feat: added record qso dvk

internal/audio/mp3.go

@@ -0,0 +1,54 @@
+//go:build windows
+
+package audio
+
+import (
+	"os"
+
+	"github.com/braheezy/shine-mp3/pkg/mp3"
+)
+
+// mp3Rate is the encode sample rate. The capture pipeline is 16 kHz, but the
+// Shine encoder emits broken "free-format" frames at MPEG-2 rates (16/22/24
+// kHz) that most players reject. Encoding at an MPEG-1 rate (we upsample ×2 to
+// 32 kHz) produces standard, universally-playable MP3s.
+const mp3Rate = sampleRate * 2 // 32000
+
+// writeMP3 encodes 16 kHz mono 16-bit PCM to a standard MP3 file using the
+// pure-Go Shine encoder (no CGO). Two quirks are worked around:
+//   - 16 kHz (MPEG-2) yields broken free-format frames → upsample ×2 to 32 kHz.
+//   - Shine's Write only encodes half the samples for MONO input (its loop
+//     advances by samples_per_pass*2). Feeding STEREO interleaved data (the
+//     encoder reads samples_per_pass*channels per pass) encodes everything, so
+//     we duplicate mono → L=R stereo.
+func writeMP3(path string, pcm []byte) error {
+	f, err := os.Create(path)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+	mono32 := upsample2(bytesToInt16(pcm)) // 16 kHz → 32 kHz mono
+	stereo := make([]int16, len(mono32)*2) // L=R interleaved
+	for i, v := range mono32 {
+		stereo[2*i], stereo[2*i+1] = v, v
+	}
+	enc := mp3.NewEncoder(mp3Rate, 2)
+	return enc.Write(f, stereo)
+}
+
+// upsample2 doubles the sample rate with linear interpolation (16 kHz → 32 kHz).
+func upsample2(in []int16) []int16 {
+	if len(in) == 0 {
+		return in
+	}
+	out := make([]int16, len(in)*2)
+	for i := range in {
+		out[2*i] = in[i]
+		if i+1 < len(in) {
+			out[2*i+1] = int16((int32(in[i]) + int32(in[i+1])) / 2)
+		} else {
+			out[2*i+1] = in[i]
+		}
+	}
+	return out
+}