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#include <stdio.h>
#include <memory.h>
#define BUF 220 // input buffer size for test main
#define OUT2(o,i,t0,t1,t2,t3,t4,t5,t6,t7,t8,t9,t10,t11,t12,t13) \
t = ( (int)in[i]*t0 + (int)in[i+1]*t1 + \ (int)in[i+2]*t2 + (int)in[i+3]*t3 + \ (int)in[i+4]*t4 + (int)in[i+5]*t5 + \ (int)in[i+6]*t6 + (int)in[i+7]*t7 + \ (int)in[i+8]*t8 + (int)in[i+9]*t9 + \ (int)in[i+8]*t10 + (int)in[i+9]*t11 + \ (int)in[i+8]*t12 + (int)in[i+9]*t13 ) >> 10; \ if (t < -32768) out[o] = -32768; else if (t > 32767) out[o]=32767; else out[o] = t;
//--------------------------------------------------------
void segment11to8(short *in, short *out){ int t; OUT2( 0, 0, 7, -20, 12, 42,-140, 238, 745, 238,-140, 42, 12, -20, 7, 0); OUT2( 1, 1, 4, -4, -18, 62, -85, 0, 654, 510, -99, -27, 49, -26, 3, 1); OUT2( 2, 2, 0, 6, -26, 38, 0,-126, 422, 704, 70,-110, 61, -10, -9, 5); OUT2( 3, 4, 6, -15, 0, 55,-129, 150, 734, 330,-140, 24, 25, -24, 7, 0); OUT2( 4, 5, 2, 0, -24, 58, -56, -57, 589, 589, -57, -56, 58, -24, 0, 2); OUT2( 5, 7, 7, -24, 25, 24,-140, 330, 734, 150,-129, 55, 0, -15, 6, 0); OUT2( 6, 8, 5, -9, -10, 61,-110, 70, 704, 422,-126, 0, 38, -26, 6, 0); OUT2( 7, 9, 1, 3, -26, 49, -27, -99, 510, 654, 0, -85, 62, -18, -4, 4);}//--------------------------------------------------------
void convert11to8(short *in, short *out, short *prev, int len){/*
Convert a buffer from 11KHz to 8KHz.
Note: len is number of shorts in input buffer, which MUST be a multiple of 11 and at least 44.
How the overhang works: The filter kernel for 1 section of 11 input samples requires KERNEL (=25) samples of the input. So we use 14 samples of overhang from the previous frame, which means the beginning of this frame looks like:
ppppppppppp ppp01234567 89abcdefghi 19.... 30.... / 41 42 43 X X x x
So we first have to do two special segments (the ones starting at X) then we do the rest (the x's) in a loop. For the example length=44 shown above, we'll do up & including 44-25=19, stopping on the last x shown. Then we save 30-43 in the overhang buffer so that 30 is the first group done on the next frame.*/
#define OVERHANG2 14
#define KERNEL2 25
int i,k; short tmp[KERNEL2+11]; // Convert the first two segments, where segment= 11 samples of input
memcpy(tmp,prev,sizeof(short)*OVERHANG2); memcpy(tmp+OVERHANG2,in,sizeof(short)*(KERNEL2+11-OVERHANG2));
segment11to8(tmp,out); segment11to8(tmp+11,out+8);
// Loop through the remaining segments
k = 16; for (i=22-OVERHANG2; i<=len-KERNEL2; i+=11) { segment11to8(in+i,out+k); k += 8; }
// Save OVERHANG2 samples for next time
memcpy(prev,in+len-OVERHANG2,sizeof(short)*OVERHANG2);}
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