MIDI Protocol

This page documents the MIDI specifications that Timbre implements: MIDI 1.0 (1983), MPE (2018), and USB-MIDI (1999). Use it as a development reference when writing firmware. For Timbre-specific mappings and design decisions, see the concept pages: MIDI & MPE and On the Wire. For Timbre’s consolidated spec sheet, see MIDI Implementation.

Serial Transport

The MIDI 1.0 electrical specification defines a current-loop serial interface at 31,250 baud, 8-N-1 (8 data bits, no parity, 1 stop bit). Each byte on the wire takes 320 microseconds.

Every MIDI byte is either a status byte (bit 7 = 1, range 0x80–0xFF) or a data byte (bit 7 = 0, range 0x00–0x7F). This single-bit distinction is the foundation of the entire protocol — a receiver can always determine whether it is looking at a command or a parameter.

Running Status

Running status allows a sender to omit the status byte when consecutive messages share the same status. The rules:

Rule	Detail
Applies to	Channel messages only (0x80–0xEF)
Canceled by	System Common (0xF0–0xF7) and System Exclusive
Not affected by	System Real-Time (0xF8–0xFF) — these may be interleaved freely
Receiver behavior	Store the last received channel status byte; apply it to subsequent data bytes until a new status arrives

Running status is a bandwidth optimization from the era of 31,250-baud serial links. Over USB it saves nothing meaningful, but receivers must handle it correctly because controllers still use it.

Channel Voice Messages

Channel Voice messages carry performance data. The low nibble of the status byte encodes the channel (0–15).

Status Byte	Binary	Data Bytes	Description
0x8n	1000nnnn	2: note, velocity	Note Off — velocity often ignored but part of the spec
0x9n	1001nnnn	2: note, velocity	Note On — velocity 0 = Note Off (running status optimization)
0xAn	1010nnnn	2: note, pressure	Polyphonic Aftertouch — per-key pressure
0xBn	1011nnnn	2: CC#, value	Control Change — see CC table below
0xCn	1100nnnn	1: program	Program Change — patch selection
0xDn	1101nnnn	1: pressure	Channel Pressure — single pressure value for the whole channel
0xEn	1110nnnn	2: LSB, MSB	Pitch Bend — 14-bit value, center = 8192 (0x2000)

Key Reference Values

Constant	Value	Note
Middle C	Note 60	MIDI standard (some DAWs display as C3, others C4)
A440	Note 69	Concert pitch reference
Pitch Bend center	8192 (0x00, 0x40)	LSB first, then MSB
Velocity range	1–127	0 = Note Off when sent as Note On

Control Change Numbers

CC messages (0xBn) use the second byte as a controller number (0–127) and the third byte as the value (0–127). The MIDI specification assigns specific functions to most of these numbers.

CC 0–31: MSB Controllers

These 32 controllers support 14-bit resolution when paired with their LSB counterparts (CC 32–63).

CC	Name	Typical Use
0	Bank Select MSB	Bank switching (paired with Program Change)
1	Modulation Wheel	LFO depth, vibrato
2	Breath Controller	Wind controller input
4	Foot Controller	Expression pedal
5	Portamento Time	Glide speed
6	Data Entry MSB	Sets value for selected RPN/NRPN
7	Channel Volume	Main volume for the channel
8	Balance	Stereo balance
10	Pan	Stereo position (64 = center)
11	Expression	Sub-volume (scales within Volume)
12–15	Effect Control 1–4	General purpose
16–19	General Purpose 1–4	Undefined by default
20–31	Undefined	Available for custom use

CC 32–63: LSB Controllers

Fine adjustment for 14-bit resolution. CC 32 is the LSB of CC 0, CC 33 is the LSB of CC 1, and so on.

CC	Name
32	Bank Select LSB
33	Modulation Wheel LSB
38	Data Entry LSB
34–63	LSBs for CC 2–31

CC 64–69: On/Off Switches

Values 0–63 = off, 64–127 = on.

