1  /* ----------------------------------------------------------------------
 2   * Project:      CMSIS DSP Library
 3   * Title:        arm_std_f32.c
 4   * Description:  Standard deviation of the elements of a floating-point vector
 5   *
 6   * $Date:        23 April 2021
 7   * $Revision:    V1.9.0
 8   *
 9   * Target Processor: Cortex-M and Cortex-A cores
10   * -------------------------------------------------------------------- */
11  /*
12   * Copyright (C) 2010-2021 ARM Limited or its affiliates. All rights reserved.
13   *
14   * SPDX-License-Identifier: Apache-2.0
15   *
16   * Licensed under the Apache License, Version 2.0 (the License); you may
17   * not use this file except in compliance with the License.
18   * You may obtain a copy of the License at
19   *
20   * www.apache.org/licenses/LICENSE-2.0
21   *
22   * Unless required by applicable law or agreed to in writing, software
23   * distributed under the License is distributed on an AS IS BASIS, WITHOUT
24   * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
25   * See the License for the specific language governing permissions and
26   * limitations under the License.
27   */
28  
29  #include "dsp/statistics_functions.h"
30  
31  /**
32    @ingroup groupStats
33   */
34  
35  /**
36    @defgroup STD Standard deviation
37  
38    Calculates the standard deviation of the elements in the input vector.
39  
40    The float implementation is relying on arm_var_f32 which is using a two-pass algorithm
41    to avoid problem of numerical instabilities and cancellation errors.
42  
43    Fixed point versions are using the standard textbook algorithm since the fixed point
44    numerical behavior is different from the float one.
45  
46    Algorithm for fixed point versions is summarized below:
47  
48  
49    <pre>
50        Result = sqrt((sumOfSquares - sum<sup>2</sup> / blockSize) / (blockSize - 1))
51  
52        sumOfSquares = pSrc[0] * pSrc[0] + pSrc[1] * pSrc[1] + ... + pSrc[blockSize-1] * pSrc[blockSize-1]
53        sum = pSrc[0] + pSrc[1] + pSrc[2] + ... + pSrc[blockSize-1]
54    </pre>
55  
56    There are separate functions for floating point, Q31, and Q15 data types.
57   */
58  
59  /**
60    @addtogroup STD
61    @{
62   */
63  
64  /**
65    @brief         Standard deviation of the elements of a floating-point vector.
66    @param[in]     pSrc       points to the input vector
67    @param[in]     blockSize  number of samples in input vector
68    @param[out]    pResult    standard deviation value returned here
69    @return        none
70   */
71  void arm_std_f32(
72    const float32_t * pSrc,
73          uint32_t blockSize,
74          float32_t * pResult)
75  {
76    float32_t var;
77    arm_var_f32(pSrc,blockSize,&var);
78    arm_sqrt_f32(var, pResult);
79  }
80  
81  /**
82    @} end of STD group
83   */