371 lines
12 KiB
Go
371 lines
12 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, a pitch LOCK that follows a single tone (so QRM
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// at other pitches is ignored), an adaptive envelope/threshold on the LOCKED
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// tone (level-independent, so weak or strong signals both key cleanly), an
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// adaptive dot-length (WPM) estimate, and a timing state machine that maps
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// marks/spaces to Morse and 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. As with every audio CW decoder, weak signals
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// and very heavy QRM still degrade it; the pitch lock keeps QRM on other tones
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// out of the decode.
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package cwdecode
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import (
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"math"
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"sort"
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"sync/atomic"
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)
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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 (0 = not locked)
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Level float64 `json:"level"` // 0..1 input audio level (RMS) for the meter
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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 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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mags []float64 // per-bin magnitude this hop
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nbuf []float64 // scratch for the noise percentile
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// Fixed-pitch target (Hz). 0 = auto-search; >0 = lock to the nearest bin and
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// ignore everything else (e.g. follow the radio's CW pitch). Set live from
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// another goroutine, so it's atomic.
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targetHz atomic.Int32
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// Pitch lock.
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lockIdx int // index of the locked tone bin, -1 = unlocked
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candIdx int // current argmax candidate while unlocked
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candHops int // consecutive hops the candidate has been dominant
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quietHops int // consecutive key-up hops while locked
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noise float64 // broadband noise estimate (percentile of bins)
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relockHops int // quiet hops before the lock is released
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acqSNR float64 // tone/noise ratio to acquire after a few stable hops
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strongSNR float64 // tone/noise ratio to lock immediately (1 hop)
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// Adaptive keying envelope, on the LOCKED bin's magnitude.
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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
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wordEmitted bool
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lastPitch float64
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lastRMS float64
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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 resolution
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win: sampleRate / 72, // ~14 ms Goertzel window (selective, fairly snappy)
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dotHops: 15, // ~20 WPM seed
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acqSNR: 1.9, // ignore noise spikes (looser locked onto noise = garbage)
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strongSNR: 3.2, // only a genuinely strong tone locks in 1 hop
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lockIdx: -1,
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candIdx: -1,
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statusEvery: 25, // ~10 Hz
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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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d.relockHops = int(0.8 * float64(d.fs) / float64(d.hop)) // release lock after ~0.8 s quiet
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// Candidate CW tones: 400–1000 Hz every 25 Hz. Deliberately NOT lower: strong
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// low-frequency noise/hum (pink/red noise rises toward DC) would otherwise win
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// the argmax and lock the decoder onto ~250 Hz junk instead of the signal.
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for f := 400.0; f <= 1000.0; 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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d.mags = make([]float64, len(d.freqs))
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d.nbuf = make([]float64, len(d.freqs))
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return d
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}
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// SetTarget fixes the decode pitch to hz (lock to the nearest bin, ignore other
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// tones), or returns to auto-search when hz <= 0. Safe to call concurrently.
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func (d *Decoder) SetTarget(hz int) { d.targetHz.Store(int32(hz)) }
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// nearestBin returns the bin index closest to hz.
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func (d *Decoder) nearestBin(hz float64) int {
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best, bestD := 0, math.Inf(1)
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for i, f := range d.freqs {
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if dd := math.Abs(f - hz); dd < bestD {
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bestD, best = dd, i
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}
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}
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return best
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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.lockIdx, d.candIdx, d.candHops, d.quietHops = -1, -1, 0, 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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d.analyze()
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d.step()
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}
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}
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}
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// analyze runs the Goertzel bank, estimates the noise floor, and maintains the
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// pitch lock (which tone the envelope detector then follows).
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func (d *Decoder) analyze() {
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n := float64(len(d.ring))
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var sumSq float64
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maxIdx, maxMag := 0, -1.0
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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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m := math.Sqrt(math.Max(s1*s1+s2*s2-coeff*s1*s2, 0)) / n
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d.mags[i] = m
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if m > maxMag {
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maxMag = m
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maxIdx = 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)
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// Fixed-pitch mode: lock straight to the target bin, skip the auto search.
