[linux-audio-dev] XAP - VVID Extended Midi Protocal

Following the discussion on VVIDs, I've been thinking about how the MIDI protocol could be modified to encompass explicit contexts. To my surprise, this would quite simple. I'll call the new protocol ECMP (Explicit Context Midi Protocol -- just a working name). Key features are: 1. Easy conversion MIDI <--> ECMP. To convert a MIDI stream to ECMP, just insert a zero byte as the second byte of each voice message (or as the first byte if running status is used). To convert ECMP to MIDI, just skip the inserted bytes which are easy to find, and what remains is standard MIDI. So porting existing instruments to use ECMP but ignore the extensions is almost trivial. 2. Allows to have a 'set of mono synths' (up to 127 at a time) on a single channel. 3. Caters for the needs of continuous control synths (I think). These are the standard MIDI voice messages, with the new byte inserted: 0x80+c z k v note off 0x90+c z k v note on 0xA0+c z k p aftertouch 0xB0+c z n x controller 0xC0+c z a program change 0xD0+c z p channel pressure 0xE0+c z b c pitch wheel c = channel number z = inserted context number k = key (pitch) p = pressure v = velocity x = controller value a = program number b, c = pitch wheel value If the context number is 0, this means standard MIDI behaviour, all contexts numbered zero are independent of each other. A 'note on' with a non-zero context number creates the context if it does not already exist. If it does already exist, the new note is started in the existing context. What that means is defined by the patch. Each such context is either in the 'on' or 'off' state, depending on which one of 'note on' or 'note off' was most recently received. When a context is 'off' and it reaches an internal state where its further existence does no longer matter (e.g. all envelopes are at the end of their release phase), it is destroyed. If a context never reaches such a point, it can be explicitly destroyed by 'note off' received in the 'off' state. Control messages (including aftertouch, pitch wheel, etc.) are ignored if they refer to a non-existing context. For the 'channel pressure' message, z = 0 means 'channel pressure', which means all contexts, and z > 0 refers to a specific context. 'Channel program' with z = 0 has its normal meaning. If z > 0, it could be used to associate a context number with a specific patch variation or static parameter set. For example, a patch that contains a simulation of four different violin strings could use this message to associate a context number with a particular string. If z > 0 in the 'aftertouch' message, then the key (pitch) parameter is effectively redundant. I propose to keep it anyway, as this allows conversion to standard MIDI (and you may get 'almost' what you want). For continuous control instruments, a context or voice is created by a 'note-on' message. The interpretation of the key and velocity params is up to the patch, but normally no sound should be produced at this point. The context will continue to exist until the end of the performance, when it is removed by a double 'note off'. Comments invited !! -- Fons Adriaensen