fm-rds-tx/internal/dsp/iqresample.go

package dsp

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

// ResampleIQ resamples a CompositeFrame from its native sample rate to
// the target device rate using linear interpolation. Returns a new frame.
// If rates are equal (within 0.5 Hz), returns the original frame unchanged.
func ResampleIQ(frame *output.CompositeFrame, targetRateHz float64) *output.CompositeFrame {
	if frame == nil || len(frame.Samples) == 0 {
		return frame
	}

	srcRate := frame.SampleRateHz
	if srcRate <= 0 || targetRateHz <= 0 {
		return frame
	}

	// No resampling needed if rates match
	ratio := targetRateHz / srcRate
	if ratio > 0.999 && ratio < 1.001 {
		return frame
	}

	srcLen := len(frame.Samples)
	dstLen := int(float64(srcLen) * ratio)
	if dstLen <= 0 {
		return frame
	}

	dst := make([]output.IQSample, dstLen)
	step := 1.0 / ratio // position step in source samples per output sample

	pos := 0.0
	for i := 0; i < dstLen; i++ {
		idx := int(pos)
		frac := float32(pos - float64(idx))

		if idx+1 < srcLen {
			s0 := frame.Samples[idx]
			s1 := frame.Samples[idx+1]
			dst[i] = output.IQSample{
				I: s0.I*(1-frac) + s1.I*frac,
				Q: s0.Q*(1-frac) + s1.Q*frac,
			}
		} else if idx < srcLen {
			dst[i] = frame.Samples[idx]
		}
		pos += step
	}

	return &output.CompositeFrame{
		Samples:      dst,
		SampleRateHz: targetRateHz,
		Timestamp:    frame.Timestamp,
		GeneratedAt:  frame.GeneratedAt,
		Sequence:     frame.Sequence,
	}
}