276 lines
7.8 KiB
Go
276 lines
7.8 KiB
Go
// Package cwdecode is a real-time CW (Morse) decoder: it turns a stream of
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// mono PCM samples into decoded text. The pipeline is the classic one — a bank
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// of Goertzel tone detectors (auto-picking the dominant pitch), an adaptive
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// envelope/threshold to recover key-down/key-up, an adaptive dot-length (WPM)
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// estimate, and a timing state machine that maps marks/spaces to Morse and
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// then to characters.
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//
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// It is deliberately self-contained and dependency-free so it can be unit
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// tested with synthetic signals. Robustness on weak/QRM/QSB signals is limited
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// (as with every audio CW decoder); it does well on clean signals.
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package cwdecode
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import "math"
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// Status is a periodic snapshot for the UI (pitch lock, speed, signal).
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type Status struct {
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WPM int `json:"wpm"`
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Pitch int `json:"pitch"` // Hz of the locked tone
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Level float64 `json:"level"` // 0..1 rough signal strength (SNR proxy)
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Active bool `json:"active"` // a tone is currently keyed down
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}
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// Decoder consumes PCM and emits decoded characters via onChar (one or more
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// characters at a time, including " " for word gaps) and periodic onStatus.
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type Decoder struct {
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fs int
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hop int // samples between envelope updates
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win int // Goertzel window length
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freqs []float64
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coeffs []float64 // precomputed 2*cos(w) per freq
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ring []float64 // last win samples
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acc int // samples since last hop
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// Adaptive envelope (relative, so absolute gain is irrelevant).
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peak, floor float64
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state bool // true = mark (key down)
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stateHops int
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dotHops float64 // adaptive dot length, in hops
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elem []byte // current "." / "-" run for the in-progress character
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charEmitted bool // current space already flushed a character
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wordEmitted bool // current space already flushed a word gap
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lastPitch float64
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lastRMS float64 // 0..1 input level of the current window (for the UI meter)
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statusEvery int
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sinceStatus int
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onChar func(string)
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onStatus func(Status)
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}
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var morse = map[string]byte{
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".-": 'A', "-...": 'B', "-.-.": 'C', "-..": 'D', ".": 'E', "..-.": 'F',
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"--.": 'G', "....": 'H', "..": 'I', ".---": 'J', "-.-": 'K', ".-..": 'L',
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"--": 'M', "-.": 'N', "---": 'O', ".--.": 'P', "--.-": 'Q', ".-.": 'R',
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"...": 'S', "-": 'T', "..-": 'U', "...-": 'V', ".--": 'W', "-..-": 'X',
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"-.--": 'Y', "--..": 'Z',
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"-----": '0', ".----": '1', "..---": '2', "...--": '3', "....-": '4',
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".....": '5', "-....": '6', "--...": '7', "---..": '8', "----.": '9',
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".-.-.-": '.', "--..--": ',', "..--..": '?', "-..-.": '/', "-...-": '=',
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".-.-.": '+', "-.-.--": '!', "---...": ':', "-....-": '-', ".--.-.": '@',
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}
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// New builds a decoder for the given sample rate. onChar receives decoded text
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// incrementally; onStatus receives ~10 snapshots/second. Either may be nil.
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func New(sampleRate int, onChar func(string), onStatus func(Status)) *Decoder {
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if sampleRate <= 0 {
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sampleRate = 16000
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}
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d := &Decoder{
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fs: sampleRate,
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hop: sampleRate / 250, // ~4 ms envelope resolution
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win: sampleRate / 62, // ~16 ms Goertzel window
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dotHops: 15, // ~20 WPM seed (15 hops * 4 ms = 60 ms)
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statusEvery: 25, // ~10 Hz status
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onChar: onChar,
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onStatus: onStatus,
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}
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if d.hop < 1 {
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d.hop = 1
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}
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// Candidate CW tones: 250–1200 Hz every 25 Hz (wide enough for most rigs'
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// audio offset). The dominant bin is the pitch (auto), and its magnitude
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// drives the envelope.
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for f := 250.0; f <= 1200.0; f += 25 {
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d.freqs = append(d.freqs, f)
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d.coeffs = append(d.coeffs, 2*math.Cos(2*math.Pi*f/float64(d.fs)))
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}
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return d
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}
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// Reset clears decode state (e.g. when the user re-arms the decoder).
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func (d *Decoder) Reset() {
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d.ring = d.ring[:0]
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d.acc = 0
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d.peak, d.floor = 0, 0
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d.state = false
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d.stateHops = 0
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d.dotHops = 15
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d.elem = d.elem[:0]
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d.charEmitted, d.wordEmitted = false, false
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}
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// Process feeds a block of mono samples through the decoder.
