FFMPEG + SDL音频播放分析

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目录 [hide]

抽象流程：

设置SDL的音频参数 —-> 打开声音设备，播放静音 —-> ffmpeg读取音频流中数据放入队列 —-> SDL调用用户设置的函数来获取音频数据 —-> 播放音频

SDL内部维护了一个buffer来存放解码后的数据，这个buffer中的数据来源是我们注册的回调函数（audio_callback），audio_callback调用audio_decode_frame来做具体的音频解码工作，需要引起注意的是：从流中读取出的一个音频包(avpacket)可能含有多个音频桢(avframe)，所以需要多次调用avcodec_decode_audio4来完成整个包的解码，解码出来的数据存放在我们自己的缓冲中（audio_buf2）。SDL每一次回调都会引起数据从audio_buf2拷贝到SDL内部缓冲区，当audio_buf2中的数据大于SDL的缓冲区大小时，需要分多次拷贝。

关键实现：

main()函数

1 int main(int argc, char **argv){ 2 SDL_Event event; //SDL事件变量 3 VideoState *is; // 纪录视频及解码器等信息的大结构体 4 is = (VideoState*) av_mallocz(sizeof(VideoState)); 5 if(argc < 2){ 6 fprintf(stderr, "Usage: play <file>\n"); 7 exit(1); 8 } 9 av_register_all(); //注册所有ffmpeg的解码器 10 /* 初始化SDL，这里只实用了AUDIO，如果有视频，好需要SDL_INIT_VIDEO等等 */ 11 if(SDL_Init(SDL_INIT_AUDIO)){ 12 fprintf(stderr, "Count not initialize SDL - %s\n", SDL_GetError()); 13 exit(1); 14 } 15 is_strlcpy(is->filename, argv[1], sizeof(is->filename)); 16 /* 创建一个SDL线程来做视频解码工作，主线程进入SDL事件循环 */ 17 is->parse_tid = SDL_CreateThread(decode_thread, is); 18 if(!is->parse_tid){ 19 SDL_WaitEvent(&event); 20 switch(event.type){ 21 case FF_QUIT_EVENT: 22 case SDL_QUIT: 23 is->quit = 1; 24 SDL_Quit(); 25 exit(0); 26 break; 27 default: 28 break; 29 } 30 } 31 return 0; 32 }

decode_thread()读取文件信息和音频包

1 static int decode_thread(void *arg){ 2 VideoState *is = (VideoState*)arg; 3 AVFormatContext *ic = NULL; 4 AVPacket pkt1, *packet = &pkt1; 5 int ret, i, audio_index = -1; 6 7 is->audioStream = -1; 8 global_video_state = is; 9 /* 使用ffmpeg打开视频，解码器等常规工作 */ 10 if(avFormat_open_input(&ic, is->filename, NULL, NULL) != 0) { 11 fprintf(stderr, "open file error: %s\n", is->filename); 12 return -1; 13 } 14 is->ic = ic; 15 if(avformat_find_stream_info(ic, NULL) < 0){ 16 fprintf(stderr, "find stream info error\n"); 17 return -1; 18 } 19 av_dump_format(ic, 0, is->filename, 0); 20 for(i = 0; i < ic->nb_streams; i++){ 21 if(ic->streams[i])->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index == -1){ 22 audio_index = i; 23 break; 24 } 25 } 26 if(audio_index >= 0) { 27 /* 所有设置SDL音频流信息的步骤都在这个函数里完成 */ 28 stream_component_open(is, audio_index); 29 } 30 if(is->audioStream < 0){ 31 fprintf(stderr, "could not open codecs for file: %s\n", is->filename); 32 goto fail; 33 } 34 /* 读包的主循环， av_read_frame不停的从文件中读取数据包（这里只取音频包）*/ 35 for(;;){ 36 if(is->quit) break; 37 /* 这里audioq.size是指队列中的所有数据包带的音频数据的总量，并不是包的数量 */ 38 if(is->audioq.size > MAX_AUDIO_SIZE){ 39 SDL_Delay(10); // 毫秒 40 continue; 41 } 42 ret = av_read_frame(is->ic, packet); 43 if(ret < 0){ 44 if(ret == AVERROR_EOF || url_feof(is->ic->pb)) break; 45 if(is->ic->pb && is->ic->pb->error) break; 46 contiue; 47 } 48 if(packet->stream_index == is->audioStream){ 49 packet_queue_put(&is->audioq, packet); 50 } else{ 51 av_free_packet(packet); 52 } 53 } 54 while(!is->quit) SDL_Delay(100); 55 fail: { 56 SDL_Event event; 57 event.type = FF_QUIT_EVENT; 58 event.user.data1 = is; 59 SDL_PushEvent(&event); 60 } 61 return 0; 62 }

