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62 lines
2.4 KiB
C
62 lines
2.4 KiB
C
/*
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* FILE: resample.h
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* BY: Julius Smith (at CCRMA, Stanford U)
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* C BY: translated from SAIL to C by Christopher Lee Fraley
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* (cf0v@andrew.cmu.edu)
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* DATE: 7-JUN-88
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* VERS: 2.0 (17-JUN-88, 3:00pm)
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*/
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/*
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* October 29, 1999
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* Various changes, bugfixes(?), increased precision, by Stan Brooks.
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*
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* This source code is distributed in the hope that it will be useful,
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* but WITHOUT ANY WARRANTY; without even the implied warranty of
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* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
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*
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*/
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/* Conversion constants */
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#define Lc 7
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#define Nc (1<<Lc)
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#define La 16
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#define Na (1<<La)
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#define Lp (Lc+La)
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#define Np (1<<Lp)
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#define Amask (Na-1)
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#define Pmask (Np-1)
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#define MAXNWING (80<<Lc)
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/* Description of constants:
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*
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* Nc - is the number of look-up values available for the lowpass filter
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* between the beginning of its impulse response and the "cutoff time"
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* of the filter. The cutoff time is defined as the reciprocal of the
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* lowpass-filter cut off frequence in Hz. For example, if the
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* lowpass filter were a sinc function, Nc would be the index of the
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* impulse-response lookup-table corresponding to the first zero-
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* crossing of the sinc function. (The inverse first zero-crossing
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* time of a sinc function equals its nominal cutoff frequency in Hz.)
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* Nc must be a power of 2 due to the details of the current
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* implementation. The default value of 128 is sufficiently high that
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* using linear interpolation to fill in between the table entries
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* gives approximately 16-bit precision, and quadratic interpolation
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* gives about 23-bit (float) precision in filter coefficients.
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*
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* Lc - is log base 2 of Nc.
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*
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* La - is the number of bits devoted to linear interpolation of the
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* filter coefficients.
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*
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* Lp - is La + Lc, the number of bits to the right of the binary point
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* in the integer "time" variable. To the left of the point, it indexes
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* the input array (X), and to the right, it is interpreted as a number
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* between 0 and 1 sample of the input X. The default value of 23 is
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* about right. There is a constraint that the filter window must be
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* "addressable" in a int32_t, more precisely, if Nmult is the number
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* of sinc zero-crossings in the right wing of the filter window, then
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* (Nwing<<Lp) must be expressible in 31 bits.
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*
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*/
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