OpsLog/internal/cat/tci_tx_probe.go

package cat

// Sending audio TO the radio over TCI.
//
// Three transmissions on a real SunSDR settled how this works, and none of it
// was guessable from the documentation:
//
//  1. The radio asks for audio only when the transmission is the CLIENT'S. With
//     the operator keying the microphone it sent 282 receive frames and nothing
//     else, over six seconds.
//  2. It asks only in a DIGITAL mode. Keyed from here in SSB: nothing, four
//     times over. The same button in DIGU: chrono frames immediately. In SSB the
//     modulator is wired to the microphone and no amount of network audio will
//     reach it — which is also the honest answer to "why can I hear myself but
//     not the tone".
//  3. The chrono is a REQUEST, not a clock to follow. It carries no payload —
//     the message itself is the ask — and it names the size it wants in the
//     header's length field: 2048 samples, two channels interleaved, arriving
//     47 times a second. Which is 1024 sample-pairs at 48 kHz, exactly real
//     time, measured rather than assumed.
//
// So audio is sent in ANSWER to chrono, never on a timer of our own. A timer
// was the first attempt and the radio ignored every frame of it: 234 sent, none
// used. Answering the request is what makes the difference, and it also means
// the radio sets the pace — no drift, no buffer to tune.
//
// What remains here is the probe: a tone, on demand, to prove the path end to
// end on real hardware. The voice keyer will use the same feed mechanism with
// WAV samples in place of the sine.

import (
	"encoding/binary"
	"fmt"
	"math"
	"time"

	"github.com/gorilla/websocket"
)

// tciTXProbeMaxSeconds caps the pass. Long enough to read a power meter, short
// enough that a carrier left running by a defect is a mistake and not an
// incident.
const tciTXProbeMaxSeconds = 10

// sendBinaryFrame writes one TCI binary frame: the 16-word header the radio's
// own frames carry, then the payload.
func (t *TCI) sendBinaryFrame(stype, rx, rate, length int, payload []byte) error {
	t.mu.Lock()
	c := t.conn
	t.mu.Unlock()
	if c == nil {
		return fmt.Errorf("tci: not connected")
	}
	buf := make([]byte, tciHeaderBytes+len(payload))
	le := binary.LittleEndian
	le.PutUint32(buf[0:], uint32(rx))
	le.PutUint32(buf[4:], uint32(rate))
	// format=3, codec=0: mirrored from what this radio SENDS. The field is
	// documented as an enumeration whose numbering did not survive contact with
	// the firmware — the receive stream answers 3 for four-byte floats — so the
	// only defensible choice is to speak back exactly what was spoken to us.
	le.PutUint32(buf[8:], 3)
	le.PutUint32(buf[12:], 0)
	le.PutUint32(buf[16:], 0) // crc — the radio sends 0 and does not check ours
	le.PutUint32(buf[20:], uint32(length))
	le.PutUint32(buf[24:], uint32(stype))
	copy(buf[tciHeaderBytes:], payload)

	t.wmu.Lock()
	defer t.wmu.Unlock()
	_ = c.SetWriteDeadline(time.Now().Add(3 * time.Second))
	return c.WriteMessage(websocket.BinaryMessage, buf)
}

// serveChrono answers one request for transmit audio.
//
// Called from the reader goroutine, so it does the least it can: take the
// frame from whatever is feeding, and write it. A feed that has run out returns
// nil and the request is counted rather than answered with silence — silence
// would be indistinguishable from a working stream on a meter.
func (t *TCI) serveChrono(rate, samples int) {
	t.audio.mu.Lock()
	feed := t.audio.txFeed
	t.audio.mu.Unlock()
	if feed == nil {
		return
	}
	if samples <= 0 {
		samples = 2048
	}
	payload := feed(samples)
	if payload == nil {
		t.audio.mu.Lock()
		t.audio.txShort++
		t.audio.mu.Unlock()
		return
	}
	if rate <= 0 {
		rate = 48000
	}
	if err := t.sendBinaryFrame(tciStreamTXAudio, 0, rate, samples, payload); err != nil {
		debugLog.Printf("TCI: could not send transmit audio: %v", err)
		return
	}
	t.audio.mu.Lock()
	t.audio.txSent++
	t.audio.mu.Unlock()
}

// setTXFeed installs (or clears) the source of transmit audio.
func (t *TCI) setTXFeed(fn func(samples int) []byte) {
	t.audio.mu.Lock()
	t.audio.txFeed = fn
	t.audio.txSent, t.audio.txShort = 0, 0
	t.audio.mu.Unlock()
}

