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Diffstat (limited to 'third_party/resample/src/resample.h')
-rw-r--r-- | third_party/resample/src/resample.h | 67 |
1 files changed, 67 insertions, 0 deletions
diff --git a/third_party/resample/src/resample.h b/third_party/resample/src/resample.h new file mode 100644 index 00000000..35212512 --- /dev/null +++ b/third_party/resample/src/resample.h @@ -0,0 +1,67 @@ +/* + * The configuration constants below govern + * the number of bits in the input sample and filter coefficients, the + * number of bits to the right of the binary-point for fixed-point math, etc. + * + */ + +/* Conversion constants */ +#define Nhc 8 +#define Na 7 +#define Np (Nhc+Na) +#define Npc (1<<Nhc) +#define Amask ((1<<Na)-1) +#define Pmask ((1<<Np)-1) +#define Nh 16 +#define Nb 16 +#define Nhxn 14 +#define Nhg (Nh-Nhxn) +#define NLpScl 13 + +/* Description of constants: + * + * Npc - is the number of look-up values available for the lowpass filter + * between the beginning of its impulse response and the "cutoff time" + * of the filter. The cutoff time is defined as the reciprocal of the + * lowpass-filter cut off frequence in Hz. For example, if the + * lowpass filter were a sinc function, Npc would be the index of the + * impulse-response lookup-table corresponding to the first zero- + * crossing of the sinc function. (The inverse first zero-crossing + * time of a sinc function equals its nominal cutoff frequency in Hz.) + * Npc must be a power of 2 due to the details of the current + * implementation. The default value of 512 is sufficiently high that + * using linear interpolation to fill in between the table entries + * gives approximately 16-bit accuracy in filter coefficients. + * + * Nhc - is log base 2 of Npc. + * + * Na - is the number of bits devoted to linear interpolation of the + * filter coefficients. + * + * Np - is Na + Nhc, the number of bits to the right of the binary point + * in the integer "time" variable. To the left of the point, it indexes + * the input array (X), and to the right, it is interpreted as a number + * between 0 and 1 sample of the input X. Np must be less than 16 in + * this implementation. + * + * Nh - is the number of bits in the filter coefficients. The sum of Nh and + * the number of bits in the input data (typically 16) cannot exceed 32. + * Thus Nh should be 16. The largest filter coefficient should nearly + * fill 16 bits (32767). + * + * Nb - is the number of bits in the input data. The sum of Nb and Nh cannot + * exceed 32. + * + * Nhxn - is the number of bits to right shift after multiplying each input + * sample times a filter coefficient. It can be as great as Nh and as + * small as 0. Nhxn = Nh-2 gives 2 guard bits in the multiply-add + * accumulation. If Nhxn=0, the accumulation will soon overflow 32 bits. + * + * Nhg - is the number of guard bits in mpy-add accumulation (equal to Nh-Nhxn) + * + * NLpScl - is the number of bits allocated to the unity-gain normalization + * factor. The output of the lowpass filter is multiplied by LpScl and + * then right-shifted NLpScl bits. To avoid overflow, we must have + * Nb+Nhg+NLpScl < 32. + */ + |