fm-rds-tx/internal/ingest/convert.go

package ingest

import (
	"fmt"
	"math"

	"github.com/jan/fm-rds-tx/internal/audio"
)

const int32AbsMax = 2147483648.0

func ChunkToFrames(chunk PCMChunk) ([]audio.Frame, error) {
	if chunk.Channels != 1 && chunk.Channels != 2 {
		return nil, fmt.Errorf("unsupported channel count: %d", chunk.Channels)
	}
	if chunk.Channels <= 0 {
		return nil, fmt.Errorf("invalid channel count: %d", chunk.Channels)
	}
	if len(chunk.Samples)%chunk.Channels != 0 {
		return nil, fmt.Errorf("invalid interleaved sample count: %d for channels=%d", len(chunk.Samples), chunk.Channels)
	}

	frames := make([]audio.Frame, len(chunk.Samples)/chunk.Channels)
	switch chunk.Channels {
	case 1:
		for i := range frames {
			s := normalizePCM(chunk.Samples[i])
			frames[i] = audio.NewFrame(s, s)
		}
	case 2:
		for i := range frames {
			off := i * 2
			l := normalizePCM(chunk.Samples[off])
			r := normalizePCM(chunk.Samples[off+1])
			frames[i] = audio.NewFrame(l, r)
		}
	}
	return frames, nil
}

func normalizePCM(v int32) audio.Sample {
	norm := float64(v) / int32AbsMax
	norm = math.Max(float64(audio.SampleMin), math.Min(float64(audio.SampleMax), norm))
	return audio.Sample(norm)
}