关于音频重采样库libsamplerate的使用

关于resample的库有多个，比如ffmpeg中的libswresample等等。alsa中使用是libsamplerate这个库，github链接地址为：https://github.com/erikd/libsamplerate 在alsaloop以及aplay中都广泛使用。今天写出一个DEMO出来解决一下实际问题。实际问题就是如何将16bit 2通道的48000采样率的PCM转换成16bit 2通道的44100采样率。

再废话一下，为什么要将不同的采样率进行转换呢？因为如果同时播放不同的采样率的音频文件，声卡其实只能是一种采样率配置，多个不同采样率要转换成同一种采样率并进行混音传递给声卡播放。这是就是这个问题的来源。

1. Ubuntu 14.04上安装libsamplerate

sudo apt-get install libsamplerate-dev

安装完成后可以查询到：

tony@tony-E431:~$ find /usr/ -name samplerate.h
/usr/include/samplerate.h
tony@tony-E431:~$ 

2. samplerate.h分析

多看几遍注释能了解个差不多。解决的我们的问题需要三步

(1) src_new 初始化
(2) src_process 开始转换
(3) src_delete 反初始化

完整源码

/*
** Copyright (C) 2002-2011 Erik de Castro Lopo <erikd@mega-nerd.com>
**
** This program is free software; you can redistribute it and/or modify
** it under the terms of the GNU General Public License as published by
** the Free Software Foundation; either version 2 of the License, or
** (at your option) any later version.
**
** This program is distributed in the hope that it will be useful,
** but WITHOUT ANY WARRANTY; without even the implied warranty of
** MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
** GNU General Public License for more details.
**
** You should have received a copy of the GNU General Public License
** along with this program; if not, write to the Free Software
** Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307, USA.
*/

/*
** This code is part of Secret Rabbit Code aka libsamplerate. A commercial
** use license for this code is available, please see:
**      http://www.mega-nerd.com/SRC/procedure.html
*/

/*
** API documentation is available here:
**     http://www.mega-nerd.com/SRC/api.html
*/

#ifndef SAMPLERATE_H
#define SAMPLERATE_H

#ifdef __cplusplus
extern "C" {
#endif  /* __cplusplus */


/* Opaque data type SRC_STATE. */
typedef struct SRC_STATE_tag SRC_STATE ;

/* SRC_DATA is used to pass data to src_simple() and src_process(). */
typedef struct
{   float   *data_in, *data_out ;

    long    input_frames, output_frames ;
    long    input_frames_used, output_frames_gen ;

    int     end_of_input ;

    double  src_ratio ;
} SRC_DATA ;

/* SRC_CB_DATA is used with callback based API. */
typedef struct
{   long    frames ;
    float   *data_in ;
} SRC_CB_DATA ;

/*
** User supplied callback function type for use with src_callback_new()
** and src_callback_read(). First parameter is the same pointer that was
** passed into src_callback_new(). Second parameter is pointer to a
** pointer. The user supplied callback function must modify *data to
** point to the start of the user supplied float array. The user supplied
** function must return the number of frames that **data points to.
*/

typedef long (*src_callback_t) (void *cb_data, float **data) ;

/*
**  Standard initialisation function : return an anonymous pointer to the
**  internal state of the converter. Choose a converter from the enums below.
**  Error returned in *error.
*/

SRC_STATE* src_new (int converter_type, int channels, int *error) ;

/*
**  Initilisation for callback based API : return an anonymous pointer to the
**  internal state of the converter. Choose a converter from the enums below.
**  The cb_data pointer can point to any data or be set to NULL. Whatever the
**  value, when processing, user supplied function "func" gets called with
**  cb_data as first parameter.
*/

SRC_STATE* src_callback_new (src_callback_t func, int converter_type, int channels,
                int *error, void* cb_data) ;

/*
**  Cleanup all internal allocations.
**  Always returns NULL.
*/

SRC_STATE* src_delete (SRC_STATE *state) ;

/*
**  Standard processing function.
**  Returns non zero on error.
*/

int src_process (SRC_STATE *state, SRC_DATA *data) ;

/*
**  Callback based processing function. Read up to frames worth of data from
**  the converter int *data and return frames read or -1 on error.
*/
long src_callback_read (SRC_STATE *state, double src_ratio, long frames, float *data) ;

/*
**  Simple interface for performing a single conversion from input buffer to
**  output buffer at a fixed conversion ratio.
**  Simple interface does not require initialisation as it can only operate on
**  a single buffer worth of audio.
*/

int src_simple (SRC_DATA *data, int converter_type, int channels) ;

/*
** This library contains a number of different sample rate converters,
** numbered 0 through N.
**
** Return a string giving either a name or a more full description of each
** sample rate converter or NULL if no sample rate converter exists for
** the given value. The converters are sequentially numbered from 0 to N.
*/

const char *src_get_name (int converter_type) ;
const char *src_get_description (int converter_type) ;
const char *src_get_version (void) ;

/*
**  Set a new SRC ratio. This allows step responses
**  in the conversion ratio.
**  Returns non zero on error.
*/

int src_set_ratio (SRC_STATE *state, double new_ratio) ;

/*
**  Reset the internal SRC state.
**  Does not modify the quality settings.
**  Does not free any memory allocations.
**  Returns non zero on error.
*/

int src_reset (SRC_STATE *state) ;

/*
** Return TRUE if ratio is a valid conversion ratio, FALSE
** otherwise.
*/

int src_is_valid_ratio (double ratio) ;

/*
**  Return an error number.
*/

int src_error (SRC_STATE *state) ;

/*
**  Convert the error number into a string.
*/
const char* src_strerror (int error) ;

/*
** The following enums can be used to set the interpolator type
** using the function src_set_converter().
*/

enum
{
    SRC_SINC_BEST_QUALITY       = 0,
    SRC_SINC_MEDIUM_QUALITY     = 1,
    SRC_SINC_FASTEST            = 2,
    SRC_ZERO_ORDER_HOLD         = 3,
    SRC_LINEAR                  = 4,
} ;

/*
** Extra helper functions for converting from short to float and
** back again.
*/

void src_short_to_float_array (const short *in, float *out, int len) ;
void src_float_to_short_array (const float *in, short *out, int len) ;

void src_int_to_float_array (const int *in, float *out, int len) ;
void src_float_to_int_array (const float *in, int *out, int len) ;


#ifdef __cplusplus
}       /* extern "C" */
#endif  /* __cplusplus */

#endif  /* SAMPLERATE_H */

3. 转换音频文件例子

sudo apt-get install samplerate-programs