fix(dmr): calibrate slicer from sync symbols

DMRDecoder.pas

@@ -108,12 +108,15 @@ type
     FOuterLevel: array[0..DMR_SAMPLES_PER_SYM - 1] of Double;
     FDibitHistory: array[0..DMR_SAMPLES_PER_SYM - 1,
                          0..DMR_SYNC_END_DIBIT] of Byte;
+    FSymbolHistory: array[0..DMR_SAMPLES_PER_SYM - 1,
+                          0..DMR_SYNC_END_DIBIT] of Double;
     FHistoryPos: array[0..DMR_SAMPLES_PER_SYM - 1] of Integer;
     FHistoryCount: array[0..DMR_SAMPLES_PER_SYM - 1] of Integer;
     FMagHistory: array[0..DMR_SAMPLES_PER_SYM - 1, 0..23] of Double;
     FMagPos: array[0..DMR_SAMPLES_PER_SYM - 1] of Integer;
     FMagSum: array[0..DMR_SAMPLES_PER_SYM - 1] of Double;
     FBestSyncQuality: Double;
+    FFixedOuterLevel: Double;
     FTrackPhase: Integer;
     FCurrentBurst: TDMRDibitBurst;
     FCurrentBurstPos: Integer;
@@ -150,6 +153,8 @@ type
     procedure NoteSync(Phase: Integer; Kind: TDMRSyncKind; Inverted: Boolean);
     procedure LoseSync;
     function Digitize(Phase: Integer; Symbol: Double): Byte;
+    function SliceSymbol(Symbol, OuterLevel: Double): Byte;
+    function SyncEyeOpening(Phase: Integer): Double;
     procedure StartBurstTracking(Phase: Integer);
     procedure CompleteBurst;
     procedure DecodeVoiceBurst;
@@ -302,12 +307,14 @@ begin
     FillChar(FSyncShift, SizeOf(FSyncShift), 0);
     FillChar(FOuterLevel, SizeOf(FOuterLevel), 0);
     FillChar(FDibitHistory, SizeOf(FDibitHistory), 0);
+    FillChar(FSymbolHistory, SizeOf(FSymbolHistory), 0);
     FillChar(FHistoryPos, SizeOf(FHistoryPos), 0);
     FillChar(FHistoryCount, SizeOf(FHistoryCount), 0);
     FillChar(FMagHistory, SizeOf(FMagHistory), 0);
     FillChar(FMagPos, SizeOf(FMagPos), 0);
     FillChar(FMagSum, SizeOf(FMagSum), 0);
     FBestSyncQuality := 0;
+    FFixedOuterLevel := 0;
     FillChar(FCurrentBurst, SizeOf(FCurrentBurst), 0);
     FillChar(FLastBurst, SizeOf(FLastBurst), 0);
     FillChar(FLastBurstInfo, SizeOf(FLastBurstInfo), 0);
@@ -456,6 +463,7 @@ begin
   // Keep the 90 dibits ending at sync. Once the timing phase is selected this
   // supplies burst positions 0..89 without waiting for another frame.
   FDibitHistory[Phase, FHistoryPos[Phase]] := Dibit;
+  FSymbolHistory[Phase, FHistoryPos[Phase]] := V;
   FHistoryPos[Phase] := (FHistoryPos[Phase] + 1) mod (DMR_SYNC_END_DIBIT + 1);
   if FHistoryCount[Phase] < DMR_SYNC_END_DIBIT + 1 then Inc(FHistoryCount[Phase]);
 
