Hi all,
I think I solved a bug in gri_iir.h
I think the index numbers are wrong when you decrement the latest_n and
latest_m after you set prev_output and prev_input.
I created a simple standalone program to test the index numbers.
The patch and the test program are at:
http://www.olifantasia.com/pub/projects/gnuradio/mdvh/mypatches/gri_iir/
Because these things are tricky to understand I would appreciate some
feedback if the algorithm is correct now.
If it is, let me know and I can apply the patch.
I think gri_iir is only used in gr_iir_filter_ffd which is only used in
the emphasis filter of the wfm modulator.
And there it isn’t even used properly because the filter taps are not
set.
Greetings,
Martin
Index: gri_iir.h
— gri_iir.h (revision 5766)
+++ gri_iir.h (working copy)
@@ -143,12 +143,6 @@
for (i = 1; i < m; i ++)
acc += (d_fbtaps[i] * d_prev_output[latest_m + i]);
- // store the values twice to avoid having to handle wrap-around in
the loop - d_prev_output[latest_m] = acc;
- d_prev_output[latest_m+m] = acc;
- d_prev_input[latest_n] = input;
- d_prev_input[latest_n+n] = input;
- latest_n–;
latest_m–;
if (latest_n < 0)
@@ -156,6 +150,13 @@
if (latest_m < 0)
latest_m += m;
- // store the values twice to avoid having to handle wrap-around in
the loop - d_prev_output[latest_m] = acc;
- d_prev_output[latest_m+m] = acc;
- d_prev_input[latest_n] = input;
- d_prev_input[latest_n+n] = input;
- d_latest_m = latest_m;
d_latest_n = latest_n;
return (o_type) acc;