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1 /*
2  *  Copyright (c) 2012 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 #include "webrtc/common_audio/signal_processing/include/real_fft.h"
12 
13 #include <stdlib.h>
14 
15 #include "webrtc/common_audio/signal_processing/include/signal_processing_library.h"
16 
17 struct RealFFT {
18   int order;
19 };
20 
WebRtcSpl_CreateRealFFT(int order)21 struct RealFFT* WebRtcSpl_CreateRealFFT(int order) {
22   struct RealFFT* self = NULL;
23 
24   if (order > kMaxFFTOrder || order < 0) {
25     return NULL;
26   }
27 
28   self = malloc(sizeof(struct RealFFT));
29   if (self == NULL) {
30     return NULL;
31   }
32   self->order = order;
33 
34   return self;
35 }
36 
WebRtcSpl_FreeRealFFT(struct RealFFT * self)37 void WebRtcSpl_FreeRealFFT(struct RealFFT* self) {
38   if (self != NULL) {
39     free(self);
40   }
41 }
42 
43 // The C version FFT functions (i.e. WebRtcSpl_RealForwardFFT and
44 // WebRtcSpl_RealInverseFFT) are real-valued FFT wrappers for complex-valued
45 // FFT implementation in SPL.
46 
WebRtcSpl_RealForwardFFT(struct RealFFT * self,const int16_t * real_data_in,int16_t * complex_data_out)47 int WebRtcSpl_RealForwardFFT(struct RealFFT* self,
48                              const int16_t* real_data_in,
49                              int16_t* complex_data_out) {
50   int i = 0;
51   int j = 0;
52   int result = 0;
53   int n = 1 << self->order;
54   // The complex-value FFT implementation needs a buffer to hold 2^order
55   // 16-bit COMPLEX numbers, for both time and frequency data.
56   int16_t complex_buffer[2 << kMaxFFTOrder];
57 
58   // Insert zeros to the imaginary parts for complex forward FFT input.
59   for (i = 0, j = 0; i < n; i += 1, j += 2) {
60     complex_buffer[j] = real_data_in[i];
61     complex_buffer[j + 1] = 0;
62   };
63 
64   WebRtcSpl_ComplexBitReverse(complex_buffer, self->order);
65   result = WebRtcSpl_ComplexFFT(complex_buffer, self->order, 1);
66 
67   // For real FFT output, use only the first N + 2 elements from
68   // complex forward FFT.
69   memcpy(complex_data_out, complex_buffer, sizeof(int16_t) * (n + 2));
70 
71   return result;
72 }
73 
WebRtcSpl_RealInverseFFT(struct RealFFT * self,const int16_t * complex_data_in,int16_t * real_data_out)74 int WebRtcSpl_RealInverseFFT(struct RealFFT* self,
75                              const int16_t* complex_data_in,
76                              int16_t* real_data_out) {
77   int i = 0;
78   int j = 0;
79   int result = 0;
80   int n = 1 << self->order;
81   // Create the buffer specific to complex-valued FFT implementation.
82   int16_t complex_buffer[2 << kMaxFFTOrder];
83 
84   // For n-point FFT, first copy the first n + 2 elements into complex
85   // FFT, then construct the remaining n - 2 elements by real FFT's
86   // conjugate-symmetric properties.
87   memcpy(complex_buffer, complex_data_in, sizeof(int16_t) * (n + 2));
88   for (i = n + 2; i < 2 * n; i += 2) {
89     complex_buffer[i] = complex_data_in[2 * n - i];
90     complex_buffer[i + 1] = -complex_data_in[2 * n - i + 1];
91   }
92 
93   WebRtcSpl_ComplexBitReverse(complex_buffer, self->order);
94   result = WebRtcSpl_ComplexIFFT(complex_buffer, self->order, 1);
95 
96   // Strip out the imaginary parts of the complex inverse FFT output.
97   for (i = 0, j = 0; i < n; i += 1, j += 2) {
98     real_data_out[i] = complex_buffer[j];
99   }
100 
101   return result;
102 }
103