1 /* 2 * Copyright (c) 2011 The WebRTC project authors. All Rights Reserved. 3 * 4 * Use of this source code is governed by a BSD-style license 5 * that can be found in the LICENSE file in the root of the source 6 * tree. An additional intellectual property rights grant can be found 7 * in the file PATENTS. All contributing project authors may 8 * be found in the AUTHORS file in the root of the source tree. 9 */ 10 11 /****************************************************************** 12 13 iLBC Speech Coder ANSI-C Source Code 14 15 WebRtcIlbcfix_Chebyshev.c 16 17 ******************************************************************/ 18 19 #include "defines.h" 20 #include "constants.h" 21 22 /*------------------------------------------------------------------* 23 * Calculate the Chevyshev polynomial series 24 * F(w) = 2*exp(-j5w)*C(x) 25 * C(x) = (T_0(x) + f(1)T_1(x) + ... + f(4)T_1(x) + f(5)/2) 26 * T_i(x) is the i:th order Chebyshev polynomial 27 *------------------------------------------------------------------*/ 28 29 int16_t WebRtcIlbcfix_Chebyshev( 30 /* (o) Result of C(x) */ 31 int16_t x, /* (i) Value to the Chevyshev polynomial */ 32 int16_t *f /* (i) The coefficients in the polynomial */ 33 ) { 34 int16_t b1_high, b1_low; /* Use the high, low format to increase the accuracy */ 35 int32_t b2; 36 int32_t tmp1W32; 37 int32_t tmp2W32; 38 int i; 39 40 b2 = (int32_t)0x1000000; /* b2 = 1.0 (Q23) */ 41 /* Calculate b1 = 2*x + f[1] */ 42 tmp1W32 = (x << 10) + (f[1] << 14); 43 44 for (i = 2; i < 5; i++) { 45 tmp2W32 = tmp1W32; 46 47 /* Split b1 (in tmp1W32) into a high and low part */ 48 b1_high = (int16_t)(tmp1W32 >> 16); 49 b1_low = (int16_t)((tmp1W32 - ((int32_t)b1_high << 16)) >> 1); 50 51 /* Calculate 2*x*b1-b2+f[i] */ 52 tmp1W32 = ((b1_high * x + ((b1_low * x) >> 15)) << 2) - b2 + (f[i] << 14); 53 54 /* Update b2 for next round */ 55 b2 = tmp2W32; 56 } 57 58 /* Split b1 (in tmp1W32) into a high and low part */ 59 b1_high = (int16_t)(tmp1W32 >> 16); 60 b1_low = (int16_t)((tmp1W32 - ((int32_t)b1_high << 16)) >> 1); 61 62 /* tmp1W32 = x*b1 - b2 + f[i]/2 */ 63 tmp1W32 = ((b1_high * x) << 1) + (((b1_low * x) >> 15) << 1) - 64 b2 + (f[i] << 13); 65 66 /* Handle overflows and set to maximum or minimum int16_t instead */ 67 if (tmp1W32>((int32_t)33553408)) { 68 return(WEBRTC_SPL_WORD16_MAX); 69 } else if (tmp1W32<((int32_t)-33554432)) { 70 return(WEBRTC_SPL_WORD16_MIN); 71 } else { 72 return (int16_t)(tmp1W32 >> 10); 73 } 74 } 75
