如何通过 c++ 中的 FFmpeg 将原始文件转换为音频文件 (.wav)

Question

I want to covert mp4 format to wav format with different sample rate in my c++ application. First of all I have extracted audio from mp4 file by ffmpeg in c++, then i have converted that to a raw file, but I down not know how can I convert raw file to a wav file with different sample rate. How can I solve this?

 #include "ffmpeg.h"
    int decode_packet(int *got_frame, int cached)
    {
        int ret = 0;
        int decoded = pkt.size;
        *got_frame = 0;
        if (pkt.stream_index == video_stream_idx) {
            /* decode video frame */
            ret = avcodec_decode_video2(video_dec_ctx, frame, got_frame, &pkt);
            if (ret < 0) {
    //            fprintf(stderr, "Error decoding video frame (%s)\n", av_err2str(ret));
                return ret;
            }
            if (*got_frame) {
                if (frame->width != width || frame->height != height ||
                    frame->format != pix_fmt) {
                    /* To handle this change, one could call av_image_alloc again and
                     * decode the following frames into another rawvideo file. */
    //                fprintf(stderr, "Error: Width, height and pixel format have to be "
    //                        "constant in a rawvideo file, but the width, height or "
    //                        "pixel format of the input video changed:\n"
    //                        "old: width = %d, height = %d, format = %s\n"
    //                        "new: width = %d, height = %d, format = %s\n",
    //                        width, height, av_get_pix_fmt_name(pix_fmt),
    //                        frame->width, frame->height,
    //                        av_get_pix_fmt_name(frame->format));
                    return -1;
                }
                printf("video_frame%s n:%d coded_n:%d\n",
                       cached ? "(cached)" : "",
                       video_frame_count++, frame->coded_picture_number);
                /* copy decoded frame to destination buffer:
                 * this is required since rawvideo expects non aligned data */
                av_image_copy(video_dst_data, video_dst_linesize,
                              (const uint8_t **)(frame->data), frame->linesize,
                              pix_fmt, width, height);
                /* write to rawvideo file */
                fwrite(video_dst_data[0], 1, video_dst_bufsize, video_dst_file);
            }
        } else if (pkt.stream_index == audio_stream_idx) {
            /* decode audio frame */
            ret = avcodec_decode_audio4(audio_dec_ctx, frame, got_frame, &pkt);
            if (ret < 0) {
    //            fprintf(stderr, "Error decoding audio frame (%s)\n", av_err2str(ret));
                return ret;
            }
            /* Some audio decoders decode only part of the packet, and have to be
             * called again with the remainder of the packet data.
             * Sample: fate-suite/lossless-audio/luckynight-partial.shn
             * Also, some decoders might over-read the packet. */
            decoded = FFMIN(ret, pkt.size);
            if (*got_frame) {
                size_t unpadded_linesize = frame->nb_samples * av_get_bytes_per_sample((AVSampleFormat)frame->format);
    //            printf("audio_frame%s n:%d nb_samples:%d pts:%s\n",
    //                   cached ? "(cached)" : "",
    //                   audio_frame_count++, frame->nb_samples,
    //                   av_ts2timestr(frame->pts, &audio_dec_ctx->time_base));
                /* Write the raw audio data samples of the first plane. This works
                 * fine for packed formats (e.g. AV_SAMPLE_FMT_S16). However,
                 * most audio decoders output planar audio, which uses a separate
                 * plane of audio samples for each channel (e.g. AV_SAMPLE_FMT_S16P).
                 * In other words, this code will write only the first audio channel
                 * in these cases.
                 * You should use libswresample or libavfilter to convert the frame
                 * to packed data. */
    //            fwrite(frame->extended_data[0], 1, unpadded_linesize, audio_dst_file);
                //encode function
                encode(cOut, frame, &pktout, audio_dst_file);


    //            av_init_packet(&pktout);
    //            pktout.data = NULL; // packet data will be allocated by the encoder
    //            pktout.size = 0;
    //            /* encode the samples */
    //            ret = avcodec_encode_audio2(cOut, &pktout, frame, &got_outputOut);
    //            if (ret < 0) {
    //                fprintf(stderr, "Error encoding audio frame\n");
    //                exit(1);
    //            }
    //            if (got_outputOut) {
    //                fwrite(pktout.data, 1, pktout.size, audio_dst_file);
    //                av_free_packet(&pktout);
    //            }

            }
        }
        /* If we use frame reference counting, we own the data and need
         * to de-reference it when we don't use it anymore */
        if (*got_frame && refcount)
            av_frame_unref(frame);
        return decoded;
    }

Answer 1

First you should use Libswresample to resample audio data.

Then you can save audio raw data with wav format.