README change

1 files changed, 35 insertions, 0 deletions

diff --git a/gnuradio-core/src/utils/db_width.m b/gnuradio-core/src/utils/db_width.m
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+++ b/gnuradio-core/src/utils/db_width.m
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+%
+% Copyright 2001 Free Software Foundation, Inc.
+%
+% This file is part of GNU Radio
+%
+% GNU Radio is free software; you can redistribute it and/or modify
+% it under the terms of the GNU General Public License as published by
+% the Free Software Foundation; either version 3, or (at your option)
+% any later version.
+%
+% GNU Radio is distributed in the hope that it will be useful,
+% but WITHOUT ANY WARRANTY; without even the implied warranty of
+% MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+% GNU General Public License for more details.
+%
+% You should have received a copy of the GNU General Public License
+% along with GNU Radio; see the file COPYING.  If not, write to
+% the Free Software Foundation, Inc., 51 Franklin Street,
+% Boston, MA 02110-1301, USA.
+
+%% find the N-dB width of the given filter
+%% E.g., to find the 3-dB width, use width = db_width (taps, -3.0)
+%% the result is normalized to nyquist == 1
+
+function width = db_width (taps, db)
+  [H,w] = freqz (taps, 1, 4096);
+  Habs = abs(H);
+  max_H = max(Habs);
+  min_H = min(Habs);
+  threshold = max_H * 10^(db/20);
+  if (min_H > threshold)
+    error ("The %g dB point is never reached", db);
+  end
+  above = Habs >= threshold;
+  width = sum(above) / length (above);