function CalcFreqGoertzel(Freq: Double; SampleRate: Double; PData: PSmallInt; SampleCount: Longint): Double;
var  i     : Longint;
     Sk0   : Double;
     Sk1   : Double;
     Sk2   : Double;
     d     : Double;
begin
     Sk0:= 0.0;
     Sk1:= 0.0;
     for i := 0 to SampleCount - 1 do
     begin
          Sk2:= Sk1;
          Sk1:= Sk0;
          Sk0:= 2 * cos( 2 * Pi * Freq / SampleRate ) * Sk1 - Sk2 + pData^;
          Inc(pData);
     end;
     d:= exp( -2 * Pi * Freq / SampleRate );
     Result:= abs( Sk0 - d * Sk1 ) / SampleCount;
end;