gortty/rawdemodulator.go

// Copyright (C) 2017 Keelan Lightfoot
// Copyright (C) 2007-2008 Board of Regents of the University of Wisconsin
//             System (Univ. of Wisconsin-Madison, Trace R&D Center)
// Copyright (C) 2007-2008 Omnitor AB
// Copyright (C) 2007-2008 Voiceriver Inc
//
// This library is free software; you can redistribute it and/or modify it
// under the terms of the GNU Lesser General Public License as published by
// the Free Software Foundation; either version 2.1 of the License, or (at
// your option) any later version.

package gortty

import (
	"context"
	"math"
)

// RawDemodulator is fun
type RawDemodulator struct {
	input      chan int16
	output     chan DCSignal
	dsp        *dsp
	settings   *ModemSettings
	sigOut     bool
	samplerate int
	div        int
}

// NewRawDemodulator is fun
func NewRawDemodulator(ctx context.Context, s *ModemSettings, input chan int16, output chan DCSignal) *RawDemodulator {

	d := new(RawDemodulator)
	d.input = input
	d.output = output

	d.settings = s

	// the bandpass filter is picky about the sample rate, and the output is perfectly
	// balanced when the sample rate is 3x the center frequency, so lets divide the sample
	// rate down to as close to that as we can get.

	centerFrequency := math.Abs(float64(d.settings.OneFreq)+float64(d.settings.ZeroFreq)) / 2 * 3

	d.div = int(math.Floor(float64(d.settings.SampleRate) / float64(centerFrequency)))
	if d.div == 0 {
		d.div = 1
	}

	d.samplerate = d.settings.SampleRate / d.div

	d.dsp = newDsp(d.settings.OneFreq, d.settings.ZeroFreq, d.samplerate)

	go d.run(ctx)
	return d
}

func (d *RawDemodulator) run(ctx context.Context) {

	count := 0
	for {
		select {
		case <-ctx.Done():
			return
		case sam := <-d.input:
			// sample rate reduction
			count++
			if count%d.div > 0 {
				continue
			}

			d.dsp.demod(float64(sam))

			var signal int8
			var quality int8

			if math.Abs(d.dsp.demLpf) > d.dsp.demTotal*d.dsp.minThresh && d.dsp.demTotal*d.dsp.minThresh != 0 {
				quality = 1
			} else {
				quality = 0
			}

			if d.dsp.demLpf-d.dsp.minThresh*d.dsp.demTotal > 0 {
				signal = 1 * quality
			} else {
				signal = -1 * quality
			}

			d.output <- DCSignal{
				Signal:  signal,
				Quality: quality,
			}
		}
	}
}