@@ -571,21 +579,33 @@ begin
   end;
   FTrackPhase := -1;
   FCurrentBurstPos := 0;
+  FFixedOuterLevel := 0;
   FMobileActiveBurst := True;
   DMRMbeReset(FMbe);
 end;
 
 function TDMRDecoder.Digitize(Phase: Integer; Symbol: Double): Byte;
 var
-  A, Threshold: Double;
+  A: Double;
 begin
+  if (Phase = FTrackPhase) and (FFixedOuterLevel > 1.0e-9) then
+    Exit(SliceSymbol(Symbol, FFixedOuterLevel));
+
   A := Abs(Symbol);
   if FOuterLevel[Phase] <= 1.0e-9 then FOuterLevel[Phase] := A
   else if A > FOuterLevel[Phase] then
     FOuterLevel[Phase] := A
   else
     FOuterLevel[Phase] := FOuterLevel[Phase] + 0.001 * (A - FOuterLevel[Phase]);
-  Threshold := 0.625 * FOuterLevel[Phase];
+  Result := SliceSymbol(Symbol, FOuterLevel[Phase]);
+end;
+
+function TDMRDecoder.SliceSymbol(Symbol, OuterLevel: Double): Byte;
+var
+  A, Threshold: Double;
+begin
+  A := Abs(Symbol);
+  Threshold := (2.0 / 3.0) * OuterLevel;
   if Symbol >= 0 then
   begin
     if A >= Threshold then Result := 1 else Result := 0; // +3 / +1
@@ -596,6 +616,15 @@ begin
   end;
 end;
 
+function TDMRDecoder.SyncEyeOpening(Phase: Integer): Double;
+var
+  i: Integer;
+begin
+  Result := 1.0e300;
+  for i := 0 to 23 do
+    Result := Min(Result, FMagHistory[Phase, i]);
+end;
+
 procedure TDMRDecoder.StartBurstTracking(Phase: Integer);
 var
   i, P: Integer;
@@ -605,7 +634,10 @@ begin
   P := FHistoryPos[Phase]; // oldest item; ring is exactly 90 dibits full
   for i := 0 to DMR_SYNC_END_DIBIT do
   begin
-    FCurrentBurst[i] := FDibitHistory[Phase, P];
+    // 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],
+                                    FFixedOuterLevel);
     P := (P + 1) mod (DMR_SYNC_END_DIBIT + 1);
   end;
   FCurrentBurstPos := DMR_SYNC_END_DIBIT + 1;
@@ -718,11 +750,13 @@ begin
     if (FLastSyncSample <> 0) and
        (FSampleNo - FLastSyncSample <= DMR_SAMPLES_PER_SYM * 2) then
     begin
-      if FMagSum[Phase] > FBestSyncQuality then
+      if SyncEyeOpening(Phase) > FBestSyncQuality then
       begin
-        FBestSyncQuality := FMagSum[Phase];
+        FBestSyncQuality := SyncEyeOpening(Phase);
         FSyncKind := Kind;
         FInverted := Inverted;
+        FFixedOuterLevel := FMagSum[Phase] / 24.0;
+        FOuterLevel[Phase] := FFixedOuterLevel;
         StartBurstTracking(Phase);
       end;
       Exit;
@@ -732,7 +766,9 @@ begin
     Inc(FSyncCount);
     FLastSyncTick := NowTick;
     FLastSyncSample := FSampleNo;
-    FBestSyncQuality := FMagSum[Phase];
+    FBestSyncQuality := SyncEyeOpening(Phase);
+    FFixedOuterLevel := FMagSum[Phase] / 24.0;
+    FOuterLevel[Phase] := FFixedOuterLevel;
     if Kind in [dskMSData, dskMSVoice] then FMobileActiveBurst := True;
     StartBurstTracking(Phase);
   finally
@@ -795,7 +831,9 @@ begin
       S.BestSyncDistance := FBestSyncDistance;
       S.BestSyncPhase := FBestSyncPhase;
       S.BestCandidateKind := FBestCandidateKind;
-      if FBestSyncPhase >= 0 then
+      if FFixedOuterLevel > 0 then
+        S.SymbolOuterLevel := FFixedOuterLevel
+      else if FBestSyncPhase >= 0 then
         S.SymbolOuterLevel := FOuterLevel[FBestSyncPhase];
       S.SignalMean := FSignalMean;
     finally