TIA audio sampled every colour clock

sum of samples is averaged and output twice per scanline for an output
sample rate of 31.4KHz

this fixes issues with ROMs that change the volume of the audio multiple
times per scanline

added *.wav to .gitignore

.gitignore

@@ -36,3 +36,4 @@ armcode.map
 go.work
 go.work.sum
 REDUX_PLAYBACK_FILES
+*.wav

Makefile

@@ -5,7 +5,7 @@ goBinary = go
 gcflags = -c 3 -B -wb=false
 ldflags = -s -w
 ldflags_version = $(ldflags) -X 'github.com/jetsetilly/gopher2600/version.number=$(version)'
-profilingRom = ./MattressMonkeys20240530rc4.bin
+profilingRom = roms/Pitfall.bin
 
 # the renderer to use for the GUI
 #

README.md

@@ -150,6 +150,10 @@ The TIA Audio implementation is based almost entirely on the work of Chris Brenn
 
 https://atariage.com/forums/topic/249865-tia-sounding-off-in-the-digital-domain/
 
+Additional work on volume sampling a result of this thread:
+
+https://forums.atariage.com/topic/370460-8-bit-digital-audio-from-2600/
+
 Musical information as seen in the tracker window taken from Random Terrain.
 
 https://www.randomterrain.com/atari-2600-memories-music-and-sound.html
@@ -265,6 +269,6 @@ http://www.festvox.org/docs/manual-2.4.0/festival_toc.html
 At various times during the development of this project, the following people
 have provided advice and encouragement: Andrew Rice, David Kelly. And those
 from AtariAge who have provided testing, advice and most importantly,
-encouragement (alphabetically): alex_79; Al Nafuur; Andrew Davie; DirtyHairy;
-John Champeau; MarcoJ; MrSQL; Rob Bairos; Spiceware; Thomas Jenztsch; Zachary
-Scolaro; ZeroPageHomebrew
+encouragement (alphabetically): alex_79; Al Nafuur; Andrew Davie; Batari;
+DirtyHairy; John Champeau; MarcoJ; MrSQL; Rob Bairos; Spiceware; Thomas
+Jenztsch; Zachary Scolaro; ZeroPageHomebrew

gui/sdlaudio/audio.go

@@ -42,7 +42,7 @@ const bufferLength = 628
 const realtimeDemand = bufferLength * 2
 
 // if queued audio ever exceeds this value then push the audio into the SDL buffer
-const maxQueueLength = 16384
+const maxQueueLength = audio.SampleFreq / 2
 
 // Audio outputs sound using SDL.
 type Audio struct {

gui/sdlimgui/win_dbgscr.go

@@ -575,11 +575,11 @@ func (win *winDbgScr) drawOverlayComboTooltip() {
 		}, true)
 	case reflection.OverlayLabels[reflection.OverlayAudio]:
 		win.img.imguiTooltip(func() {
+			imguiColorLabelSimple("Change", win.img.cols.reflectionColors[reflection.AudioChanged])
+			imgui.Spacing()
 			imguiColorLabelSimple("Phase 0", win.img.cols.reflectionColors[reflection.AudioPhase0])
 			imgui.Spacing()
 			imguiColorLabelSimple("Phase 1", win.img.cols.reflectionColors[reflection.AudioPhase1])
-			imgui.Spacing()
-			imguiColorLabelSimple("Change", win.img.cols.reflectionColors[reflection.AudioChanged])
 		}, true)
 	case reflection.OverlayLabels[reflection.OverlayCoproc]:
 		win.img.imguiTooltip(func() {
@@ -759,16 +759,16 @@ func (win *winDbgScr) drawReflectionTooltip() {
 			// no RSYNC specific hover information
 		case reflection.OverlayLabels[reflection.OverlayAudio]:
 			imguiSeparator()
-			if ref.AudioPhase0 || ref.AudioPhase1 || ref.AudioChanged {
+			if ref.AudioChanged || ref.AudioPhase0 || ref.AudioPhase1 {
+				if ref.AudioChanged {
+					reg := strings.Split(e.Operand.Resolve(), ",")[0]
+					imguiColorLabelSimple(fmt.Sprintf("%s updated", reg), win.img.cols.reflectionColors[reflection.AudioChanged])
+				}
 				if ref.AudioPhase0 {
 					imguiColorLabelSimple("Audio phase 0", win.img.cols.reflectionColors[reflection.AudioPhase0])
 				} else if ref.AudioPhase1 {
 					imguiColorLabelSimple("Audio phase 1", win.img.cols.reflectionColors[reflection.AudioPhase1])
 				}
-				if ref.AudioChanged {
-					reg := strings.Split(e.Operand.Resolve(), ",")[0]
-					imguiColorLabelSimple(fmt.Sprintf("%s updated", reg), win.img.cols.reflectionColors[reflection.AudioChanged])
-				}
 			} else {
 				imgui.Text("Audio unchanged")
 			}

