ffmpeg实现音频重採样的核心函数swr_convert功能很强大，但是ffmpeg文档对它的凝视太过简单。在应用中往往会出这样那样的问题，事实上在读取数据->重採样->编码数据的循环中在第一次运行swr_convert后还应用swr_convert再作个缓存检測看看是否还有数据。假设有就要把它写到FIFO中去，留在下次再使用，这点在转码和由低向高转换採样率时特别重要。
以下一段简单的代码，摘自我自已写的測试程序.
const int frame_size = FFMIN(fifo_size, m_Opt->encode_pCodecCtx->frame_size);
if ((ret = av_audio_fifo_read(m_fifo, (void **)m_fifo_samples_array, frame_size)) < frame_size) {
fprintf(stderr, "Could not read data from FIFO\n");
return AVERROR_EXIT;
}
int out_samples = av_rescale_rnd(swr_get_delay(m_Opt->out_resample_context, 48000) + 1536, 44100, 48000, AV_ROUND_UP);
int conver_samples= swr_convert(m_Opt->out_resample_context, m_fifo_conver_samples_array, frame_size,
(const uint8_t **)m_fifo_samples_array, frame_size);
ret = av_audio_fifo_size(m_conver_fifo);
if ((ret = av_audio_fifo_realloc(m_conver_fifo, av_audio_fifo_size(m_conver_fifo) + frame_size)) < 0) {
fprintf(stderr, "Could not reallocate FIFO\n");
return ret;
}
ret = av_audio_fifo_write(m_conver_fifo, (void **)m_fifo_conver_samples_array, conver_samples);
fifo_size = av_audio_fifo_size(m_conver_fifo);
if ((ret = av_audio_fifo_read(m_conver_fifo, (void **)m_Opt->out_samples_array, frame_size)) < frame_size) {
fprintf(stderr, "Could not read data from FIFO\n");
return AVERROR_EXIT;
}
ret = Encode_audio(ret);
int ret1 = 0;
while ((ret1 = swr_convert(m_Opt->out_resample_context, m_fifo_conver_samples_array, frame_size, NULL, 0)) > 0)
{
if ((ret = av_audio_fifo_realloc(m_conver_fifo, av_audio_fifo_size(m_conver_fifo) + ret1)) < 0) {
fprintf(stderr, "Could not reallocate FIFO\n");
return ret;
}
ret = av_audio_fifo_write(m_conver_fifo, (void **)m_fifo_conver_samples_array, ret1);
fifo_size = av_audio_fifo_size(m_conver_fifo);
if (fifo_size > m_Opt->encode_pFrame->nb_samples)
{
if ((ret = av_audio_fifo_read(m_conver_fifo, (void **)m_Opt->out_samples_array, frame_size)) < frame_size) {
fprintf(stderr, "Could not read data from FIFO\n");
return AVERROR_EXIT;
}
ret = Encode_audio(ret);
}由于是測试代码，有点乱中间也有一些不必要的步骤。主要是为了调试好用。
重点在：
if ((r = swr_convert(swr_ctx, output, output_nb_samples,(const uint8_t**)input, nb_samples)) < 0)
return -1; while ((r = swr_convert(swr_ctx, output, output_nb_samples, NULL, 0)) > 0) { }