stream_component_open()：设置音频参数和打开设备

1 int stream_component_open(videoState *is, int stream_index){ 2 AVFormatContext *ic = is->ic; 3 AVCodecContext *codecCtx; 4 AVCodec *codec; 5 /* 在用SDL_OpenAudio()打开音频设备的时候需要这两个参数*/ 6 /* wanted_spec是我们期望设置的属性，spec是系统最终接受的参数 */ 7 /* 我们需要检查系统接受的参数是否正确 */ 8 SDL_AudioSpec wanted_spec, spec; 9 int64_t wanted_channel_layout = 0; // 声道布局(SDL中的具体定义见“FFMPEG结构体”部分) 10 int wanted_nb_channels; // 声道数 11 /* SDL支持的声道数为 1, 2, 4, 6 */ 12 /* 后面我们会使用这个数组来纠正不支持的声道数目 */ 13 const int next_nb_channels[] = { 0, 0, 1, 6, 2, 6, 4, 6 }; 14 15 if(stream_index < 0 || stream_index >= ic->nb_streams) return -1; 16 codecCtx = ic->streams[stream_index]->codec; 17 wanted_nb_channels = codecCtx->channels; 18 if(!wanted_channel_layout || wanted_nb_channels != av_get_channel_layout_nb_channels(wanted_channel_layout)) { 19 wanted_channel_layout = av_get_default_channel_lauout(wanted_channel_nb_channels); 20 wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX; 21 } 22 wanted_spec.channels = av_get_channels_layout_nb_channels(wanted_channel_layout); 23 wanted_spec.freq = codecCtx->sample_rate; 24 if(wanted_spec.freq <= 0 || wanted_spec.channels <=0){ 25 fprintf(stderr, "Invaild sample rate or channel count!\n"); 26 return -1; 27 } 28 wanted_spec.format = AUDIO_S16SYS; // 具体含义请查看“SDL宏定义”部分 29 wanted_spec.silence = 0; // 0指示静音 30 wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE; // 自定义SDL缓冲区大小 31 wanted_spec.callback = audio_callback; // 音频解码的关键回调函数 32 wanted_spec.userdata = is; // 传给上面回调函数的外带数据 33 34 /* 打开音频设备，这里使用一个while来循环尝试打开不同的声道数(由上面 */ 35 /* next_nb_channels数组指定）直到成功打开，或者全部失败 */ 36 while(SDL_OpenAudio(&wanted_spec, &spec) < 0){ 37 fprintf(stderr, "SDL_OpenAudio(%d channels): %s\n", wanted_spec.channels, SDL_GetError()); 38 wanted_spec.channels = next_nb_channels[FFMIN(7, wanted_spec.channels)]; // FFMIN()由ffmpeg定义的宏，返回较小的数 39 if(!wanted_spec.channels){ 40 fprintf(stderr, "No more channel to try\n"); 41 return -1; 42 } 43 wanted_channel_layout = av_get_default_channel_layout(wanted_spec.channels); 44 } 45 /* 检查实际使用的配置（保存在spec,由SDL_OpenAudio()填充） */ 46 if(spec.format != AUDIO_S16SYS){ 47 fprintf(stderr, "SDL advised audio format %d is not supported\n", spec.format); 48 return -1; 49 } 50 if(spec.channels != wanted_spec.channels) { 51 wanted_channel_layout = av_get_default_channel_layout(spec.channels); 52 if(!wanted_channel_layout){ 53 fprintf(stderr, "SDL advised channel count %d is not support\n", spec.channels); 54 return -1; 55 } 56 } 57 /* 把设置好的参数保存到大结构中 */ 58 is->audio_src_fmt = is->audio_tgt_fmt = AV_SAMPLE_FMT_S16; 59 is->audio_src_freq = is->audio_tgt_freq = spec.freq; 60 is->audio_src_channel_layout = is->audio_tgt_layout = wanted_channel_layout; 61 is->audio_src_channels = is->audio_tat_channels = spec.channels; 62 63 codec = avcodec_find_decoder(codecCtx>codec_id); 64 if(!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)){ 65 fprintf(stderr, "Unsupported codec!\n"); 66 return -1; 67 } 68 ic->streams[stream_index]->discard = AVDISCARD_DEFAULT; //具体含义请查看“FFMPEG宏定义”部分 69 is->audioStream = stream_index; 70 is->audio_st = ic->streams[stream_index]; 71 is->audio_buf_size = 0; 72 is->audio_buf_index = 0; 73 memset(&is->audio_pkt, 0, sizeof(is->audio_pkt)); 74 packet_queue_init(&is->audioq); 75 SDL_PauseAudio(0); // 开始播放静音 76 }

