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

package rds

import (
	"math"
)

const (
	defaultBitRate    = 1187.5
	defaultSubcarrier = 57000
	defaultAmplitude  = 0.02
)

// Encoder emits a simple BPSK-like RDS subcarrier stream for offline MPX builds.
type Encoder struct {
	config     RDSConfig
	sampleRate float64
	bits       []float64
	bitRate    float64
	subFreq    float64
	amplitude  float64

	bitPhase float64
	bitIndex int
	subPhase float64
}

// NewEncoder builds a new encoder for the provided configuration and sample rate.
func NewEncoder(cfg RDSConfig) (*Encoder, error) {
	if cfg.SampleRate <= 0 {
		cfg.SampleRate = 48000
	}

	bits := buildBits(cfg)
	if len(bits) == 0 {
		bits = []float64{1}
	}

	return &Encoder{
		config:     cfg,
		sampleRate: cfg.SampleRate,
		bits:       bits,
		bitRate:    defaultBitRate,
		subFreq:    defaultSubcarrier,
		amplitude:  defaultAmplitude,
	}, nil
}

// Reset restarts the encoder phases so Generate outputs from the beginning of the bit stream again.
func (e *Encoder) Reset() {
	e.bitPhase = 0
	e.bitIndex = 0
	e.subPhase = 0
}

// Generate produces the requested number of RDS samples.
func (e *Encoder) Generate(samples int) []float64 {
	out := make([]float64, samples)
	if len(e.bits) == 0 || samples == 0 {
		return out
	}

	for i := 0; i < samples; i++ {
		out[i] = e.nextSample()
	}
	return out
}

func (e *Encoder) nextSample() float64 {
	symbol := e.bits[e.bitIndex]
	value := e.amplitude * symbol * math.Sin(2*math.Pi*e.subPhase)
	e.subPhase += e.subFreq / e.sampleRate
	if e.subPhase >= 1 {
		e.subPhase -= math.Floor(e.subPhase)
	}

	e.bitPhase += e.bitRate / e.sampleRate
	if e.bitPhase >= 1 {
		steps := int(e.bitPhase)
		e.bitIndex = (e.bitIndex + steps) % len(e.bits)
		e.bitPhase -= float64(steps)
	}

	return value
}

func buildBits(cfg RDSConfig) []float64 {
	var bits []float64
	bits = append(bits, wordToBits(cfg.PI)...)
	status := uint8(cfg.PTY&0x1F) | boolToBit(cfg.TP)<<7 | boolToBit(cfg.TA)<<6
	bits = append(bits, byteToBits(status)...)
	bits = append(bits, stringToBits(cfg.PS)...)
	bits = append(bits, stringToBits(cfg.RT)...)
	return bits
}

func wordToBits(word uint16) []float64 {
	bits := make([]float64, 0, 16)
	for i := 15; i >= 0; i-- {
		bits = append(bits, bitToSymbol(uint8((word>>i)&1)))
	}
	return bits
}

func byteToBits(b uint8) []float64 {
	bits := make([]float64, 0, 8)
	for i := 7; i >= 0; i-- {
		bits = append(bits, bitToSymbol(uint8((b>>i)&1)))
	}
	return bits
}

func stringToBits(text string) []float64 {
	bits := make([]float64, 0, len(text)*8)
	for i := 0; i < len(text); i++ {
		for bit := 7; bit >= 0; bit-- {
			bits = append(bits, bitToSymbol(uint8((text[i]>>bit)&1)))
		}
	}
	return bits
}

func bitToSymbol(bit uint8) float64 {
	if bit == 0 {
		return -1
	}
	return 1
}

func boolToBit(value bool) uint8 {
	if value {
		return 1
	}
	return 0
}