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.
// All outputs are unity-normalized. The combiner controls actual injection levels.
type Components struct {
	Mono   float64 // (L+R)/2 baseband
	Stereo float64 // (L-R)/2 * sin(2π·38kHz·t), unity subcarrier
	Pilot  float64 // sin(2π·19kHz·t), unity amplitude
}

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

// NewStereoEncoder creates a StereoEncoder configured for the provided sample rate.
func NewStereoEncoder(sampleRate float64) StereoEncoder {
	return StereoEncoder{
		pilot:      dsp.NewPilotGenerator(sampleRate),
		subcarrier: dsp.Oscillator{Frequency: 38000, SampleRate: sampleRate},
	}
}

// Encode converts a stereo frame into MPX components.
func (s *StereoEncoder) Encode(frame audio.Frame) Components {
	pilot := s.pilot.Sample()
	sub38 := s.subcarrier.Tick()

	return Components{
		Mono:   float64(frame.Mono()),
		Stereo: float64(frame.Difference()) * sub38,
		Pilot:  pilot,
	}
}

// Reset restarts the pilot and subcarrier generators.
func (s *StereoEncoder) Reset() {
	s.pilot.Reset()
	s.subcarrier.Reset()
}