本文主要梳理 ffmpeg 中的音频格式转换。由于采集的音频数据和编码器支持的音频格式可能不一样，所以经常需要进行格式转换。
API 调用
常用 API
struct SwrContext *swr_alloc(void);
int swr_init(struct SwrContext *s);
struct SwrContext *swr_alloc_set_opts(struct SwrContext *s,int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate,int64_t in_ch_layout, enum AVSampleFormat in_sample_fmt, int in_sample_rate,int log_offset, void *log_ctx);
void swr_free(struct SwrContext **s);
int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,const uint8_t **in , int in_count);
int swr_convert_frame(SwrContext *swr,AVFrame *output, const AVFrame *input);
初始化和销毁相关
swr_alloc 创建 context 上下文结构体
swr_init 初始化 context 结构体
swr_free 销毁结构体
SwsContext class 定义 libswresample\options.c
static const AVClass av_class = {.class_name = "SWResampler",.item_name = context_to_name,.option = options,.version = LIBAVUTIL_VERSION_INT,.log_level_offset_offset = OFFSET(log_level_offset),.parent_log_context_offset = OFFSET(log_ctx),.category = AV_CLASS_CATEGORY_SWRESAMPLER,
};
swr_init 之前需要配置 context 一些参数，才能正确初始化。
if (av_opt_set_int(s, "ocl", out_ch_layout, 0) < 0)goto fail;if (av_opt_set_int(s, "osf", out_sample_fmt, 0) < 0)goto fail;if (av_opt_set_int(s, "osr", out_sample_rate, 0) < 0)goto fail;if (av_opt_set_int(s, "icl", in_ch_layout, 0) < 0)goto fail;if (av_opt_set_int(s, "isf", in_sample_fmt, 0) < 0)goto fail;if (av_opt_set_int(s, "isr", in_sample_rate, 0) < 0)goto fail;if (av_opt_set_int(s, "ich", av_get_channel_layout_nb_channels(s-> user_in_ch_layout), 0) < 0)goto fail;if (av_opt_set_int(s, "och", av_get_channel_layout_nb_channels(s->user_out_ch_layout), 0) < 0)goto fail;
为了简化调用所以有了 swr_alloc_set_opts接口，其主要就是做了 1. swr_alloc调用；2.参数设置。
由于其不像视频转换的 sws_getContext 接口，内部不会调用 swr_init，所以还需要调用一下初始化接口。
类型转换
swr_convert
swr_convert_frame
输出结果是直接写在输入的内存上的，所以 data 需要提前分配好内存
demo 调用
m_asc = swr_alloc_set_opts(m_asc,m_ac->channel_layout, m_ac->sample_fmt, m_ac->sample_rate,av_get_default_channel_layout(m_inChannels), (AVSampleFormat)m_inSampleFmt, m_inSampleRate,0, 0);
ret = swr_init(m_asc);
const uint8_t* data[1];
data[0] = (uint8_t*)pcm;
int len = swr_convert(m_asc, m_pcm->data, m_pcm->nb_samples,data, m_pcm->nb_samples);
if (m_asc)
{swr_free(&m_asc);
}
其他
所有接口
const AVClass *swr_get_class(void);
struct SwrContext *swr_alloc(void);
int swr_init(struct SwrContext *s);
int swr_is_initialized(struct SwrContext *s);
struct SwrContext *swr_alloc_set_opts(struct SwrContext *s,int64_t out_ch_layout, enum AVSampleFormat out_sample_fmt, int out_sample_rate,int64_t in_ch_layout, enum AVSampleFormat in_sample_fmt, int in_sample_rate,int log_offset, void *log_ctx);
void swr_free(struct SwrContext **s);
void swr_close(struct SwrContext *s);
int swr_convert(struct SwrContext *s, uint8_t **out, int out_count,const uint8_t **in , int in_count);
int64_t swr_next_pts(struct SwrContext *s, int64_t pts);
int swr_set_compensation(struct SwrContext *s, int sample_delta, int compensation_distance);
int swr_set_channel_mapping(struct SwrContext *s, const int *channel_map);
int swr_build_matrix(uint64_t in_layout, uint64_t out_layout,double center_mix_level, double surround_mix_level,double lfe_mix_level, double rematrix_maxval,double rematrix_volume, double *matrix,int stride, enum AVMatrixEncoding matrix_encoding,void *log_ctx);
int swr_set_matrix(struct SwrContext *s, const double *matrix, int stride);
int swr_drop_output(struct SwrContext *s, int count);
int swr_inject_silence(struct SwrContext *s, int count);
int64_t swr_get_delay(struct SwrContext *s, int64_t base);
int swr_get_out_samples(struct SwrContext *s, int in_samples);
unsigned swresample_version(void);
const char *swresample_configuration(void);
const char *swresample_license(void);
int swr_convert_frame(SwrContext *swr,AVFrame *output, const AVFrame *input);
int swr_config_frame(SwrContext *swr, const AVFrame *out, const AVFrame *in);