fix(dmr): use flat discriminator and reject noise sync

DMRDecoder.pas

@@ -140,12 +140,15 @@ type
     FSyncCount: QWord;
     FLastSyncTick: QWord;
     FLastSyncSample: QWord;
+    FPendingSyncSample: array[TDMRSyncKind] of QWord;
+    FPendingSyncCount: array[TDMRSyncKind] of Byte;
     FMobileActiveBurst: Boolean;
 
     function PopSamples(var Buf: array of Single): Integer;
     procedure ProcessSample(S: Single);
     procedure ProcessSymbol(Phase: Integer; Symbol: Double);
     procedure NoteSync(Phase: Integer; Kind: TDMRSyncKind; Inverted: Boolean);
+    procedure LoseSync;
     function Digitize(Phase: Integer; Symbol: Double): Byte;
     procedure StartBurstTracking(Phase: Integer);
     procedure CompleteBurst;
@@ -174,6 +177,7 @@ const
   DMR_SYNC_HOLD_MS = 500;
   DMR_SYNC_MAX_ERRORS = 2;
   DMR_SUPERFRAME_SAMPLES = DMR_INPUT_RATE * 360 div 1000;
+  DMR_SYNC_CADENCE_TOLERANCE = DMR_SAMPLES_PER_SYM * 3;
 
   // One bit per DSD-FME sync character: '1'=0, '3'=1. These are the eight
   // ETSI DMR 48-bit sync patterns after collapsing each dibit to its sign.
@@ -337,6 +341,8 @@ begin
       FSyncCount := 0;
       FLastSyncTick := 0;
       FLastSyncSample := 0;
+      FillChar(FPendingSyncSample, SizeOf(FPendingSyncSample), 0);
+      FillChar(FPendingSyncCount, SizeOf(FPendingSyncCount), 0);
       FMobileActiveBurst := True;
     finally
       FStatusLock.Leave;
@@ -432,11 +438,19 @@ var
   Pat: LongWord;
   Dibit: Byte;
   V: Double;
-  DeltaSamples, CycleOffset: QWord;
+  DeltaSamples, CycleOffset, PendingSample: QWord;
   CandidateAllowed: Boolean;
 begin
   V := Symbol;
   if FInputInverted then V := -V;
+
+  // Stop assembling/decoding an endless stream of noise after the carrier
+  // vanishes.  Two superframes allow one damaged sync word without losing a
+  // valid call, while still closing the decoder in about 720 ms.
+  if (FLastSyncSample <> 0) and
+     (FSampleNo - FLastSyncSample > 2 * DMR_SUPERFRAME_SAMPLES +
+      DMR_SYNC_CADENCE_TOLERANCE) then
+    LoseSync;
   Dibit := Digitize(Phase, V);
 
   // Keep the 90 dibits ending at sync. Once the timing phase is selected this
@@ -479,7 +493,6 @@ begin
       // match lost almost every superframe in recorded handheld-radio IQ.
       DeltaSamples := FSampleNo - FLastSyncSample;
       CandidateAllowed := (FLastSyncSample = 0) or
-        (DeltaSamples > DMR_INPUT_RATE * 2) or
         (SYNC_KIND[i] = FSyncKind);
       // MS voice/data sync repeats every 360 ms. Cadence gating rejects
       // chance <=2-bit matches inside AMBE payload while still accepting a
@@ -494,13 +507,74 @@ begin
       end;
       if (Distance <= DMR_SYNC_MAX_ERRORS) and CandidateAllowed then
       begin
-        NoteSync(Phase, SYNC_KIND[i], FInputInverted);
+        // Fuzzy 24-symbol matches occur often enough in open-channel noise to
+        // make mbelib emit occasional garbage.  Real voice sync repeats every
+        // 360 ms, so require three words of the same kind on that cadence
+        // before opening burst and vocoder processing.
+        if FLastSyncSample = 0 then
+        begin
+          PendingSample := FPendingSyncSample[SYNC_KIND[i]];
+          if PendingSample <> 0 then
+          begin
+            DeltaSamples := FSampleNo - PendingSample;
+            CycleOffset := DeltaSamples mod DMR_SUPERFRAME_SAMPLES;
+            if (DeltaSamples >= DMR_SUPERFRAME_SAMPLES -
+                 DMR_SYNC_CADENCE_TOLERANCE) and
+               ((CycleOffset <= DMR_SYNC_CADENCE_TOLERANCE) or
+                (CycleOffset >= DMR_SUPERFRAME_SAMPLES -
+                 DMR_SYNC_CADENCE_TOLERANCE)) then
+            begin
+              Inc(FPendingSyncCount[SYNC_KIND[i]]);
+              FPendingSyncSample[SYNC_KIND[i]] := FSampleNo;
+              // Three consecutive superframe syncs make accidental capture
+              // in random discriminator noise vanishingly unlikely.
+              if FPendingSyncCount[SYNC_KIND[i]] >= 3 then
+                NoteSync(Phase, SYNC_KIND[i], FInputInverted);
+            end
+            else if DeltaSamples > DMR_SUPERFRAME_SAMPLES +
+                    DMR_SYNC_CADENCE_TOLERANCE then
+            begin
+              FPendingSyncSample[SYNC_KIND[i]] := FSampleNo;
+              FPendingSyncCount[SYNC_KIND[i]] := 1;
+            end;
+          end;
+          if FLastSyncSample = 0 then
+          begin
+            if PendingSample = 0 then
+            begin
+              FPendingSyncSample[SYNC_KIND[i]] := FSampleNo;
+              FPendingSyncCount[SYNC_KIND[i]] := 1;
+            end;
+            Exit;
+          end;
+        end
+        else
+          NoteSync(Phase, SYNC_KIND[i], FInputInverted);
         Exit;
       end;
     end;
   end;
 end;
 
+procedure TDMRDecoder.LoseSync;
+begin
+  FStatusLock.Enter;
+  try
+    FSyncKind := dskNone;
+    FInverted := False;
+    FLastSyncTick := 0;
+    FLastSyncSample := 0;
+    FillChar(FPendingSyncSample, SizeOf(FPendingSyncSample), 0);
+    FillChar(FPendingSyncCount, SizeOf(FPendingSyncCount), 0);
+  finally
+    FStatusLock.Leave;
+  end;
+  FTrackPhase := -1;
+  FCurrentBurstPos := 0;
+  FMobileActiveBurst := True;
+  DMRMbeReset(FMbe);
+end;
+
 function TDMRDecoder.Digitize(Phase: Integer; Symbol: Double): Byte;
 var
   A, Threshold: Double;