update m2.md

docs/formats/m2.md

@@ -20,7 +20,7 @@ Pre-implementation versions included:
 
 # Magic bytes
 
-Every binary m2 file must begin with `MIDI2.0`, and a file version identifier byte (zero for first initial version). Textual m2 files begin with `MIDI2.0 VER 1`, where `1` must be substituted with current version of the file.
+Every binary m2 file must begin with `MIDI2.0B`, and a file version identifier word (32 bits, zero for first initial version). Textual m2 files begin with `MIDI2.0 VER 1`, where `1` must be substituted with current version of the file.
 
 # Human-readable format
 
@@ -35,7 +35,27 @@ The note describing character sequence consists of
 * a symbol (either `-` or `#`),
 * one or more numbers (-1 octave is symbolized as `00`)
 
-Examples: `c#000`, `d-5`, `A#3`.
+Examples: `c#00`, `d-5`, `A#3`.
+
+## Velocity format
+
+Velocity can be entered as:
+* a decimal number,
+* a hexadecimal number,
+* a letter combination (case insensitive) borrowed from music notation.
+
+### Recognized letter combinations
+
+* pppp = 0x00 / 0x/00_00
+* ppp = 0x0f / 0x00_ff
+* pp = 0x1f / 0x22_ff
+* p = 0x2f / 0x44_ff
+* mp = 0x3f / 0x66_ff
+* mf = 0x4f / 0x88_ff
+* f = 0x5f / 0xaa_ff
+* ff 0x6f / 0xcc_ff
+* fff = 0x70 / 0xee_ff
+* ffff = 0x7f / 0xff_ff
 
 ## Rhythm format