audio_callback(): 回调函数，向SDL缓冲区填充数据

1 void audio_callback(void *userdata, Uint8 *stream, int len){ 2 VideoState *is = (VideoState*)userdata; 3 int len1, audio_data_size; 4 5 /* len是由SDL传入的SDL缓冲区的大小，如果这个缓冲未满，我们就一直往里填充数据 */ 6 while(len > 0){ 7 /* audio_buf_index 和 audio_buf_size 标示我们自己用来放置解码出来的数据的缓冲区，*/ 8 /* 这些数据待copy到SDL缓冲区，当audio_buf_index >= audio_buf_size的时候意味着我*/ 9 /* 们的缓冲为空，没有数据可供copy，这时候需要调用audio_decode_frame来解码出更 10 /* 多的桢数据 */ 11 if(is->audio_buf_index >= is->audio_buf_size){ 12 audio_data_size = audio_decode_frame(is); 13 /* audio_data_size < 0 标示没能解码出数据，我们默认播放静音 */ 14 is(audio_data_size < 0){ 15 is->audio_buf_size = 1024; 16 /* 清零，静音 */ 17 memset(is->audio_buf, 0, is->audio_buf_size); 18 } else{ 19 is->audio_buf_size = audio_data_size; 20 } 21 is->audio_buf_index = 0; 22 } 23 /* 查看stream可用空间，决定一次copy多少数据，剩下的下次继续copy */ 24 len1 = is->audio_buf_size - is->audio_buf_index; 25 if(len1 > len) len1 = len; 26 27 memcpy(stream, (uint8_t*)is->audio_buf + is->audio_buf_index, len1); 28 len -= len1; 29 stream += len1; 30 is->audio_buf_index += len1; 31 } 32 }

audio_decode_frame()：解码音频

1 int audio_decode_frame(VideoState *is){ 2 int len1, len2, decoded_data_size; 3 AVPacket *pkt = &is->audio_pkt; 4 int got_frame = 0; 5 int64_t dec_channel_layout; 6 int wanted_nb_samples, resampled_data_size; 7 8 for(;;){ 9 while(is->audio_pkt_size > 0){ 10 if(!is->audio_frame){ 11 if(!(is->audio_frame = avacodec_alloc_frame())){ 12 return AVERROR(ENOMEM); 13 } 14 } else 15 avcodec_get_frame_defaults(is->audio_frame); 16 17 len1 = avcodec_decode_audio4(is->audio_st_codec, is->audio_frame, got_frame, pkt); 18 /* 解码错误，跳过整个包 */ 19 if(len1 < 0){ 20 is->audio_pkt_size = 0; 21 break; 22 } 23 is->audio_pkt_data += len1; 24 is->audio_pkt_size -= len1; 25 if(!got_frame) continue; 26 /* 计算解码出来的桢需要的缓冲大小 */ 27 decoded_data_size = av_samples_get_buffer_size(NULL, 28 is->audio_frame_channels, 29 is->audio_frame_nb_samples, 30 is->audio_frame_format, 1); 31 dec_channel_layout = (is->audio_frame->channel_layout && is->audio_frame->channels 32 == av_get_channel_layout_nb_channels(is->audio_frame->channel_layout)) 33

? is->audio_frame->channel_layout : av_get_default_channel_layout(is->audio_frame->channels);

