From 8bd467598c3441d2e6a51b32851d5fbbe831b0ea Mon Sep 17 00:00:00 2001 From: Cort Buffington Date: Sun, 9 Aug 2026 11:42:20 -0500 Subject: [PATCH 1/2] Debounce link-mode external-audio underrun before dropping PTT Under [FM] LinkMode=1, linkStateMachine() set m_extSignal false the instant a single ext-audio sample went missing, with no debounce -- unlike the non-link duplex/simplex paths, which already tolerate a brief gap via the RELAYING_EXT -> RELAYING_WAIT_EXT / m_ackDelayTimer pattern before declaring a real loss. Since IO::process() drops PTT the moment the downstream TX buffer runs dry, that zero-debounce cutoff meant even a very brief, late-but-recoverable gap from the gateway (scheduling jitter, a late UDP frame, etc.) could cascade into a full PTT drop-and-rekey cycle -- far more audible than the underlying gap itself. Adds m_extGapTimer, mirroring the existing pattern: on the first missed sample, start the timer instead of immediately clearing m_extSignal; a gap that recovers before FM_LINK_EXT_GAP_MS (default 60ms) expires never affects m_extSignal at all, so the transmitter never notices. Only a gap that outlasts the timer is treated as a genuine end of transmission, exactly as before. linkSamples()'s currentExtSample is now zero-initialized, since m_extSignal can now stay true for a few samples past an underrun -- silence, not stack garbage, must be what gets played out to the modem during that window. --- FM.cpp | 52 +++++++++++++++++++++++++++++++++++++++++++--------- FM.h | 1 + 2 files changed, 44 insertions(+), 9 deletions(-) diff --git a/FM.cpp b/FM.cpp index dfa9418..c8ac3a2 100644 --- a/FM.cpp +++ b/FM.cpp @@ -28,6 +28,13 @@ const uint16_t FM_SERIAL_BLOCK_SIZE = 80U;//this is the number of sample pairs t //three times this value shall never exceed 252 const uint16_t FM_SERIAL_BLOCK_SIZE_BYTES = FM_SERIAL_BLOCK_SIZE * 3U; +// How long a link-mode external-audio underrun is tolerated before it is +// treated as a real end of transmission (see linkStateMachine()). A gap +// shorter than this is inaudible on its own; the PTT drop-and-rekey it +// would otherwise trigger is not. Tune here if needed -- no other code +// depends on this value. +const uint16_t FM_LINK_EXT_GAP_MS = 60U; + const uint8_t FS_LISTENING = 0U; const uint8_t FS_KERCHUNK_RF = 1U; const uint8_t FS_RELAYING_RF = 2U; @@ -62,6 +69,7 @@ m_ackMinTimer(), m_ackDelayTimer(), m_hangTimer(), m_reverseTimer(), +m_extGapTimer(), m_needReverse(false), m_filterStage1( 724, 1448, 724, 32768, -37895, 21352),//3rd order Cheby Filter 300 to 2700Hz, 0.2dB passband ripple, sampling rate 24kHz m_filterStage2(32768, 0,-32768, 32768, -50339, 19052), @@ -85,6 +93,7 @@ m_rssiAccum(0U), m_rssiCount(0U) { m_reverseTimer.setTimeout(0U, 150U); + m_extGapTimer.setTimeout(0U, FM_LINK_EXT_GAP_MS); insertDelay(100U); } @@ -250,13 +259,17 @@ void CFM::linkSamples(bool cos, q15_t* samples, uint8_t length) uint8_t i = 0U; for (; i < length; i++) { - // ARMv7-M has hardware integer division + // ARMv7-M has hardware integer division q15_t currentRFSample = q15_t((q31_t(samples[i]) << 8) / m_rxLevel); if (m_noiseSquelch) cos = m_squelch.process(currentRFSample); - q15_t currentExtSample; + // Zero-initialized: getSample() leaves this untouched on underrun, and + // with the ext-gap debounce below, m_extSignal can now stay true for a + // few samples past that point -- silence, not stack garbage, must be + // what gets played out to the modem in that window. + q15_t currentExtSample = 0; bool inputExt = m_inputExtRB.getSample(currentExtSample);//always consume the external input data so it does not overflow inputExt = inputExt && m_extEnabled; @@ -601,6 +614,7 @@ void CFM::clock(uint8_t length) m_ackDelayTimer.clock(length); m_hangTimer.clock(length); m_reverseTimer.clock(length); + m_extGapTimer.clock(length); } void CFM::listeningStateDuplex(bool validRFSignal, bool validExtSignal) @@ -1231,6 +1245,12 @@ void CFM::linkStateMachine(bool validRFSignal, bool validExtSignal) m_extSignal = true; } + if (validExtSignal && m_extGapTimer.isRunning()) { + // Recovered before the gap timer committed to a real loss below -- + // m_extSignal was never flipped, so there is nothing else to undo. + m_extGapTimer.stop(); + } + if (!validRFSignal && m_rfSignal) { io.setDecode(false); io.setADCDetection(false); @@ -1248,14 +1268,28 @@ void CFM::linkStateMachine(bool validRFSignal, bool validExtSignal) } if (!validExtSignal && m_extSignal) { - if (!m_rfSignal) { - DEBUG1("State to