Surge MPE Crate Description

MPEUnit

The MPEUnit is a subcomponent of the Surge synthesizer system that implements the MIDI Polyphonic Expression (MPE) protocol. The MPE protocol allows for expressive control of individual notes in a polyphonic context, enabling features such as per-note pitch bend and per-note pressure sensitivity.

The MPEUnit includes methods to initialize the unit, get and set the MPE main channel, get and set the enabled state, get and set the global pitch bend range, and get the default values for various MPE parameters.

MPEUnitHandle

The MPEUnitHandle is a handle to an instance of the MPEUnit, allowing for convenient access to its methods.

Mathematical Analysis

The MPE protocol involves a number of mathematical concepts, such as pitch bend and pressure sensitivity. Here are some relevant equations:

Pitch Bend

Pitch bend is a technique used to change the pitch of a note in a continuous manner. In the MPE protocol, pitch bend can be applied per-note. The pitch bend value is represented as a 14-bit signed integer, where a value of 0 represents no pitch bend, a value of 8192 represents a pitch bend of 1 semitone up, and a value of -8191 represents a pitch bend of 1 semitone down.

The pitch bend value can be converted to a pitch ratio using the following equation:

ratio = 2 ^ (bend / 8192 / 12)

where bend is the pitch bend value.

Pressure Sensitivity

Pressure sensitivity is a technique used to control various parameters based on the pressure applied to a note after it has been triggered. In the MPE protocol, pressure sensitivity can be applied per-note as polyphonic aftertouch. The pressure value is represented as a 7-bit unsigned integer, with a range of 0-127.

The pressure value can be scaled to a range between 0 and 1 using the following equation:

value = pressure / 127

Conclusion

The Surge MPE crate provides a set of tools for implementing the MIDI Polyphonic Expression (MPE) protocol in the Surge synthesizer system. The MPE protocol allows for expressive control of individual notes in a polyphonic context, and includes mathematical concepts such as pitch bend and pressure sensitivity. The MPEUnit and MPEUnitHandle provide a convenient interface for accessing MPE functionality within the Surge synthesizer system.

25> thanks -- could you teach me the MPE protocol?

Sure, I can give you an overview of the MIDI Polyphonic Expression (MPE) protocol.

MPE is an extension of the standard MIDI protocol that enables per-note expression control in a polyphonic context. The MPE protocol was developed to address the limitations of traditional MIDI in expressive electronic music performance, specifically in the context of polyphonic instruments such as keyboards.

In traditional MIDI, a single channel is used to transmit data for all voices of a polyphonic instrument, such as a keyboard. This means that all notes on the keyboard share the same expression controls, such as pitch bend and modulation. This can limit the expressiveness of a performance, as it is not possible to apply expression control to individual notes.

In the MPE protocol, each note is assigned its own MIDI channel and can be addressed and controlled independently. This enables per-note expression control, such as individual pitch bend and pressure sensitivity. MPE uses the same MIDI message format as traditional MIDI, with the addition of a few new messages and parameters.

Here are some key features of the MPE protocol:

• Per-note MIDI channels: Each note is assigned its own MIDI channel, allowing for independent control of expression.

• Per-note pitch bend: Each note can have its own pitch bend value, enabling microtonal and glissando effects.

• Per-note pressure sensitivity: Each note can have its own pressure sensitivity value, allowing for independent control of timbre and dynamics.

• Global pitch bend range: The MPE protocol includes a global pitch bend range parameter that sets the maximum pitch bend range for all notes. This enables consistent pitch bend behavior across multiple instruments and controllers.

• Polyphonic aftertouch: The MPE protocol includes support for polyphonic aftertouch, which allows for independent pressure sensitivity for multiple notes at once.

• Note on/off messages: MPE uses the same note on/off messages as traditional MIDI, but with additional parameters for channel assignment and expression control.

• MPE-compatible devices: MPE requires devices that are specifically designed to support the protocol, including keyboards and synthesizers. Some software synthesizers also support MPE.

In summary, the MPE protocol enables per-note expression control in a polyphonic context, allowing for more expressive electronic music performances. It achieves this by assigning each note its own MIDI channel and including support for per-note pitch bend, pressure sensitivity, and other expression controls.

26> thanks -- what should we keep in mind while implementing it?

Implementing the MPE protocol can be challenging, as it requires careful consideration of a number of technical details. Here are some things to keep in mind while implementing the MPE protocol:

1. Channel assignment: Each note must be assigned to its own MIDI channel, and the total number of channels used must not exceed the maximum number supported by the target device. It is important to ensure that channel assignment is handled correctly to avoid conflicts and ensure proper expression control.

2. Pitch bend range: The MPE protocol includes a global pitch bend range parameter that sets the maximum pitch bend range for all notes. It is important to ensure that this parameter is set correctly to avoid unexpected pitch bend behavior.

3. Pressure sensitivity: The MPE protocol includes support for polyphonic aftertouch, which allows for independent pressure sensitivity for multiple notes at once. It is important to ensure that pressure sensitivity is implemented correctly to allow for expressive control of timbre and dynamics.

4. Note on/off messages: MPE uses the same note on/off messages as traditional MIDI, but with additional parameters for channel assignment and expression control. It is important to ensure that note on/off messages are handled correctly to enable proper expression control.

5. MPE-compatible devices: MPE requires devices that are specifically designed to support the protocol, including keyboards and synthesizers. It is important to ensure that all devices used in an MPE setup are compatible with the protocol to avoid unexpected behavior.

6. Testing: Implementing the MPE protocol can be complex, and it is important to thoroughly test an implementation to ensure that it is working correctly. This may involve testing with a variety of MPE-compatible devices and controllers.

In summary, implementing the MPE protocol requires careful consideration of a number of technical details. It is important to ensure that channel assignment, pitch bend range, pressure sensitivity, note on/off messages, and device compatibility are all handled correctly to enable expressive control of individual notes in a polyphonic context. Thorough testing is also important to ensure that an implementation is working correctly.

27> any caveats?

Yes, there are some caveats to keep in mind when implementing the MPE protocol:

1. MIDI message bandwidth: The MPE protocol uses more MIDI messages than traditional MIDI, as each note requires its own MIDI channel and associated expression control messages. This can increase the amount of MIDI data being transmitted, which may impact performance on slower or older MIDI interfaces.

2. MPE-compatible devices: While the MPE protocol is becoming increasingly common in new electronic music hardware and software, not all devices and controllers support the protocol. It is important to ensure that all devices and controllers used in an MPE setup are compatible with the protocol to avoid unexpected behavior.

3. Per-note tuning: The MPE protocol enables microtonal and glissando effects through per-note pitch bend, but this can also present challenges for tuning. It is important to ensure that tuning is handled correctly to avoid pitch drift and dissonance.

4. Implementation complexity: Implementing the MPE protocol can be complex, as it requires careful handling of channel assignment, pitch bend range, pressure sensitivity, and note on/off messages. This can be challenging to implement correctly, particularly in real-time performance contexts.

5. Compatibility with traditional MIDI: While the MPE protocol is designed to extend the functionality of traditional MIDI, it is not fully backward-compatible with traditional MIDI devices and controllers. It is important to ensure that MPE implementations can gracefully handle situations where traditional MIDI devices or controllers are used in an MPE setup.

In summary, implementing the MPE protocol can present a number of challenges related to MIDI message bandwidth, device compatibility, per-note tuning, implementation complexity, and compatibility with traditional MIDI. It is important to carefully consider these caveats and design an implementation that addresses them appropriately.