ffmpeg實現音訊重取樣的核心函式swr_convert功能非常強大，可是ffmpeg文件對它的註釋太過簡單，在應用中往往會出這樣那樣的問題，其實在讀取資料->重取樣->編碼資料的迴圈中在第一次執行swr_convert後還應用swr_convert再作個快取檢測看看是否還有資料，如果有就要把它寫到FIFO中去，留在下次再使用，這點在轉碼和由低向高轉換取樣率時特別重要。

     下面一段簡單的程式碼，摘自我自已寫的測試程式.

	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);

		}
 

因為是測試程式碼，有點亂中間也有一些不必要的步驟，主要是為了除錯好用。

重點在：

    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) {
 
    }