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 modulated onto 38 kHz DSB-SC
	Pilot  float64 // 19 kHz pilot tone
}

// StereoEncoder generates stereo MPX primitives from stereo audio frames.
// It internally maintains phase-coherent 19 kHz pilot and 38 kHz subcarrier
// oscillators so that block-boundary phase continuity is guaranteed.
type StereoEncoder struct {
	pilot       dsp.PilotGenerator
	subcarrier  dsp.Oscillator // 38 kHz, phase-locked to pilot (2× pilot)
	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),
		subcarrier:  dsp.Oscillator{Frequency: 38000, SampleRate: sampleRate},
		LevelStereo: 1.0,
	}
}

// Encode converts a stereo frame into MPX components.
// The 38 kHz subcarrier is generated from the internal oscillator,
// maintaining continuous phase across calls.
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()) * s.LevelStereo * sub38,
		Pilot:  pilot,
	}
}

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