midi refactor and document things a little

technomid.py

@@ -3,11 +3,12 @@
 # Home page: https://github.com/ScrelliCopter/TECHNO.COM
 # SPDX-License-Identifier: Zlib  (https://opensource.org/license/Zlib)
 
+import os
 import struct
 import math
 from abc import ABC, abstractmethod
 from pathlib import Path
-from typing import BinaryIO, List, Tuple
+from typing import BinaryIO, Iterable, Tuple
 from enum import IntEnum
 
 
@@ -54,7 +55,7 @@ class MIDIProgramChange(MIDIEvent):
 		if channel < 0 or channel > 0xF:
 			raise ValueError("MIDI Channel out of range")
 		if patch < 0 or patch >= 0x80:
-			raise ValueError("Program out of range")
+			raise ValueError("MIDI Program out of range")
 		self._channel = channel
 		self._patch = patch
 
@@ -95,74 +96,78 @@ class MIDIWriter:
 		self._file.write(b"MThd")
 		self._file.write(struct.pack(">IHHH", 6, fmt, track_count, division))
 
-	def write_track(self, events: List[Tuple[int, MIDIEvent]]):
-		payload = b""
-		for event in events:
-			payload += self.encode_varint(event[0])
-			payload += event[1].serialise()
+	def write_track(self, events: Iterable[Tuple[int, MIDIEvent]]):
 		self._file.write(b"MTrk")
-		self._file.write(len(payload).to_bytes(4, byteorder="big"))
-		self._file.write(payload)
+		ofs = self._file.tell()
+		self._file.write(b"\0\0\0\0")  # Blank length field to write later
 
+		# Serialise and write out events
+		payload_len = 0
+		for event in events:
+			data = event[1].serialise()
+			self._file.write(self.encode_varint(event[0]) + data)
+			payload_len += 4 + len(data)
+
+		# Fill in track length field
+		self._file.seek(ofs)
+		self._file.write(payload_len.to_bytes(4, byteorder="big"))
+		self._file.seek(0, os.SEEK_END)
+
+	# Variable integer, used by event deltas.
+	# Up to 4 bytes can encode 7 bits each by setting the continuation bit (bit 8)
 	def encode_varint(self, value: int) -> bytes:
-		if value < 0:
-			raise ValueError("Variable integer must be positive")
-		if value >= 0x10000000:
-			raise ValueError("Variable integer is larger than 0FFFFFFF")
 		if value < 0x80:
 			return value.to_bytes()
+		if value < 0x4000:
+			return bytes([0x80 | (value >> 7), value & 0x7F])
+		if value < 0x200000:
+			return bytes([0x80 | (value >> 14), 0x80 | (value >> 7) & 0x7F, value & 0x7F])
+		if value < 0x10000000:
+			return bytes([0x80 | (value >> 21), 0x80 | (value >> 14) & 0x7F, 0x80 | (value >> 7) & 0x7F, value & 0x7F])
 		else:
-			a = (value & 0xFE00000) >> 21
-			b = (value & 0x01FC000) >> 14
-			c = (value & 0x0003F80) >> 7
-			d = (value & 0x000007F)
-			if a != 0:
-				return bytes([a | 0x80, b | 0x80, c | 0x80, d])
-			elif b != 0:
-				return bytes([b | 0x80, c | 0x80, d])
-			elif c != 0:
-				return bytes([c | 0x80, d])
+			raise ValueError("Variable integer out of range")
 
 
-def generate(outpath: Path):
-	timer = round((1000000 * 14.31818) / 12)
+def generate(f: BinaryIO):
+	def note_from_period(period: int, reference: int) -> (int, int):
+		frequency = reference / max(1, period)        # Convert period to hz
+		fnote = 69 + 12 * math.log2(frequency / 440)  # Convert pitch to MIDI note
+		note = int(round(fnote))                      # Snap to nearest semitone
+		bend = int(round((fnote - note) * 0x1000))    # Error is encoded as pitch bend
+		return min(0x7F, note), min(0x1FFF, bend)
 
-	def note_from_period(period: int) -> (int, int):
-		frequency = timer / max(1, period)
-		fnote = 69 + 12 * math.log2(frequency / 440)
-		note = int(round(fnote))
-		bend = min(0x1FFF, int(round((fnote - note) * 0x1000)))
-		return min(0x7F, note), bend
-
-	def techno():
+	def techno(length: int):
+		timer = int(round((1000000 * 1260 / 88) / 12))  # Intel 8253 (PIC) clock in Mhz
+		# Music tables from disassembly
 		phrase = [2, *[1, 0, 0] * 3] * 3 + [2, 3] + [0, 3] * 3
 		freq_tbl = [5424, 2712, 2416, 2280]
 		mangler = 0x0404
 
+		yield 0, MIDIMetaTempo(int(round((1000000 * 0x80000) / timer)))  # 16th note every PIC tick
+		yield 0, MIDIProgramChange(0, 80)  # Set GM patch to #81 "Lead 1 (Square)"
+
 		i = 0
 		bend = 0
 		while True:
 			for sixteenth in phrase:
-				note, new_bend = note_from_period(freq_tbl[sixteenth])
+				note, new_bend = note_from_period(freq_tbl[sixteenth], timer)
 				if new_bend != bend:
 					yield 0, MIDIPitchWheel(0, new_bend)
 					bend = new_bend
 				yield 0, MIDINoteOn(0, note)
 				yield 32, MIDINoteOff(0, note)
 				i += 2
-				if i >= 80 * 25:
+				if i >= length:
 					return
+			# Scramble pitch table at the end of each measure
 			mangler = (mangler + len(phrase) * 2) & 0xFFFF
 			freq_tbl = [freq ^ mangler for freq in freq_tbl]
 
-	with outpath.open("wb") as f:
-		mid = MIDIWriter(f)
-		mid.write_header(128)
-		mid.write_track([
-			(0, MIDIProgramChange(0, 80)),
-			(0, MIDIMetaTempo(int(round((1000000 * 0x80000) / timer))))
-		] + list(techno()))
+	mid = MIDIWriter(f)
+	mid.write_header(128)
+	mid.write_track(techno(80 * 25))
 
 
 if __name__ == "__main__":
-	generate(Path("techno.mid"))
+	with Path("techno.mid").open("wb") as f:
+		generate(f)