Improve multirate section.

applications/lofar2/model/pfb_os/dsp_study_erko.txt

@@ -1029,7 +1029,7 @@ c) s-plane and z-plane
   . Up sampling   : x[n] --> up Q --> H(z^Q) --> y[m], is equivalent to:
                              H(z) --> up Q
 
-  . Down sampling : x[n] --> H(z^Q) --> down --> y[m], is equivalent to:
+  . Down sampling : x[n] --> H(z^Q) --> down Q --> y[m], is equivalent to:
                     x[n] --> down Q --> H(z)   --> y[m]
 
   . H_i(z^Q) is upsampled-by-Q version of H(z), so with Q-1 zero coefficients
@@ -1100,8 +1100,8 @@ c) s-plane and z-plane
     the original spectrum must be less than pi / D. This is the non-aliasing
     condition for the downsampling operator.
 
-  . Discarding samples folds the spectrum around multiples of pi / D =
-    fxNyquist / D. First the LPF has to remove all folds.
+  . Discarding samples copies the spectrum around multiples of 2pi / D. First
+    the LPF has to remove all copies.
   . Do not calculate samples that will be thrown away.
 
 - Upsampling + LPF = interpolation:
@@ -1118,9 +1118,9 @@ c) s-plane and z-plane
 
       X_U(e^jw) = X(exp(jw U))
 
-  . Inserting zeros replicates the spectrum around multiples of pi / U. Then
+  . Inserting zeros replicates the spectrum around multiples of 2pi / U. Then
     the LPF has to remove all replicas and by that it interpolates to fill in
-    the zeros.
+    the zeros. To remove the replicas, the LPF pass band is pi / L.
   . Do not calculate samples that will be inserted as zeros.
   . Using zero order hold would be a naive approach, because then all samples
     need to be calculated and the LPF then needs to compensate for the non-flat