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// A narrow filter at the known pitch is exactly how a skimmer avoids QRM.
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if th := int(d.targetHz.Load()); th > 0 {
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d.lockIdx = d.nearestBin(float64(th))
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d.lastPitch = d.freqs[d.lockIdx]
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return
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}
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// Noise floor = 40th percentile of the bins (robust to a few strong tones).
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copy(d.nbuf, d.mags)
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sort.Float64s(d.nbuf)
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d.noise = d.nbuf[int(0.4*float64(len(d.nbuf)-1)+0.5)]
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if d.lockIdx < 0 {
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if maxIdx == d.candIdx {
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d.candHops++
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} else {
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d.candIdx, d.candHops = maxIdx, 1
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}
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snr := maxMag / (d.noise + 1e-9)
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// Tiered acquisition: a clearly strong tone locks on the FIRST hop (so we
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// don't eat the first element of a strong signal), a marginal/weak tone
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// locks after a couple of stable hops (so we don't lock onto pure noise).
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if snr > d.strongSNR || (d.candHops >= 3 && snr > d.acqSNR) {
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d.lockIdx = maxIdx
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d.peak, d.floor = maxMag, d.noise // seed the envelope to this bin
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d.quietHops = 0
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}
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}
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if d.lockIdx >= 0 {
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d.lastPitch = d.freqs[d.lockIdx]
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} else {
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d.lastPitch = 0
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}
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}
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// step runs the adaptive envelope on the locked bin and the timing state
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// machine, one hop. The envelope adapts to the signal level (not an absolute
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// threshold), so weak and strong signals both key correctly.
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func (d *Decoder) step() {
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on := false
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if d.lockIdx >= 0 {
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m := d.mags[d.lockIdx]
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// Peak: fast attack, slow release.
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if m > d.peak {
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d.peak += (m - d.peak) * 0.4
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} else {
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d.peak += (m - d.peak) * 0.02
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}
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// Floor: drops fast toward the signal, but only RISES between marks (when
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// keyed up). Letting the floor rise during a long dash would shrink the
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// span until the dash drops below the threshold and fragments into dots —
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// the cause of the "all dots" garbage on a strong clean signal.
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if m < d.floor {
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d.floor += (m - d.floor) * 0.4
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} else if !d.state {
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d.floor += (m - d.floor) * 0.02
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}
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span := d.peak - d.floor
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// The frozen floor already stops dashes fragmenting, so keep balanced
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// thresholds: low enough that short inter-element GAPS are still seen
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// (otherwise elements merge into >7-symbol runs that decode to nothing).
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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 = m > offTh
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} else {
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on = m > onTh
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}
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}
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// Release the lock after a long quiet so we can retune to a new signal.
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if on {
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d.quietHops = 0
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} else {
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d.quietHops++
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if d.quietHops > d.relockHops {
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d.lockIdx, d.candIdx, d.candHops = -1, -1, 0
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}
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}
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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()
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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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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. Runs shorter than a third of a dot are rejected as
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// clicks/noise.
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func (d *Decoder) endMark(hops int) {
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h := float64(hops)
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// Reject clicks/noise: shorter than a third of a dot AND an absolute floor
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// of ~4 hops (~16 ms, i.e. faster than ~75 WPM) so noise can't drag the
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// dot-length estimate down to the clamp (which produced 100 WPM garbage).
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if h < d.dotHops*0.35 || h < 4 {
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return
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}
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if h > d.dotHops*2 {
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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*0.8 + obs*0.2 // slower: a few odd marks can't yank it
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if d.dotHops < 5 { // 5 hops ≈ 60 WPM ceiling — never 100
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d.dotHops = 5
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}
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if d.dotHops > 55 {
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d.dotHops = 55
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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 && len(d.elem) <= 7 {
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// Only flag a genuinely Morse-shaped but unknown char with "?". An
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// over-long element run is noise — drop it silently rather than spam "?".
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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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