[sbx2wav] Start adding a tool to generate tape audio

This tool will take a .studybox ROM and encode the data in an MFM audio
wave.  Currently, this just handles the data channel and not the
recorded audio.  Output is a mono .wav file.

Also, some timing and settings are probably wrong. (Generated audio
doesn't decode with Sour's decoder)

Makefile

@@ -1,10 +1,11 @@
 .PHONY: all
 
-all: bin/script-decode bin/sbutil bin/just-stats bin/extract-imgs
+all: bin/script-decode bin/sbutil bin/just-stats bin/extract-imgs bin/sbx2wav
 
 bin/script-decode: script/*.go
 bin/sbutil: rom/*.go
 bin/just-stats: script/*.go
+bin/sbx2wav: rom/*.go audio/*.go
 
 bin/%: cmd/%.go
 	go build -o $@ $<

audio/bitdata.go

@@ -0,0 +1,55 @@
+package audio
+
+type BitData struct {
+	data    []byte
+	next    int
+	current byte
+	left    int // bits left in current
+}
+
+func NewBitData(data []byte) *BitData {
+	if len(data) == 0 {
+		panic("no data")
+	}
+
+	return &BitData{
+		data: data,
+		next: 1,
+		current: data[0],
+		left: 7,
+	}
+}
+
+// Returns the bit in the lowest position, and end of data.  false if nothing left.
+func (b *BitData) Next() (byte, bool) {
+	if b.left < 0 {
+		if len(b.data) <= b.next {
+			return 0, false
+		}
+
+		b.current = b.data[b.next]
+		b.next++
+		b.left = 7
+		return 0, true
+	}
+
+	ret := (b.current >> b.left) & 0x01
+	b.left--
+	return ret, true
+}
+
+func (b *BitData) Peek() byte {
+	left := b.left
+	current := b.current
+
+	if left < 0 {
+		if len(b.data) <= b.next {
+			return 0
+		}
+		current = b.data[b.next]
+		left = 7
+	}
+
+	return (current >> left) & 0x01
+}
+

