fm-rds-tx/internal/stereo/encoder.go

package stereo

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

// Components holds the individual MPX components produced by the stereo encoder.
type Components struct {
	Mono   float64 // L+R baseband
	Stereo float64 // L-R baseband on suppressed carrier
	Pilot  float64 // 19 kHz pilot tone
}

// StereoEncoder generates stereo MPX primitives from stereo audio frames.
type StereoEncoder struct {
	pilot       dsp.PilotGenerator
	LevelStereo float64
}

// NewStereoEncoder creates a StereoEncoder configured for the provided sample rate.
func NewStereoEncoder(sampleRate float64) StereoEncoder {
	return StereoEncoder{
		pilot:       dsp.NewPilotGenerator(sampleRate, 0.1),
		LevelStereo: 0.75,
	}
}

// Encode converts a stereo frame into MPX components.
func (s *StereoEncoder) Encode(frame audio.Frame) Components {
	return Components{
		Mono:   float64(frame.Mono()),
		Stereo: float64(frame.Difference()) * s.LevelStereo,
		Pilot:  s.pilot.Sample(),
	}
}

// Reset restarts the pilot generator phase.
func (s *StereoEncoder) Reset() {
	s.pilot.Reset()
}