CC	Name	Notes
64	Sustain Pedal	The most common CC after Mod Wheel
65	Portamento On/Off	Enables glide between notes
66	Sostenuto	Sustains only notes held at pedal-down
67	Soft Pedal	Reduces volume/brightness
68	Legato Footswitch	Enables legato mode
69	Hold 2	Extended sustain variation

CC 70–79: Sound Controllers

CC	Name	Common Assignment
70	Sound Variation	Timbre variation
71	Resonance / Timbre	Filter Q / resonance
72	Release Time	Amplitude envelope release
73	Attack Time	Amplitude envelope attack
74	Brightness / Cutoff	Filter cutoff frequency
75	Decay Time	Amplitude envelope decay (GM2)
76	Vibrato Rate	LFO rate
77	Vibrato Depth	LFO depth
78	Vibrato Delay	LFO onset delay
79	Sound Controller 10	Undefined

MPE note: In MPE mode, CC 74 is the per-note Slide dimension — the third axis of expression alongside Pitch Bend and Channel Pressure. On Timbre, Slide controls oscillation mode morphing rather than simple filter cutoff. See MIDI & MPE.

CC 80–95: General Purpose and Effects

CC	Name	Notes
80–83	General Purpose 5–8	Undefined by default
84	Portamento Control	Source note for portamento
85–90	Undefined
91	Reverb Send	Effects depth 1
92	Tremolo Depth	Effects depth 2
93	Chorus Send	Effects depth 3
94	Celeste Depth	Effects depth 4 (detune)
95	Phaser Depth	Effects depth 5

CC 96–101: Data and Parameter Selection

CC	Name	Function
96	Data Increment	Bump RPN/NRPN value up by 1
97	Data Decrement	Bump RPN/NRPN value down by 1
98	NRPN LSB	Non-Registered Parameter Number (LSB)
99	NRPN MSB	Non-Registered Parameter Number (MSB)
100	RPN LSB	Registered Parameter Number (LSB)
101	RPN MSB	Registered Parameter Number (MSB)

CC 102–119: Undefined

All 18 controllers in this range are unassigned by the MIDI specification and available for custom use.

CC 120–127: Channel Mode Messages

These are sent as Control Change messages but function as mode commands.

CC	Name	Function
120	All Sound Off	Immediately silence all voices on this channel
121	Reset All Controllers	Return all CCs to default values
122	Local Control On/Off	Connect/disconnect keyboard from internal sound engine
123	All Notes Off	Release all held notes (respects sustain pedal)
124	Omni Mode Off	Respond only to assigned channel
125	Omni Mode On	Respond to all channels
126	Mono Mode On	One voice per channel (+ optional voice count)
127	Poly Mode On	Normal polyphonic operation

Registered Parameter Numbers (RPN)

RPNs provide a standardized way to control parameters that do not fit into the 128-controller CC space. The protocol uses four CC messages in sequence:

1. CC 101 (RPN MSB) — selects the parameter (MSB)
2. CC 100 (RPN LSB) — selects the parameter (LSB)
3. CC 6 (Data Entry MSB) — sets the value (MSB)
4. CC 38 (Data Entry LSB) — sets the value (LSB, optional for 7-bit parameters)

After setting a parameter, transmit RPN NULL (CC 101 = 127, CC 100 = 127) to deselect and prevent accidental data entry changes from affecting the wrong parameter.

Standard RPN Table

RPN (MSB, LSB)	Name	Data Entry Value	Range
0, 0	Pitch Bend Sensitivity	MSB = semitones, LSB = cents	Typically 2 (standard) or 48 (MPE)
0, 1	Fine Tuning	14-bit, center = 8192	±100 cents
0, 2	Coarse Tuning	MSB only, center = 64	±64 semitones
0, 3	Tuning Program Change	MSB = program	0–127
0, 4	Tuning Bank Select	MSB = bank	0–127
0, 5	Modulation Depth Range	MSB = semitones, LSB = cents	MPE-defined
0, 6	MPE Configuration	MSB = member channel count	See MPE section
127, 127	RPN NULL	—	Deselects active RPN

System Messages

System messages are not channel-specific — they apply globally to the MIDI bus.