hardware/tia/audio/audio.go

@@ -34,14 +34,10 @@ type Tracker interface {
 	AudioTick(env TrackerEnvironment, channel int, reg Registers)
 }
 
-// SampleFreq represents the number of samples generated per second. This is
-// the 30Khz reference frequency desribed in the Stella Programmer's Guide.
-const SampleFreq = 31400
+// SampleFreq represents the number of samples generated per second
+const SampleFreq = 15700 * 2
 
-// Audio is the implementation of the TIA audio sub-system, using Ron Fries'
-// method. Reference source code here:
-//
-// https://raw.githubusercontent.com/alekmaul/stella/master/emucore/TIASound.c
+// Audio is the implementation of the TIA audio sub-system
 type Audio struct {
 	env *environment.Environment
 
@@ -51,6 +47,11 @@ type Audio struct {
 	// twice in that time
 	clock228 int
 
+	// the volume is sampled every colour clock and the volume at each clock is
+	// summed. at fixed points, the volume is averaged
+	sampleSum   []int
+	sampleSumCt int
+
 	// From the "Stella Programmer's Guide":
 	//
 	// "There are two audio circuits for generating sound. They are identical but
@@ -62,30 +63,27 @@ type Audio struct {
 	Vol0 uint8
 	Vol1 uint8
 
+	// the addition of a tracker is not required
 	tracker          Tracker
 	registersChanged bool
+	samplePoint      bool
 }
 
 // NewAudio is the preferred method of initialisation for the Audio sub-system.
 func NewAudio(env *environment.Environment) *Audio {
-	return &Audio{
-		env: env,
+	au := &Audio{
+		env:       env,
+		sampleSum: make([]int, 2),
 	}
+
+	return au
 }
 
 // Plumb audio into emulation
 func (au *Audio) Plumb(env *environment.Environment) {
 	au.env = env
 }
 
-func (au *Audio) Reset() {
-	au.clock228 = 0
-	au.channel0 = channel{}
-	au.channel1 = channel{}
-	au.Vol0 = 0
-	au.Vol1 = 0
-}
-
 // SetTracker adds a Tracker implementation to the Audio sub-system.
 func (au *Audio) SetTracker(tracker Tracker) {
 	au.tracker = tracker
@@ -115,6 +113,8 @@ func (au *Audio) UpdateTracker() {
 // 30Khz clock.
 func (au *Audio) Step() bool {
 	au.registersChanged = false
+	au.samplePoint = false
+
 	if au.tracker != nil {
 		// it's impossible for both channels to have changed in a single video cycle
 		if au.channel0.registersChanged {
@@ -128,35 +128,42 @@ func (au *Audio) Step() bool {
 		}
 	}
 
-	au.clock228++
-	if au.clock228 >= 228 {
-		au.clock228 = 0
-		return false
-	}
+	var changed bool
+
+	// sum volume bits
+	au.sampleSum[0] += int(au.channel0.actualVolume())
+	au.sampleSum[1] += int(au.channel1.actualVolume())
+	au.sampleSumCt++
 
 	switch au.clock228 {
 	case 10:
-		au.channel0.phase0()
-		au.channel1.phase0()
-		return false
+		fallthrough
 	case 82:
 		au.channel0.phase0()
 		au.channel1.phase0()
-		return false
 	case 38:
-		au.channel0.phase1()
-		au.channel1.phase1()
+		fallthrough
 	case 150:
 		au.channel0.phase1()
 		au.channel1.phase1()
-	default:
-		return false
+
+		// take average of sum of volume bits
+		au.Vol0 = uint8(au.sampleSum[0] / au.sampleSumCt)
+		au.Vol1 = uint8(au.sampleSum[1] / au.sampleSumCt)
+		au.sampleSum[0] = 0
+		au.sampleSum[1] = 0
+		au.sampleSumCt = 0
+
+		changed = true
 	}
 
