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 * This file contains the function WebRtcSpl_LpcToReflCoef().
14 * The description header can be found in signal_processing_library.h
15 *
16 */
17
18#include "signal_processing_library.h"
19
20#define SPL_LPC_TO_REFL_COEF_MAX_AR_MODEL_ORDER 50
21
22void WebRtcSpl_LpcToReflCoef(WebRtc_Word16* a16, int use_order, WebRtc_Word16* k16)
23{
24    int m, k;
25    WebRtc_Word32 tmp32[SPL_LPC_TO_REFL_COEF_MAX_AR_MODEL_ORDER];
26    WebRtc_Word32 tmp_inv_denom32;
27    WebRtc_Word16 tmp_inv_denom16;
28
29    k16[use_order - 1] = WEBRTC_SPL_LSHIFT_W16(a16[use_order], 3); //Q12<<3 => Q15
30    for (m = use_order - 1; m > 0; m--)
31    {
32        // (1 - k^2) in Q30
33        tmp_inv_denom32 = ((WebRtc_Word32)1073741823) - WEBRTC_SPL_MUL_16_16(k16[m], k16[m]);
34        // (1 - k^2) in Q15
35        tmp_inv_denom16 = (WebRtc_Word16)WEBRTC_SPL_RSHIFT_W32(tmp_inv_denom32, 15);
36
37        for (k = 1; k <= m; k++)
38        {
39            // tmp[k] = (a[k] - RC[m] * a[m-k+1]) / (1.0 - RC[m]*RC[m]);
40
41            // [Q12<<16 - (Q15*Q12)<<1] = [Q28 - Q28] = Q28
42            tmp32[k] = WEBRTC_SPL_LSHIFT_W32((WebRtc_Word32)a16[k], 16)
43                    - WEBRTC_SPL_LSHIFT_W32(WEBRTC_SPL_MUL_16_16(k16[m], a16[m-k+1]), 1);
44
45            tmp32[k] = WebRtcSpl_DivW32W16(tmp32[k], tmp_inv_denom16); //Q28/Q15 = Q13
46        }
47
48        for (k = 1; k < m; k++)
49        {
50            a16[k] = (WebRtc_Word16)WEBRTC_SPL_RSHIFT_W32(tmp32[k], 1); //Q13>>1 => Q12
51        }
52
53        tmp32[m] = WEBRTC_SPL_SAT(8191, tmp32[m], -8191);
54        k16[m - 1] = (WebRtc_Word16)WEBRTC_SPL_LSHIFT_W32(tmp32[m], 2); //Q13<<2 => Q15
55    }
56    return;
57}
58