System Exclusive (SysEx)

Byte Position	Value	Description
1	0xF0	Start of Exclusive
2	Manufacturer ID	1 byte (0x01–0x7C) or 3 bytes (0x00 + 2 bytes)
3…N	Data	Payload — all bytes must be 0x00–0x7F (7-bit)
N+1	0xF7	End of Exclusive

7-bit encoding constraint: Every data byte inside a SysEx message must have bit 7 = 0. To transmit 8-bit binary data (firmware images, patch dumps), it must be packed into 7-bit form — typically by splitting each group of 7 bytes into 8 bytes, with the high bits collected in a prefix byte.

Reserved Manufacturer IDs

ID	Purpose
0x7D	Development / non-commercial — free use, no registration required
0x7E	Universal Non-Real Time (MIDI-CI, sample dumps, tuning)
0x7F	Universal Real Time (MTC, notation, device control)

Identity Request / Reply

Any MIDI device should respond to a Universal SysEx Identity Request:

Request: F0 7E 7F 06 01 F7 (Universal Non-Real Time, broadcast, General Information, Identity Request)

Reply: F0 7E DD 06 02 MM FF FF DD DD SS SS SS SS F7 (DD = device ID, MM = manufacturer, FF FF = family, DD DD = model, SS SS SS SS = version)

System Common

Status	Bytes	Name	Description
0xF1	1 data	MTC Quarter Frame	SMPTE timecode fragment
0xF2	2 data	Song Position Pointer	14-bit beat count (MIDI beat = 6 clocks)
0xF3	1 data	Song Select	Select song/sequence number
0xF4	—	Undefined
0xF5	—	Undefined
0xF6	0	Tune Request	Trigger analog oscillator tuning
0xF7	0	End of Exclusive	Terminates SysEx message

System Real-Time

Single-byte messages with no data. May be interleaved anywhere in the byte stream — even between the status byte and data bytes of another message, or within a SysEx transfer. Receivers must handle this.

Status	Name	Description
0xF8	Timing Clock	24 pulses per quarter note (ppqn)
0xF9	Undefined
0xFA	Start	Begin playback from song position 0
0xFB	Continue	Resume from current song position
0xFC	Stop	Halt playback
0xFD	Undefined
0xFE	Active Sensing	Heartbeat — once started, must arrive within 300 ms or receiver assumes disconnect
0xFF	System Reset	Return device to power-on state

MPE — MIDI Polyphonic Expression

MPE (adopted by the MMA in January 2018) assigns each sounding note to its own MIDI channel, enabling per-note continuous control of multiple expression dimensions simultaneously.

Zone Architecture

MPE defines up to two zones, each with a Manager Channel and one or more Member Channels:

Zone	Manager Channel	Member Channels	Direction
Lower Zone	Channel 1	Channels 2 upward	Ascending
Upper Zone	Channel 16	Channels 15 downward	Descending

The Manager Channel carries zone-wide controls (sustain pedal, volume, program change). Member Channels carry per-note data (note on/off, pitch bend, pressure, CC 74).

MPE Configuration Message (MCM)

Zone setup is performed via RPN 6 on the Manager Channel. The Data Entry MSB specifies how many Member Channels to allocate. Setting it to 0 deactivates the zone.

Lower Zone — 15 member channels:

B0 65 00   CC#101 = 0 (RPN MSB)       on channel 1
B0 64 06   CC#100 = 6 (RPN LSB)       on channel 1
B0 06 0F   CC#6   = 15 (Data Entry)   on channel 1

Upper Zone — 7 member channels:

BF 65 00   CC#101 = 0 (RPN MSB)       on channel 16
BF 64 06   CC#100 = 6 (RPN LSB)       on channel 16
BF 06 07   CC#6   = 7 (Data Entry)    on channel 16

Deactivate Lower Zone:

B0 65 00   CC#101 = 0 (RPN MSB)       on channel 1
B0 64 06   CC#100 = 6 (RPN LSB)       on channel 1
B0 06 00   CC#6   = 0 (Data Entry)    on channel 1

Default Pitch Bend Sensitivity

After MCM, the MPE specification sets default pitch bend ranges:

Channel Type	Default Range	RPN 0 Value
Manager Channel	±2 semitones	2
Member Channels	±48 semitones	48

The wide ±48 range on member channels gives expressive controllers (Linnstrument, Seaboard, Continuum) full pitch-slide capability across the playing surface.

