使用FFmpeg库对音频进行编码

#import "avformat.h"
#import "timestamp.h"
#import "avcodec.h"
#import "opt.h"
#import "samplefmt.h"
#import "swscale.h"
#import "avutil.h"

static int select_best_sample_rate(const AVCodec *codec){
    const int *p;
    int best_samplerate = 0;

    if(!codec->supported_samplerates){
        return 44100;
    }
    p = codec->supported_samplerates;
    while(*p){
        if(!best_samplerate || abs(44100 - *p) < abs(44100 - best_samplerate)){
            best_samplerate = *p;
        }
        p++;
    }
    return best_samplerate;
}

static int check_sample_fmt(const AVCodec *codec, enum AVSampleFormat sample_fmt){
    const enum AVSampleFormat *p = codec->sample_fmts;

    while(*p != AV_SAMPLE_FMT_NONE){
        if( *p == sample_fmt) {
            return 1;
        }
        p++;
    }
    return 0;
}

static int select_channel_layout(const AVCodec *codec)
{
    const uint64_t *p;
    uint64_t best_ch_layout = 0;
    int best_nb_channels   = 0;

    if (!codec->channel_layouts)
        return AV_CH_LAYOUT_STEREO;

    p = codec->channel_layouts;
    while (*p) {
        int nb_channels = av_get_channel_layout_nb_channels(*p);

        if (nb_channels > best_nb_channels) {
            best_ch_layout    = *p;
            best_nb_channels = nb_channels;
        }
        p++;
    }
    return best_ch_layout;
}

- (IBAction)encodeAudioAction:(id)sender {
    
    // 处理好的文件存储路径
    NSString * dstpath = [NSString stringWithFormat:@"%@/Library/Caches/1-3.aac",NSHomeDirectory()];
    char  dstChar [ 1024 ];
    strcpy (dstChar ,( char  *)[dstpath  UTF8String]);

    //编码格式
    NSString * type = @"libfdk_aac";
    char  typeChar [ 1024 ];
    strcpy (typeChar ,( char  *)[type  UTF8String]);
    
    char * argv[2];
    argv[0] = dstChar;
    argv[1] = typeChar;
    
    NSLog(@"存储路径：%@", dstpath);
    
    encodeAudio(2, argv);
}

#pragma mark 音频编码
void encodeAudio(int argc, char* argv[]) {
    int ret = -1;

    FILE *f = NULL;

    char *dst = NULL;
    char *codecName = NULL;

    const AVCodec *codec = NULL;
    AVCodecContext *ctx = NULL;

    AVFrame *frame = NULL;
    AVPacket *pkt = NULL;

    uint16_t *samples = NULL;

    av_log_set_level(AV_LOG_DEBUG);

    //1. 输入参数
    if(argc < 2){
        av_log(NULL, AV_LOG_ERROR, "arguments must be more than 2\n");
        goto _ERROR;
    }

    dst = argv[0];
    codecName = argv[1];

    //2. 查找编码器
    codec = avcodec_find_encoder_by_name(codecName);
    //codec = avcodec_find_encoder(AV_CODEC_ID_AAC);
    if(!codec){
        av_log(NULL, AV_LOG_ERROR, "don't find Codec: %s", codecName);
        goto _ERROR;
    }

    //3. 创建编码器上下文
    ctx = avcodec_alloc_context3(codec);
    if(!ctx){
        av_log(NULL, AV_LOG_ERROR, "NO MEMRORY\n");
        goto _ERROR;
    }

    //4. 设置编码器参数
    ctx->bit_rate = 64000;
    ctx->sample_fmt = AV_SAMPLE_FMT_S16;//AV_SAMPLE_FMT_FLTP
    if(!check_sample_fmt(codec, ctx->sample_fmt)){
        av_log(NULL, AV_LOG_ERROR, "Encoder does not support sample format!\n");
        goto _ERROR;
    }

    ctx->sample_rate = select_best_sample_rate(codec);
//    av_channel_layout_copy(&ctx->ch_layout, &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO); //AV_CHANNEL_LAYOUT_MONO

    ctx->channel_layout = select_channel_layout(codec);
    ctx->channels       = av_get_channel_layout_nb_channels(ctx->channel_layout);
    
    //5. 编码器与编码器上下文绑定到一起
    ret = avcodec_open2(ctx, codec , NULL);
    if(ret < 0) {
        av_log(ctx, AV_LOG_ERROR, "Don't open codec: %s \n", av_err2str(ret));
        goto _ERROR;
    }

    //6. 创建输出文件
    f = fopen(dst, "wb");
    if(!f){
        av_log(NULL, AV_LOG_ERROR, "Don't open file:%s", dst);
        goto _ERROR;
    }

    //7. 创建AVFrame
    frame = av_frame_alloc();
    if(!frame){
        av_log(NULL, AV_LOG_ERROR, "NO MEMORY!\n");
        goto _ERROR;
    }

    frame->nb_samples = ctx->frame_size;
    frame->format = AV_SAMPLE_FMT_S16; //AV_SAMPLE_FMT_FLTP
//    av_channel_layout_copy(&frame->ch_layout,  &(AVChannelLayout)AV_CHANNEL_LAYOUT_STEREO); //AV_CHANNEL_LAYOUT_MONO
    frame->channel_layout = ctx->channel_layout;
    
    frame->sample_rate = ctx->sample_rate;
    ret = av_frame_get_buffer(frame, 0);
    if(ret < 0) {
        av_log(NULL, AV_LOG_ERROR, "Could not allocate the video frame \n");
        goto _ERROR;
    }

    //8. 创建AVPacket
    pkt = av_packet_alloc();
     if(!pkt){
        av_log(NULL, AV_LOG_ERROR, "NO MEMORY!\n");
        goto _ERROR;
    }

    //9. 生成音频内容
    float t = 0;
    float tincr = 4*M_PI*440/ctx->sample_rate;

    for(int i=0; i < 200; i++){
        ret = av_frame_make_writable(frame);
        if(ret < 0) {
            av_log(NULL, AV_LOG_ERROR, "Could not allocate space!\n");
            goto _ERROR;
        }

        samples = (uint16_t*)frame->data[0]; //FLTP 32 (uint32_t*)
        for(int j=0; j < ctx->frame_size; j++){
            samples[2*j] = (int)(sin(t)*10000); //4
            for(int k=1; k < ctx->channels; k++){
                samples[2*j + k] = samples[2*j]; //4
            }
            t += tincr;
        }
        encode(ctx, frame, pkt, f);
    }
    //10. 编码
    encode(ctx, NULL, pkt, f);
_ERROR:
    //ctx
    if(ctx){
        avcodec_free_context(&ctx);
    }

    //avframe
    if(frame){
        av_frame_free(&frame);
    }

    //avpacket
    if(pkt){
        av_packet_free(&pkt);
    }

    //dst
    if(f){
        fclose(f);
    }
}