fix(dmr): center discriminator eye per sync

DMRDecoder.pas

@@ -24,6 +24,7 @@ const
   DMR_INPUT_RATE       = 48000;
   DMR_SYMBOL_RATE      = 4800;
   DMR_SAMPLES_PER_SYM  = DMR_INPUT_RATE div DMR_SYMBOL_RATE;
+  DMR_INTEGRATOR_SAMPLES = 8;
   DMR_RING_SIZE        = 65536; // >1.3 s at 48 kHz, power of two
   DMR_BURST_DIBITS     = 144;
   DMR_SYNC_END_DIBIT   = 89;    // sync occupies burst dibits 66..89
@@ -100,7 +101,7 @@ type
     FReadPos, FWritePos: Integer;
     FDropped: QWord;
 
-    FSampleWindow: array[0..DMR_SAMPLES_PER_SYM - 1] of Double;
+    FSampleWindow: array[0..DMR_INTEGRATOR_SAMPLES - 1] of Double;
     FWindowPos, FWindowCount: Integer;
     FWindowSum: Double;
     FSampleNo: QWord;
@@ -117,6 +118,9 @@ type
     FMagSum: array[0..DMR_SAMPLES_PER_SYM - 1] of Double;
     FBestSyncQuality: Double;
     FFixedOuterLevel: Double;
+    FFixedPositiveOuter: Double;
+    FFixedNegativeOuter: Double;
+    FFixedCenter: Double;
     FTrackPhase: Integer;
     FCurrentBurst: TDMRDibitBurst;
     FCurrentBurstPos: Integer;
@@ -155,6 +159,7 @@ type
     function Digitize(Phase: Integer; Symbol: Double): Byte;
     function SliceSymbol(Symbol, OuterLevel: Double): Byte;
     function SyncEyeOpening(Phase: Integer): Double;
+    procedure CalibrateSyncLevels(Phase: Integer);
     procedure StartBurstTracking(Phase: Integer);
     procedure CompleteBurst;
     procedure DecodeVoiceBurst;
@@ -315,6 +320,9 @@ begin
     FillChar(FMagSum, SizeOf(FMagSum), 0);
     FBestSyncQuality := 0;
     FFixedOuterLevel := 0;
+    FFixedPositiveOuter := 0;
+    FFixedNegativeOuter := 0;
+    FFixedCenter := 0;
     FillChar(FCurrentBurst, SizeOf(FCurrentBurst), 0);
     FillChar(FLastBurst, SizeOf(FLastBurst), 0);
     FillChar(FLastBurstInfo, SizeOf(FLastBurstInfo), 0);
@@ -427,15 +435,15 @@ begin
   FSignalMean := FSignalMean + 0.001 * (S - FSignalMean);
   FWindowSum := FWindowSum - FSampleWindow[FWindowPos] + S;
   FSampleWindow[FWindowPos] := S;
-  FWindowPos := (FWindowPos + 1) mod DMR_SAMPLES_PER_SYM;
+  FWindowPos := (FWindowPos + 1) mod DMR_INTEGRATOR_SAMPLES;
-  if FWindowCount < DMR_SAMPLES_PER_SYM then Inc(FWindowCount);
+  if FWindowCount < DMR_INTEGRATOR_SAMPLES then Inc(FWindowCount);
   Inc(FSampleNo);
-  if FWindowCount < DMR_SAMPLES_PER_SYM then Exit;
+  if FWindowCount < DMR_INTEGRATOR_SAMPLES then Exit;
 
   // Ten interleaved timing hypotheses. Each receives one boxcar-integrated
   // symbol every ten input samples; the correct phase produces exact sync.
   Phase := Integer(FSampleNo mod DMR_SAMPLES_PER_SYM);
-  Sym := FWindowSum / DMR_SAMPLES_PER_SYM;
+  Sym := FWindowSum / DMR_INTEGRATOR_SAMPLES;
   ProcessSymbol(Phase, Sym);
 end;
 
@@ -580,6 +588,9 @@ begin
   FTrackPhase := -1;
   FCurrentBurstPos := 0;
   FFixedOuterLevel := 0;
+  FFixedPositiveOuter := 0;
+  FFixedNegativeOuter := 0;
+  FFixedCenter := 0;
   FMobileActiveBurst := True;
   DMRMbeReset(FMbe);
 end;
@@ -589,7 +600,7 @@ var
   A: Double;
 begin
   if (Phase = FTrackPhase) and (FFixedOuterLevel > 1.0e-9) then
-    Exit(SliceSymbol(Symbol, FFixedOuterLevel));
+    Exit(SliceSymbol(Symbol - FFixedCenter, FFixedOuterLevel));
 