audio/encode.go

@@ -0,0 +1,247 @@
+package audio
+
+// TODO:
+// - Stereo with the recorded audio, not just data.
+// - Configurable lead-in silence (from start of audio)
+// - Configurable segment gap lengths (silence between segments)
+
+import (
+	"math"
+	"slices"
+	"io"
+	"bytes"
+	"fmt"
+
+	"github.com/go-audio/audio"
+	"github.com/go-audio/wav"
+
+	"git.zorchenhimer.com/Zorchenhimer/go-studybox/rom"
+)
+
+const (
+	SampleRate uint32 = 44_100 // TODO: verify sample rate with SBX audio
+	Amplitude  int = 16_000
+)
+
+var (
+	BitRate    int = 4890
+)
+
+func EncodeRom(w io.WriteSeeker, sbx *rom.StudyBox) error {
+	if sbx == nil {
+		return fmt.Errorf("nil rom")
+	}
+
+	if sbx.Audio == nil {
+		return fmt.Errorf("Missing audio")
+	}
+
+	if sbx.Audio.Format != rom.AUDIO_WAV {
+		return fmt.Errorf("unsupported audio format: %s", sbx.Audio)
+	}
+
+	if len(sbx.Data.Pages) == 0 {
+		return fmt.Errorf("no pages")
+	}
+
+	if len(sbx.Data.Pages[0].Packets) == 0 {
+		return fmt.Errorf("no packets")
+	}
+
+	wavreader := bytes.NewReader(sbx.Audio.Data)
+	decoder := wav.NewDecoder(wavreader)
+	if !decoder.IsValidFile() {
+		return fmt.Errorf(".studybox file does not contain a valid wav file")
+	}
+
+	decoder.ReadInfo()
+
+	if decoder.SampleRate != SampleRate {
+		return fmt.Errorf("SampleRate mismatch. Expected %d; found %d", SampleRate, decoder.SampleRate)
+	}
+
+	afmt := &audio.Format{
+		NumChannels: 1,
+		SampleRate: int(decoder.SampleRate),
+	}
+
+	writer := wav.NewEncoder(
+		w,
+		int(decoder.SampleRate),
+		int(decoder.BitDepth),
+		int(decoder.NumChans),
+		1)
+	defer writer.Close()
+
+	var err error
+	runningSamples := int64(0)
+
+	prevPageLeadIn := 0
+
+	for _, page := range sbx.Data.Pages {
+		if prevPageLeadIn > page.AudioOffsetLeadIn {
+			return fmt.Errorf("out of order pages (AudioOffsetLeadIn)")
+		}
+
+		prevPageLeadIn = page.AudioOffsetLeadIn
+
+		padLen := int64(page.AudioOffsetLeadIn)-runningSamples
+		fmt.Printf("padLen: %d = %d - %d\n", padLen, int64(page.AudioOffsetLeadIn), runningSamples)
+		if padLen < 0 {
+			padLen = 100_000
+		}
+
+		err = generatePadding(writer, afmt, padLen)
+		if err != nil {
+			return fmt.Errorf("generatePadding() error: %w", err)
+		}
+		runningSamples += padLen
+
+		sampleCount, err := encodePage(writer, afmt, page)
+		if err != nil {
+			return fmt.Errorf("encodePage() error: %w", err)
+		}
+		runningSamples += sampleCount
+	}
+
+	writer.Close()
+
+	return nil
+}
+
+func generatePadding(writer *wav.Encoder, afmt *audio.Format, length int64) error {
+	fmt.Println("generatePadding() length:", length)
+	buf := &audio.IntBuffer{
+		Format: afmt,
+		SourceBitDepth: writer.BitDepth,
+		Data: make([]int, length),
+	}
+
+	return writer.Write(buf)
+}
+
+func encodePage(writer *wav.Encoder, afmt *audio.Format, page *rom.Page) (int64, error) {
+	runningFract := float64(0)
+
+	//dataLeadLen := int(float64(page.AudioOffsetData - page.AudioOffsetLeadIn) / float64(83))
+	samplesPerFlux := float64(writer.SampleRate) / float64(BitRate) / 2
+	fmt.Println("samplesPerFlux:", samplesPerFlux)
+	dataLeadLen := (page.AudioOffsetData - page.AudioOffsetLeadIn) / int((samplesPerFlux * 9)) / 2
+	fmt.Println("dataLeadLen:", dataLeadLen-9)
+	lead := &rawData{
+		data: slices.Repeat([]byte{0}, dataLeadLen-9),
+		pageOffset: int64(page.AudioOffsetData),
+	}
+
+	fmt.Println("lead length:", len(lead.data))
+
+	runningFract = lead.encode(samplesPerFlux, 0)
+	data := []*rawData{}
+	data = append(data, lead)
+
+	for _, packet := range page.Packets {
+		d := &rawData{
+			data: packet.RawBytes(),
+			pageOffset: int64(page.AudioOffsetData),
+		}
+
+		runningFract = d.encode(samplesPerFlux, runningFract)
+
+		data = append(data, d)
+	}
+
+	sampleCount := int64(0)
+	for _, d := range data {
+		buf := &audio.IntBuffer{
+			Format: afmt,
+			SourceBitDepth: writer.BitDepth,
+			Data: d.samples,
+		}
+
+		err := writer.Write(buf)
+		if err != nil {
+			return sampleCount, err
+		}
+
+		sampleCount += int64(len(d.samples))
+	}
+
+	return sampleCount, nil
+}
+
+type rawData struct {
+	pageOffset int64
+	realOffset int64
+	data []byte
+	samples []int
+}
+
+func (d *rawData) encode(samplesPerFlux, runningFract float64) float64 {
+	bits := NewBitData(d.data)
+	flux := []byte{}
+	prev := 0
+
+	for {
+		bit, more := bits.Next()
+		if !more {
+			break
+		}
+
+		peek := bits.Peek()
+
+		if bit == 1 {
+			flux = append(flux, 1)
+		} else {
+			flux = append(flux, 0)
+		}
+
+		if bit == peek && bit == 0 {
+			// clock flux change
+			flux = append(flux, 1)
+		} else {
+			// no clock flux change
+			flux = append(flux, 0)
+		}
+	}
+
+	first := true
+	for _, f := range flux {
+		amp := Amplitude
+		if f == 1 {
+			if prev <= 0 {
+				prev = 1
+			} else {
+				prev = -1
+			}
+			//amp = int(float64(Amplitude) * 1.5)
+			first = true
+		}
+
+		//if i == 0 {
+		//	amp = Amplitude + 1000
+		//} else {
+		//	amp = Amplitude
+		//}
+
+		spf, fract := math.Modf(samplesPerFlux)
+		runningFract += fract
+		if runningFract >= 1 {
+			runningFract -= 1
+			spf++
+		}
+
+		//d.samples = append(d.samples, slices.Repeat([]int{prev*Amplitude}, int(spf))...)
+		for i := 0; i < int(spf); i++ {
+			if first {
+				d.samples = append(d.samples, prev*int(float64(amp)*1.5))
+			}
+			d.samples = append(d.samples, prev*amp)
+			first = false
+			//amp -= 500
+		}
+		//d.samples = append(d.samples, slices.Repeat([]int{prev*Amplitude}, int(samplesPerFlux))...)
+		first = false
+	}
+
+	return runningFract
+}

cmd/sbx2wav.go

@@ -0,0 +1,51 @@
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/alexflint/go-arg"
+
+	"git.zorchenhimer.com/Zorchenhimer/go-studybox/audio"
+	"git.zorchenhimer.com/Zorchenhimer/go-studybox/rom"
+)
+
+type Arguments struct {
+	Input  string `arg:"positional,required"`
+	Output string `arg:"positional,required"`
+
+	BitRate int `arg:"--bit-rate", default:"4790"` // value found by trial and error
+}
+
+func run(args *Arguments) error {
+	sbx, err := rom.ReadFile(args.Input)
+	if err != nil {
+		return err
+	}
+
+	output, err := os.Create(args.Output)
+	if err != nil {
+		return err
+	}
+	defer output.Close()
+
+	audio.BitRate = args.BitRate
+
+	err = audio.EncodeRom(output, sbx)
+	if err != nil {
+		return fmt.Errorf("Encode error: %w", err)
+	}
+
+	return nil
+}
+
+func main() {
+	args := &Arguments{}
+	arg.MustParse(args)
+
+	err := run(args)
+	if err != nil {
+		fmt.Fprintln(os.Stderr, err)
+		os.Exit(1)
+	}
+}