// ProbeTXStream keys the radio, answers its chrono requests with a tone for the
// given number of seconds, unkeys, and reports what happened.
//
// INTO A DUMMY LOAD, AND IN A DIGITAL MODE. In SSB the radio takes the
// microphone and this produces nothing — which is a property of the radio, not
// a fault here, so it is said rather than worked around.
func (t *TCI) ProbeTXStream(seconds int, toneHz float64) error {
	if seconds <= 0 {
		seconds = 5
	}
	if seconds > tciTXProbeMaxSeconds {
		seconds = tciTXProbeMaxSeconds
	}
	if toneHz <= 0 {
		toneHz = 1000
	}

	t.mu.Lock()
	allowed, known, connected := t.txAllowed, t.txAllowedKnown, t.conn != nil
	mode := t.mode
	t.mu.Unlock()
	if !connected {
		return fmt.Errorf("not connected to the radio")
	}
	if known && !allowed {
		return fmt.Errorf("the radio refuses transmitting (tx_enable is false)")
	}
	if !tciDigitalMode(mode) {
		// Refused rather than attempted. A pass in SSB keys the transmitter,
		// produces nothing, and teaches nobody anything — and it is still a
		// transmission.
		return fmt.Errorf("the radio is in %s: transmit audio over TCI only reaches the modulator in a digital mode (DIGU, DIGL, or an FT8/data mode) — switch mode and try again", mode)
	}

	t.audio.mu.Lock()
	rate := t.audio.rate
	t.audio.mu.Unlock()
	if rate <= 0 {
		rate = 48000
	}

	// The tone, generated on demand: the radio asks for a size and gets exactly
	// that, at whatever pace it asks. Phase is carried across the calls, since a
	// sine restarted every frame is a click 47 times a second.
	phase := 0.0
	step := 2 * math.Pi * toneHz / float64(rate)
	// A quarter of full scale: enough to read on a meter, short of the level
	// where the radio's own processing starts deciding things for us.
	const amp = 0.25
	const chans = 2
	le := binary.LittleEndian
	t.setTXFeed(func(samples int) []byte {
		payload := make([]byte, samples*4)
		for s := 0; s+chans-1 < samples; s += chans {
			v := float32(math.Sin(phase) * amp)
			phase += step
			if phase > 2*math.Pi {
				phase -= 2 * math.Pi
			}
			bits := math.Float32bits(v)
			le.PutUint32(payload[s*4:], bits)     // left
			le.PutUint32(payload[(s+1)*4:], bits) // right
		}
		return payload
	})
	defer t.setTXFeed(nil)

	debugLog.Printf("TCI: TX PROBE starting — %d s of a %.0f Hz tone answered to the radio's own requests, mode %s, INTO A DUMMY LOAD",
		seconds, toneHz, mode)

	if err := t.SetPTT(true); err != nil {
		return fmt.Errorf("could not key the radio: %w", err)
	}
	// Every path out unkeys, including the panic that has not happened yet. A
	// transmitter left keyed by a defect is the one fault here that would reach
	// somebody else's band.
	defer func() {
		if err := t.SetPTT(false); err != nil {
			debugLog.Printf("TCI: TX PROBE — UNKEY FAILED (%v) — stop the transmission at the radio", err)
		}
	}()

	time.Sleep(time.Duration(seconds) * time.Second)

	t.audio.mu.Lock()
	sent, short := t.audio.txSent, t.audio.txShort
	chrono := t.audio.countByType[tciStreamTXChrono] - t.audio.txMark[tciStreamTXChrono]
	t.audio.mu.Unlock()
	debugLog.Printf("TCI: TX PROBE finished — the radio asked %d times, %d frames sent, %d requests unanswered",
		chrono, sent, short)
	if sent == 0 {
		debugLog.Printf("TCI: TX PROBE — the radio never asked for audio; in a digital mode it should, so check that ExpertSDR3 takes its transmit audio from TCI")
	}
	return nil
}

// tciDigitalMode says whether the radio's current mode is one where network
// audio reaches the modulator. Measured on a SunSDR: DIGU asks for audio, SSB
// never does.
func tciDigitalMode(mode string) bool {
	switch mode {
	case "digu", "digl", "DIGU", "DIGL", "FT8", "ft8", "FT4", "ft4", "DATA", "data", "RTTY", "rtty":
		return true
	}
	return false
}