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func (d *Decoder) Process(samples []int16) {
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for _, s := range samples {
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d.ring = append(d.ring, float64(s))
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if len(d.ring) > d.win {
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d.ring = d.ring[len(d.ring)-d.win:]
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}
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d.acc++
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if d.acc >= d.hop && len(d.ring) >= d.win {
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d.acc = 0
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mag, pitch := d.toneMag()
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d.step(mag, pitch)
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}
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}
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}
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// toneMag runs the Goertzel bank over the current window and returns the
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// strongest bin's magnitude and its frequency (the auto-detected pitch).
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func (d *Decoder) toneMag() (float64, float64) {
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best, bestF := 0.0, d.lastPitch
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n := float64(len(d.ring))
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var sumSq float64
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for i, coeff := range d.coeffs {
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var s1, s2 float64
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for _, x := range d.ring {
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s0 := x + coeff*s1 - s2
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s2 = s1
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s1 = s0
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}
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power := s1*s1 + s2*s2 - coeff*s1*s2
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if power > best {
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best = power
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bestF = d.freqs[i]
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}
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}
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for _, x := range d.ring {
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sumSq += x * x
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}
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d.lastRMS = math.Min(1, math.Sqrt(sumSq/n)/32768*4) // ×4 so quiet audio is visible
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// Normalise by window length so the magnitude scale is rate-independent.
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return math.Sqrt(math.Max(best, 0)) / n, bestF
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}
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// step advances the envelope follower + timing state machine by one hop.
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func (d *Decoder) step(mag, pitch float64) {
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// Envelope: fast attack / slow release for the peak, fast drop / slow rise
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// for the noise floor. Tracks the signal even through QSB.
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if mag > d.peak {
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d.peak += (mag - d.peak) * 0.4
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} else {
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d.peak += (mag - d.peak) * 0.02
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}
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if mag < d.floor {
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d.floor += (mag - d.floor) * 0.4
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} else {
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d.floor += (mag - d.floor) * 0.01
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}
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span := d.peak - d.floor
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// Hysteresis thresholds; require a minimum SNR span to call anything a tone.
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on := d.state
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if span > d.floor*0.3+1e-9 {
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onTh := d.floor + 0.55*span
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offTh := d.floor + 0.35*span
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if d.state {
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on = mag > offTh
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} else {
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on = mag > onTh
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}
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if on {
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d.lastPitch = pitch
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}
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} else {
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on = false
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}
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if on == d.state {
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d.stateHops++
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if !d.state {
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d.spaceProgress() // flush char/word as the gap grows
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}
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} else {
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if d.state {
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d.endMark(d.stateHops)
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}
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d.state = on
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d.stateHops = 1
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if on {
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// A new mark starts → the previous space is over; re-arm flushing.
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d.charEmitted, d.wordEmitted = false, false
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}
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}
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d.emitStatus(on)
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}
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// endMark classifies a finished key-down run as a dot or dash and adapts the
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// dot-length estimate.
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func (d *Decoder) endMark(hops int) {
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h := float64(hops)
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// Reject impulse noise far shorter than a dot.
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if h < d.dotHops*0.35 {
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return
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}
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dash := h > d.dotHops*2
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if dash {
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d.elem = append(d.elem, '-')
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d.adaptDot(h / 3)
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} else {
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d.elem = append(d.elem, '.')
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d.adaptDot(h)
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}
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}
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// adaptDot nudges the dot-length estimate toward an observation (EMA, clamped
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// to ~5–100 WPM).
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func (d *Decoder) adaptDot(obs float64) {
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d.dotHops = d.dotHops*0.7 + obs*0.3
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if d.dotHops < 3 {
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d.dotHops = 3
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}
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if d.dotHops > 60 {
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d.dotHops = 60
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}
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}
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// spaceProgress flushes the current character once the gap exceeds a character
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// gap, and a word space once it exceeds a word gap.
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func (d *Decoder) spaceProgress() {
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g := float64(d.stateHops)
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if !d.charEmitted && g > d.dotHops*2 {
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d.flushChar()
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d.charEmitted = true
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}
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if !d.wordEmitted && g > d.dotHops*5 {
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if d.onChar != nil {
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d.onChar(" ")
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}
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d.wordEmitted = true
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}
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}
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// flushChar looks up the accumulated element string and emits the character.
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func (d *Decoder) flushChar() {
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if len(d.elem) == 0 {
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return
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}
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if c, ok := morse[string(d.elem)]; ok {
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if d.onChar != nil {
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d.onChar(string(c))
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}
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} else if d.onChar != nil {
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d.onChar("?")
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}
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d.elem = d.elem[:0]
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}
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func (d *Decoder) emitStatus(on bool) {
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d.sinceStatus++
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if d.sinceStatus < d.statusEvery || d.onStatus == nil {
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return
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}
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d.sinceStatus = 0
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hopMs := float64(d.hop) / float64(d.fs) * 1000
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wpm := 0
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if d.dotHops > 0 {
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wpm = int(math.Round(1200 / (d.dotHops * hopMs)))
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}
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d.onStatus(Status{WPM: wpm, Pitch: int(math.Round(d.lastPitch)), Level: d.lastRMS, Active: on})
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}
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