tone: add package for producing tones using the PWM interface

tone/notes.go

@@ -0,0 +1,147 @@
+package tone
+
+// Note represents a MIDI note number. For example, Note(69) is A4 or 440Hz.
+type Note uint8
+
+// Define all the notes in a format similar to the Tone library in the Arduino
+// IDE.
+const (
+	A0 Note = iota + 21 // 27.5Hz
+	AS0
+	B0
+	C1
+	CS1
+	D1
+	DS1
+	E1
+	F1
+	FS1
+	G1
+	GS1
+	A1 // 55Hz
+	AS1
+	B1
+	C2
+	CS2
+	D2
+	DS2
+	E2
+	F2
+	FS2
+	G2
+	GS2
+	A2 // 110Hz
+	AS2
+	B2
+	C3
+	CS3
+	D3
+	DS3
+	E3
+	F3
+	FS3
+	G3
+	GS3
+	A3 // 220Hz
+	AS3
+	B3
+	C4
+	CS4
+	D4
+	DS4
+	E4
+	F4
+	FS4
+	G4
+	GS4
+	A4 // 440Hz
+	AS4
+	B4
+	C5
+	CS5
+	D5
+	DS5
+	E5
+	F5
+	FS5
+	G5
+	GS5
+	A5 // 880Hz
+	AS5
+	B5
+	C6
+	CS6
+	D6
+	DS6
+	E6
+	F6
+	FS6
+	G6
+	GS6
+	A6 // 1760Hz
+	AS6
+	B6
+	C7
+	CS7
+	D7
+	DS7
+	E7
+	F7
+	FS7
+	G7
+	GS7
+	A7 // 3520Hz
+	AS7
+	B7
+	C8
+	CS8
+	D8
+	DS8
+	E8
+	F8
+	FS8
+	G8
+	GS8
+	A8 // 7040Hz
+	AS8
+	B8
+)
+
+// Period returns the period in nanoseconds of a single wave.
+func (n Note) Period() uint64 {
+	if n == 0 {
+		// Assume that a zero note means no sound.
+		return 0
+	}
+
+	octave := (n - 9) / 12
+	note := (n - 9) - octave*12
+
+	// Start with a base period (in nanoseconds) of 6.875Hz (quarter the
+	// frequency of A0) and shift it right with the octave to get the base
+	// period of this note.
+	//     145454545 = 1e9 / 6.875
+	basePeriod := uint32(145454545) >> octave
+
+	// Make the pitch higher based on the note within the octave.
+	period := uint64(basePeriod) * uint64(tones[note]) / 32768
+
+	return period
+}
+
+// Constants to calculate the pitch within an octave. Python oneliner:
+// [round(1e9/(440*(2**(n/12))) / (1e9/440) * 0x8000) for n in range(12)]
+var tones = [12]uint16{
+	32768,
+	30929,
+	29193,
+	27554,
+	26008,
+	24548,
+	23170,
+	21870,
+	20643,
+	19484,
+	18390,
+	17358,
+}