[FFmpeg-cvslog] r14887 - trunk/libavcodec/acelp_filters.h
michael
subversion
Thu Aug 21 23:52:56 CEST 2008
Author: michael
Date: Thu Aug 21 23:52:56 2008
New Revision: 14887
Log:
Cleanup comment for ff_acelp_interp_filter.
Modified:
trunk/libavcodec/acelp_filters.h
Modified: trunk/libavcodec/acelp_filters.h
==============================================================================
--- trunk/libavcodec/acelp_filters.h (original)
+++ trunk/libavcodec/acelp_filters.h Thu Aug 21 23:52:56 2008
@@ -26,55 +26,12 @@
#include <stdint.h>
/**
- * low-pass FIR (Finite Impulse Response) filter coefficients
- *
- * A similar filter is named b30 in G.729.
- *
- * G.729 specification says:
- * b30 is based on Hamming windowed sinc functions, truncated at +/-29 and
- * padded with zeros at +/-30 b30[30]=0.
- * The filter has a cut-off frequency (-3 dB) at 3600 Hz in the oversampled
- * domain.
- *
- * After some analysis, I found this approximation:
- *
- * PI * x
- * Hamm(x,N) = 0.53836-0.46164*cos(--------)
- * N-1
- * ---
- * 2
- *
- * PI * x
- * Hamm'(x,k) = Hamm(x - k, 2*k+1) = 0.53836 + 0.46164*cos(--------)
- * k
- *
- * sin(PI * x)
- * Sinc(x) = ----------- (normalized sinc function)
- * PI * x
- *
- * h(t,B) = 2 * B * Sinc(2 * B * t) (impulse response of sinc low-pass filter)
- *
- * b(k,B, n) = Hamm'(n, k) * h(n, B)
- *
- *
- * 3600
- * B = ----
- * 8000
- *
- * 3600 - cut-off frequency
- * 8000 - sampling rate
- * k - filter order
- *
- * ff_acelp_interp_filter[6*i+j] = b(10, 3600/8000, i+j/6)
- *
- * The filter assumes the following order of fractions (X - integer delay):
- *
- * 1/3 precision: X 1/3 2/3 X 1/3 2/3 X
- * 1/6 precision: X 1/6 2/6 3/6 4/6 5/6 X 1/6 2/6 3/6 4/6 5/6 X
- *
- * The filter can be used for 1/3 precision, too, by
- * passing 2*pitch_delay_frac as third parameter to the interpolation routine.
+ * low-pass Finite Impulse Response filter coefficients.
*
+ * Hamming windowed sinc filter with cutoff freq 3/40 of the sampling freq.
+ * This array only contains the right half of the filter.
+ * This filter is likely identical to the one used in G.729, though this
+ * could not be determined from the original comments with certainity.
*/
extern const int16_t ff_acelp_interp_filter[61];
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