Define expected subband bin amplitude relative to real input amplitude, for DC and for sine.

libraries/dsp/fft/src/vhdl/fft_pkg.vhd

@@ -37,8 +37,10 @@ package fft_pkg is
   -- A/2, one at -f_bin and one at +f_bin. The power stays the same, because
   -- the power of an real input sinus is A**2 / 2 and the power of the f_bin
   -- signal complex phasors is (A/2)**2 + (A/2)**2 = A**2 / 2.
-  CONSTANT c_fft_real_input_gain_w  : INTEGER := -1;
-  CONSTANT c_fft_real_input_gain    : REAL := 2.0**REAL(c_fft_real_input_gain_w);
+  -- For DC at bin 0 the real input gain is 1.0, because DC has only one
+  -- phasor component of frequency 0.
+  CONSTANT c_fft_real_input_gain_sine : REAL := 0.5;
+  CONSTANT c_fft_real_input_gain_dc   : REAL := 1.0;
 
   -- FFT parameters for pipelined FFT (fft_pipe), parallel FFT (fft_par) and wideband FFT (fft_wide)
   type t_fft is record