How MIDI CC 91 Controls Reverb Send Depth

In General MIDI (GM) specifications, MIDI Continuous Controller 91 (CC 91) is standardly defined as "Effects 1 Depth," universally implemented as the control for reverb send level. Rather than generating a distinct reverb effect for each individual track, GM tone generators use an auxiliary send-and-return architecture with a shared, master reverb processor. Transmitting CC 91 data adjusts the proportion of a specific channel's audio routed into this central master reverb engine, allowing different instruments to share the same virtual acoustic space at varying depths.

The Auxiliary Send Architecture

General MIDI tone generators feature a global, multi-effects processor that handles reverb for all 16 MIDI channels simultaneously. CC 91 acts as an auxiliary send knob on a mixing console.

When a channel produces a sound:

1. The direct, unaffected "dry" signal is routed directly to the main stereo output.
2. A duplicate of that signal is split and routed to an auxiliary bus.
3. CC 91 scales the amplitude of the signal entering that auxiliary bus.
4. The combined auxiliary signals from all active channels are fed into the input of the master reverb engine.
5. The resulting 100% "wet" reverb output is blended back into the main stereo mix.

Because CC 91 only dictates the send level, increasing its value does not reduce the volume of the dry signal; it only increases the volume of the signal feeding the reverb engine, raising the apparent wet-to-dry balance.

Value Scaling and Defaults

Like standard 7-bit MIDI continuous controllers, CC 91 operates within a value range of 0 to 127:

• 0: Completely bypasses the reverb. No signal from the channel reaches the master reverb processor, resulting in a completely dry sound.
• 1–126: Linearly or logarithmically scales the send level into the processor.
• 127: Maximum send level, resulting in the highest possible reverberant wetness relative to the dry level.

Under the General MIDI Level 1 and Level 2 specifications, tone generators typically initialize with a default reverb send value (commonly set to 40) on most melodic channels to provide a natural acoustic room sound, while non-pitched percussion channels on MIDI Channel 10 may initialize with lower values or zero to maintain rhythmic clarity.

Per-Channel Control vs. Master Reverb Parameters

It is critical to distinguish between the per-channel send depth controlled by CC 91 and the parameters of the master reverb processor itself.

CC 91 is a channel-specific message. Sending CC 91, Value 100 on Channel 1 affects only the piano on Channel 1, leaving the bass on Channel 2 unaffected.

However, CC 91 does not alter the characteristics of the reverb itself—such as room type (Room, Hall, Plate), decay time, pre-delay, or the master return volume. In General MIDI environments, those global reverb parameters are preconfigured by the sound module's firmware or modified globally across the entire device using System Exclusive (SysEx) messages or Master Effects controls. CC 91 purely dictates how much of an individual channel's audio is immersed into that predefined master acoustic environment.