LISTENING"); - m_state = FS_LISTENING; - serial.writeFMStatus(m_state); - } + // A missed/late sample used to flip m_extSignal false right here, on + // the very first bad sample, with zero debounce -- that is what + // actually keys the transmitter off (IO::process() drops PTT the + // instant the downstream TX buffer runs dry, see IO.cpp). A gap well + // under FM_LINK_EXT_GAP_MS is inaudible on its own; the PTT + // drop-and-rekey it triggers is not. Give it a short grace period + // instead, mirroring the RELAYING_EXT/RELAYING_WAIT_EXT pattern the + // non-link duplex/simplex paths already use for exactly this + // situation (see relayingExtStateDuplex()/relayingExtWaitStateDuplex()). + if (!m_extGapTimer.isRunning()) { + m_extGapTimer.start(); + } else if (m_extGapTimer.hasExpired()) { + if (!m_rfSignal) { + DEBUG1("State to LISTENING"); + m_state = FS_LISTENING; + serial.writeFMStatus(m_state); + } - m_needReverse = true; - m_extSignal = false; + m_needReverse = true; + m_extSignal = false; + m_extGapTimer.stop(); + } } } diff --git a/FM.h b/FM.h index 2b5db51..a1a198f 100644 --- a/FM.h +++ b/FM.h @@ -75,6 +75,7 @@ private: CFMTimer m_ackDelayTimer; CFMTimer m_hangTimer; CFMTimer m_reverseTimer; + CFMTimer m_extGapTimer; // debounce for link-mode ext-audio underrun, see linkStateMachine() bool m_needReverse; CFMDirectFormI m_filterStage1; CFMDirectFormI m_filterStage2; From 8a89804fda5f5de86cebb84df083ca8ae463134c Mon Sep 17 00:00:00 2001 From: Cort Buffington Date: Tue, 11 Aug 2026 08:55:22 -0500 Subject: [PATCH 2/2] Refactor FM_LINK_EXT_GAP_MS and improve comments Updated the FM_LINK_EXT_GAP_MS constant declaration and added comments for clarity. --- FM.cpp | 20 ++++++-------------- 1 file changed, 6 insertions(+), 14 deletions(-) diff --git a/FM.cpp b/FM.cpp index c8ac3a2..efe3e05 100644 --- a/FM.cpp +++ b/FM.cpp @@ -27,14 +27,8 @@ const uint16_t FM_TX_BLOCK_SIZE = 100U; const uint16_t FM_SERIAL_BLOCK_SIZE = 80U;//this is the number of sample pairs to send over serial. One sample pair is 3bytes. //three times this value shall never exceed 252 const uint16_t FM_SERIAL_BLOCK_SIZE_BYTES = FM_SERIAL_BLOCK_SIZE * 3U; - -// How long a link-mode external-audio underrun is tolerated before it is -// treated as a real end of transmission (see linkStateMachine()). A gap -// shorter than this is inaudible on its own; the PTT drop-and-rekey it -// would otherwise trigger is not. Tune here if needed -- no other code -// depends on this value. -const uint16_t FM_LINK_EXT_GAP_MS = 60U; - +const uint16_t FM_LINK_EXT_GAP_MS = 60U;// How long a link-mode external-audio underrun is tolerated before it is + // treated as a real end of transmission (see linkStateMachine()). const uint8_t FS_LISTENING = 0U; const uint8_t FS_KERCHUNK_RF = 1U; const uint8_t FS_RELAYING_RF = 2U; @@ -259,7 +253,6 @@ void CFM::linkSamples(bool cos, q15_t* samples, uint8_t length) uint8_t i = 0U; for (; i < length; i++) { - // ARMv7-M has hardware integer division q15_t currentRFSample = q15_t((q31_t(samples[i]) << 8) / m_rxLevel); if (m_noiseSquelch) @@ -267,8 +260,7 @@ void CFM::linkSamples(bool cos, q15_t* samples, uint8_t length) // Zero-initialized: getSample() leaves this untouched on underrun, and // with the ext-gap debounce below, m_extSignal can now stay true for a - // few samples past that point -- silence, not stack garbage, must be - // what gets played out to the modem in that window. + // few samples past that point q15_t currentExtSample = 0; bool inputExt = m_inputExtRB.getSample(currentExtSample);//always consume the external input data so it does not overflow inputExt = inputExt && m_extEnabled; @@ -1272,9 +1264,9 @@ void CFM::linkStateMachine(bool validRFSignal, bool validExtSignal) // the very first bad sample, with zero debounce -- that is what // actually keys the transmitter off (IO::process() drops PTT the // instant the downstream TX buffer runs dry, see IO.cpp). A gap well - // under FM_LINK_EXT_GAP_MS is inaudible on its own; the PTT - // drop-and-rekey it triggers is not. Give it a short grace period - // instead, mirroring the RELAYING_EXT/RELAYING_WAIT_EXT pattern the + // under FM_LINK_EXT_GAP_MS is less offensive than the PTT + // drop-and-rekey it triggers. Give it a short grace period + // mirroring the RELAYING_EXT/RELAYING_WAIT_EXT pattern the // non-link duplex/simplex paths already use for exactly this // situation (see relayingExtStateDuplex()/relayingExtWaitStateDuplex()). if (!m_extGapTimer.isRunning()) {