   A := Abs(Symbol);
   if FOuterLevel[Phase] <= 1.0e-9 then FOuterLevel[Phase] := A
@@ -605,7 +616,7 @@ var
   A, Threshold: Double;
 begin
   A := Abs(Symbol);
-  Threshold := (2.0 / 3.0) * OuterLevel;
+  Threshold := 0.645 * OuterLevel;
   if Symbol >= 0 then
   begin
     if A >= Threshold then Result := 1 else Result := 0; // +3 / +1
@@ -625,6 +636,39 @@ begin
     Result := Min(Result, FMagHistory[Phase, i]);
 end;
 
+procedure TDMRDecoder.CalibrateSyncLevels(Phase: Integer);
+var
+  i, P, PositiveCount, NegativeCount: Integer;
+  V, PositiveSum, NegativeSum: Double;
+begin
+  PositiveSum := 0;
+  NegativeSum := 0;
+  PositiveCount := 0;
+  NegativeCount := 0;
+  P := (FHistoryPos[Phase] + DMR_SYNC_END_DIBIT + 1 - 24) mod
+       (DMR_SYNC_END_DIBIT + 1);
+  for i := 0 to 23 do
+  begin
+    V := FSymbolHistory[Phase, P];
+    if V >= 0 then
+    begin
+      PositiveSum := PositiveSum + V;
+      Inc(PositiveCount);
+    end
+    else
+    begin
+      NegativeSum := NegativeSum - V;
+      Inc(NegativeCount);
+    end;
+    P := (P + 1) mod (DMR_SYNC_END_DIBIT + 1);
+  end;
+  if PositiveCount > 0 then FFixedPositiveOuter := PositiveSum / PositiveCount;
+  if NegativeCount > 0 then FFixedNegativeOuter := NegativeSum / NegativeCount;
+  FFixedCenter := 0.5 * (FFixedPositiveOuter - FFixedNegativeOuter);
+  FFixedOuterLevel := 0.5 * (FFixedPositiveOuter + FFixedNegativeOuter);
+  FOuterLevel[Phase] := FFixedOuterLevel;
+end;
+
 procedure TDMRDecoder.StartBurstTracking(Phase: Integer);
 var
   i, P: Integer;
@@ -636,7 +680,7 @@ begin
   begin
     // All 24 DMR sync dibits are known outer levels.  Once they calibrate the
     // eye, re-slice the already buffered pre-sync payload with the same level.
-    FCurrentBurst[i] := SliceSymbol(FSymbolHistory[Phase, P],
+    FCurrentBurst[i] := SliceSymbol(FSymbolHistory[Phase, P] - FFixedCenter,
                                     FFixedOuterLevel);
     P := (P + 1) mod (DMR_SYNC_END_DIBIT + 1);
   end;
@@ -755,8 +799,7 @@ begin
         FBestSyncQuality := SyncEyeOpening(Phase);
         FSyncKind := Kind;
         FInverted := Inverted;
-        FFixedOuterLevel := FMagSum[Phase] / 24.0;
+        CalibrateSyncLevels(Phase);
-        FOuterLevel[Phase] := FFixedOuterLevel;
         StartBurstTracking(Phase);
       end;
       Exit;
@@ -767,8 +810,7 @@ begin
     FLastSyncTick := NowTick;
     FLastSyncSample := FSampleNo;
     FBestSyncQuality := SyncEyeOpening(Phase);
-    FFixedOuterLevel := FMagSum[Phase] / 24.0;
+    CalibrateSyncLevels(Phase);
-    FOuterLevel[Phase] := FFixedOuterLevel;
     if Kind in [dskMSData, dskMSVoice] then FMobileActiveBurst := True;
     StartBurstTracking(Phase);
   finally

tests/dmr_frontend_test.pas

@@ -67,6 +67,8 @@ begin
 end;
 
 procedure TestBurst(D: TDMRDecoder);
+const
+  DC_OFFSET = 0.12;
 var
   Want, Got: array[0..DMR_BURST_DIBITS - 1] of Byte;
   L, R: array of Single;
@@ -84,17 +86,17 @@ begin
   SetLength(R, Length(L));
   // One complete preceding symbol mirrors a continuous stream and fills the
   // 90-dibit history even for the latest timing hypothesis.
-  V := 0.70; // Want[0] = 1
+  V := 0.70 + DC_OFFSET; // Want[0] = 1
   p := 3 + DMR_SAMPLES_PER_SYM;
   for j := 0 to p - 1 do begin L[j] := V; R[j] := V; end;
   for i := 0 to High(Want) do
   begin
     case Want[i] of
-      0: V := 0.23;
+      0: V := 0.23 + DC_OFFSET;
-      1: V := 0.70;
+      1: V := 0.70 + DC_OFFSET;
-      2: V := -0.23;
+      2: V := -0.23 + DC_OFFSET;
     else
-      V := -0.70;
+      V := -0.70 + DC_OFFSET;
     end;
     for j := 0 to DMR_SAMPLES_PER_SYM - 1 do
     begin