-	au.Vol0 = au.channel0.actualVol
-	au.Vol1 = au.channel1.actualVol
+	// advance 228 clock and reset sample counter
+	au.clock228++
+	if au.clock228 >= 228 {
+		au.clock228 = 0
+	}
 
-	return true
+	return changed
 }
 
 // HasTicked returns whether the audio channels were ticked on the previous

hardware/tia/audio/channels.go

@@ -28,7 +28,7 @@ type channel struct {
 	pulseCounter uint8
 	noiseCounter uint8
 
-	actualVol uint8
+	volumeChanged bool
 }
 
 func (ch *channel) String() string {
@@ -119,6 +119,11 @@ func (ch *channel) phase1() {
 			}
 		}
 	}
+}
 
-	ch.actualVol = (ch.pulseCounter & 0x01) * ch.registers.Volume
+// the actual volume of the channel is the volume in the register multiplied by
+// the lower bit of the pulsecounter. this is then used in combination with the
+// volume of the other channel to get the actual output volume
+func (ch *channel) actualVolume() uint8 {
+	return (ch.pulseCounter & 0x01) * ch.registers.Volume
 }

hardware/tia/audio/doc.go

@@ -28,4 +28,12 @@
 //	 Stella (published under the GNU GPL v2.0 licence)
 //			https://github.com/stella-emu/stella/blob/e6af23d6c12893dd17711002971087f28f87c31f/src/emucore/tia/Audio.cxx
 //			https://github.com/stella-emu/stella/blob/e6af23d6c12893dd17711002971087f28f87c31f/src/emucore/tia/AudioChannel.cxx
+//
+// Additional work on volume sampling a result of this thread:
+//
+// https://forums.atariage.com/topic/370460-8-bit-digital-audio-from-2600/
+//
+// For reference, Ron Fries' audio method is represented here:
+//
+// https://raw.githubusercontent.com/alekmaul/stella/master/emucore/TIASound.c
 package audio

hardware/tia/audio/registers.go

@@ -56,30 +56,27 @@ func (au *Audio) ReadMemRegisters(data chipbus.ChangedRegister) bool {
 	switch data.Register {
 	case cpubus.AUDC0:
 		au.channel0.registers.Control = data.Value & 0x0f
-		au.channel0.reactAUDCx()
+		au.channel0.registersChanged = true
 	case cpubus.AUDC1:
 		au.channel1.registers.Control = data.Value & 0x0f
-		au.channel1.reactAUDCx()
+		au.channel1.registersChanged = true
 	case cpubus.AUDF0:
 		au.channel0.registers.Freq = data.Value & 0x1f
-		au.channel0.reactAUDCx()
+		au.channel0.registersChanged = true
 	case cpubus.AUDF1:
 		au.channel1.registers.Freq = data.Value & 0x1f
-		au.channel1.reactAUDCx()
+		au.channel1.registersChanged = true
 	case cpubus.AUDV0:
 		au.channel0.registers.Volume = data.Value & 0x0f
-		au.channel0.reactAUDCx()
+		au.channel0.volumeChanged = true
+		au.channel0.registersChanged = true
 	case cpubus.AUDV1:
 		au.channel1.registers.Volume = data.Value & 0x0f
-		au.channel1.reactAUDCx()
+		au.channel0.volumeChanged = true
+		au.channel1.registersChanged = true
 	default:
 		return true
 	}
 
 	return false
 }
-
-// changing the value of an AUDx registers causes some side effect.
-func (ch *channel) reactAUDCx() {
-	ch.registersChanged = true
-}