go.mod

@@ -3,8 +3,13 @@ module git.zorchenhimer.com/Zorchenhimer/go-studybox
 go 1.24.1
 
 require (
-	github.com/alexflint/go-arg v1.5.1
+	github.com/alexflint/go-arg v1.6.0
-	github.com/zorchenhimer/go-retroimg v0.0.0-20251108020316-705ea2ebacd6
+	github.com/go-audio/audio v1.0.0
+	github.com/go-audio/wav v1.1.0
+	github.com/zorchenhimer/go-retroimg v0.0.0-20251111010417-7299e86df5a9
 )
 
-require github.com/alexflint/go-scalar v1.2.0 // indirect
+require (
+	github.com/alexflint/go-scalar v1.2.0 // indirect
+	github.com/go-audio/riff v1.0.0 // indirect
+)

go.sum

@@ -1,15 +1,21 @@
-github.com/alexflint/go-arg v1.5.1 h1:nBuWUCpuRy0snAG+uIJ6N0UvYxpxA0/ghA/AaHxlT8Y=
+github.com/alexflint/go-arg v1.6.0 h1:wPP9TwTPO54fUVQl4nZoxbFfKCcy5E6HBCumj1XVRSo=
-github.com/alexflint/go-arg v1.5.1/go.mod h1:A7vTJzvjoaSTypg4biM5uYNTkJ27SkNTArtYXnlqVO8=
+github.com/alexflint/go-arg v1.6.0/go.mod h1:A7vTJzvjoaSTypg4biM5uYNTkJ27SkNTArtYXnlqVO8=
 github.com/alexflint/go-scalar v1.2.0 h1:WR7JPKkeNpnYIOfHRa7ivM21aWAdHD0gEWHCx+WQBRw=
 github.com/alexflint/go-scalar v1.2.0/go.mod h1:LoFvNMqS1CPrMVltza4LvnGKhaSpc3oyLEBUZVhhS2o=
 github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
 github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/go-audio/audio v1.0.0 h1:zS9vebldgbQqktK4H0lUqWrG8P0NxCJVqcj7ZpNnwd4=
+github.com/go-audio/audio v1.0.0/go.mod h1:6uAu0+H2lHkwdGsAY+j2wHPNPpPoeg5AaEFh9FlA+Zs=
+github.com/go-audio/riff v1.0.0 h1:d8iCGbDvox9BfLagY94fBynxSPHO80LmZCaOsmKxokA=
+github.com/go-audio/riff v1.0.0/go.mod h1:l3cQwc85y79NQFCRB7TiPoNiaijp6q8Z0Uv38rVG498=
+github.com/go-audio/wav v1.1.0 h1:jQgLtbqBzY7G+BM8fXF7AHUk1uHUviWS4X39d5rsL2g=
+github.com/go-audio/wav v1.1.0/go.mod h1:mpe9qfwbScEbkd8uybLuIpTgHyrISw/OTuvjUW2iGtE=
 github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
 github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
 github.com/stretchr/testify v1.2.2/go.mod h1:a8OnRcib4nhh0OaRAV+Yts87kKdq0PP7pXfy6kDkUVs=
 github.com/stretchr/testify v1.7.0 h1:nwc3DEeHmmLAfoZucVR881uASk0Mfjw8xYJ99tb5CcY=
 github.com/stretchr/testify v1.7.0/go.mod h1:6Fq8oRcR53rry900zMqJjRRixrwX3KX962/h/Wwjteg=
-github.com/zorchenhimer/go-retroimg v0.0.0-20251108020316-705ea2ebacd6 h1:94b43etKen/R0Ga+lxgfSMlYQicwsMvlFBizMOQ3loc=
+github.com/zorchenhimer/go-retroimg v0.0.0-20251111010417-7299e86df5a9 h1:tDALzDZa+sEzKDNDtPkbWIIxcfR3lB5X6ghUieaPLnE=
-github.com/zorchenhimer/go-retroimg v0.0.0-20251108020316-705ea2ebacd6/go.mod h1:iQUJQkvvbgycl7TS2OWdSC0+kHYypOASX129xmnv+SE=
+github.com/zorchenhimer/go-retroimg v0.0.0-20251111010417-7299e86df5a9/go.mod h1:iQUJQkvvbgycl7TS2OWdSC0+kHYypOASX129xmnv+SE=
 gopkg.in/yaml.v3 v3.0.0 h1:hjy8E9ON/egN1tAYqKb61G10WtihqetD4sz2H+8nIeA=
 gopkg.in/yaml.v3 v3.0.0/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=