Per-Note Expression Dimensions

Dimension	MIDI Message	Typical Controller Gesture
Strike velocity	Note On velocity	Initial key/pad impact
Pitch	Pitch Bend (per-channel)	Horizontal slide on playing surface
Pressure	Channel Pressure (per-channel)	Vertical force after initial strike
Slide	CC 74 (per-channel)	Vertical position on playing surface
Lift velocity	Note Off velocity	Release speed

Note Sharing

When the number of sounding notes exceeds available member channels, the MPE specification permits note sharing — multiple notes on the same channel. Per-note expression is degraded for shared notes (they receive the same pitch bend and pressure). Instruments should avoid this by voice-stealing the oldest or quietest note before resorting to channel sharing.

USB-MIDI

The USB Device Class Definition for MIDI Devices (1999) defines how MIDI travels over USB, built on top of the USB Audio Class.

USB-MIDI Event Packet

Every USB-MIDI transaction is composed of 32-bit packets:

Bits	Field	Description
31–28	Cable Number	Virtual cable / port index (0–15)
27–24	Code Index Number (CIN)	Identifies the message type and packet size
23–16	MIDI_0	First MIDI byte (usually status)
15–8	MIDI_1	Second MIDI byte (or 0x00 if unused)
7–0	MIDI_2	Third MIDI byte (or 0x00 if unused)

CIN Table

CIN	MIDI Bytes Used	Message Type
0x0	1, 2, or 3	Miscellaneous (reserved)
0x2	2	Two-byte System Common
0x3	3	Three-byte System Common
0x4	3	SysEx starts or continues (3 bytes)
0x5	1	Single-byte System Common or SysEx end
0x6	2	SysEx ends with 2 bytes
0x7	3	SysEx ends with 3 bytes
0x8	3	Note Off
0x9	3	Note On
0xA	3	Polyphonic Aftertouch
0xB	3	Control Change
0xC	2	Program Change
0xD	2	Channel Pressure
0xE	3	Pitch Bend
0xF	1	Single Byte (Real-Time)

The Cable Number field allows a single USB connection to carry up to 16 independent virtual MIDI cables (ports), each with its own 16-channel MIDI stream. Timbre uses a single cable in Phase 1–2, with dual cable (performance + configuration) planned for Phase 3.

Key Constants

Quick-lookup table for firmware development:

Constant	Value	Hex	Notes
A440	Note 69	0x45	Concert pitch reference
Middle C	Note 60	0x3C
Pitch Bend center	8192	0x2000	14-bit, LSB first on wire
Max channel	15	0x0F	0-indexed (channels 0–15 = display 1–16)
Baud rate	31,250	—	Serial MIDI
Byte time	320 us	—	At 31,250 baud, 10 bits per byte
SysEx Start	—	0xF0
SysEx End	—	0xF7
Velocity range	1–127	—	0 = Note Off (in Note On context)
14-bit max	16,383	0x3FFF	Pitch bend, 14-bit CCs
7-bit max	127	0x7F	Standard CC/velocity/note range
MIDI Clock ppqn	24	—	Pulses per quarter note
Active Sensing timeout	300 ms	—	After first 0xFE received

Sources

Resource	URL
MIDI 1.0 — Summary of Messages	midi.org/summary-of-midi-1-0-messages
MIDI Message Quick Reference	michd.me/jsmidgen/midi-message-reference
MPE Specification	midi.org/midi-polyphonic-expression-mpe
USB-MIDI Class Definition	usb.org/document-library/usb-midi-devices-10
MIDI 2.0 Specification	midi.org/midi-2-0