percussion, leaving 15 channels for melodic instruments. GM2 supports up to two simultaneous drum kits by designating both Channel 10 and Channel 11 as percussion channels, providing greater depth and flexibility for rhythm-heavy arrangements.

Expanded Instrument Palette and Drum Kits

The core of GM1 was its fixed set of 128 melodic instruments and one standard drum kit featuring 47 percussion sounds. GM2 expands this library substantially by utilizing Bank Select messages (Control Change 0 and Control Change 32):

• Melodic Voices: GM2 includes a minimum of 384 melodic instruments. It retains the standard 128 program locations from GM1 as "Capital" sounds, while adding variation banks to provide alternative timbres, articulations, and specialized instruments.
• Percussion: GM2 increases the number of drum kits from one to nine distinct kits, including Jazz, Brush, Orchestral, Room, and Electronic sets, while expanding the percussion sound map to include additional world and orchestral instruments.

Standardized Sound Editing and Effects Control

One of the largest limitations of GM1 was the inability to edit timbre dynamically using standardized MIDI messages. GM2 standardizes several Non-Registered Parameter Numbers (NRPNs) and continuous controllers (CC) for sound shaping:

• Envelope Controls: Dedicated controllers for Attack Time (CC 73) and Release Time (CC 72).
• Filter Controls: Universal control over low-pass filter Cutoff Frequency (Brightness, CC 74) and Resonance (Timbre/Harmonic Content, CC 71).
• Built-in Effects: Standardized depth controllers for Reverb (CC 91) and Chorus (CC 93).

Tuning and System Exclusive Messages

GM2 incorporates standardized Universal System Exclusive (SysEx) messages for global device management. These messages allow composers to control Master Volume, Master Fine Tuning, and Master Coarse Tuning directly from the sequence. Furthermore, GM2 introduces microtonal capabilities and scale/temperament tuning options, allowing non-Western musical scales and historical temperaments to be reliably reproduced across any GM2-compliant sound module.

Backward Compatibility

Despite its expanded capabilities, GM2 was designed with strict backward compatibility. Any standard MIDI file authored for GM1 plays seamlessly on a GM2 synthesizer. If a GM2 device encounters a sequence that calls for an unavailable variation bank, it automatically defaults to the corresponding primary capital instrument defined in the original GM1 specification.