FFmpeg编程学习笔记二：音频重采样

ffmpeg实现音频重采样的核心函数swr_convert功能非常强大，可是ffmpeg文档对它的注释太过简单，在应用中往往会出这样那样的问题，其实在读取数据->重采样->编码数据的循环中在第一次执行swr_convert后还应用swr_convert再作个缓存检测看看是否还有数据，如果有就要把它写到FIFO中去，留在下次再使用，，这点在转码和由低向高转换采样率时特别重要。

下面一段简单的代码，摘自我自已写的测试程序.

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);}

因为是测试代码，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) {}

只能昏昏沉沉地沿着青草和泥土的气息前进。

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