34 wanted_nb_samples = is->audio_frame->nb_samples; 35 if (is->audio_frame->format != is->audio_src_fmt || 36 dec_channel_layout != is->audio_src_channel_layout || 37 is->audio_frame->sample_rate != is->audio_src_freq || 38 (wanted_nb_samples != is->audio_frame->nb_samples && !is->swr_ctx)) { 39 if (is->swr_ctx) swr_free(&is->swr_ctx); 40 is->swr_ctx = swr_alloc_set_opts(NULL, 41 is->audio_tgt_channel_layout, 42 is->audio_tgt_fmt, 43 is->audio_tgt_freq, 44 dec_channel_layout, 45 is->audio_frame->format, 46 is->audio_frame->sample_rate, 47 0, NULL); 48 if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) { 49 fprintf(stderr, "swr_init() failed\n"); 50 break; 51 } 52 is->audio_src_channel_layout = dec_channel_layout; 53 is->audio_src_channels = is->audio_st->codec->channels; 54 is->audio_src_freq = is->audio_st->codec->sample_rate; 55 is->audio_src_fmt = is->audio_st->codec->sample_fmt; 56 } 57 /* 这里我们可以对采样数进行调整，增加或者减少，一般可以用来做声画同步 */ 58 if (is->swr_ctx) { 59 const uint8_t **in = (const uint8_t **)is->audio_frame->extended_data; 60 uint8_t *out[] = { is->audio_buf2 }; 61 if (wanted_nb_samples != is->audio_frame->nb_samples) { 62 if(swr_set_compensation(is->swr_ctx, 63 (wanted_nb_samples - is->audio_frame->nb_samples)*is->audio_tgt_freq/is->audio_frame->sample_rate, 64 wanted_nb_samples * is->audio_tgt_freq/is->audio_frame->sample_rate) < 0) { 65 fprintf(stderr, "swr_set_compensation() failed\n"); 66 break; 67 } 68 } 69 len2 = swr_convert(is->swr_ctx, out, 70 sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt), 71 in, is->audio_frame->nb_samples); 72 if (len2 < 0) { 73 fprintf(stderr, "swr_convert() failed\n"); 74 break; 75 } 76 if(len2 == sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt)) { 77 fprintf(stderr, "warning: audio buffer is probably too small\n"); 78 swr_init(is->swr_ctx); 79 } 80 is->audio_buf = is->audio_buf2; 81 resampled_data_size = len2*is->audio_tgt_channels*av_get_bytes_per_sample(is->audio_tgt_fmt); 82 } else { 83 resampled_data_size = decoded_data_size; 84 is->audio_buf = is->audio_frame->data[0]; 85 } 86 /* 返回得到的数据 */ 87 return resampled_data_size; 88 } 89 if (pkt->data) av_free_packet(pkt); 90 memset(pkt, 0, sizeof(*pkt)); 91 if (is->quit) return -1; 92 if (packet_queue_get(&is->audioq, pkt, 1) < 0) return -1; 93 is->audio_pkt_data = pkt->data; 94 is->audio_pkt_size = pkt->size; 95 96 } 97 }

FFMPEG结构体

channel_layout_map

1 static const struct { 2 const char *name; 3 int nb_channels; 4 uint64_t layout; 5 } channel_layout_map[] = { 6 { "mono", 1, AV_CH_LAYOUT_MONO }, 7 { "stereo", 2, AV_CH_LAYOUT_STEREO }, 8 { "2.1", 3, AV_CH_LAYOUT_2POINT1 }, 9 { "3.0", 3, AV_CH_LAYOUT_SURROUND }, 10 { "3.0(back)", 3, AV_CH_LAYOUT_2_1 }, 11 { "4.0", 4, AV_CH_LAYOUT_4POINT0 }, 12 { "quad", 4, AV_CH_LAYOUT_QUAD }, 13 { "quad(side)", 4, AV_CH_LAYOUT_2_2 }, 14 { "3.1", 4, AV_CH_LAYOUT_3POINT1 }, 15 { "5.0", 5, AV_CH_LAYOUT_5POINT0_BACK }, 16 { "5.0(side)", 5, AV_CH_LAYOUT_5POINT0 }, 17 { "4.1", 5, AV_CH_LAYOUT_4POINT1 }, 18 { "5.1", 6, AV_CH_LAYOUT_5POINT1_BACK }, 19 { "5.1(side)", 6, AV_CH_LAYOUT_5POINT1 }, 20 { "6.0", 6, AV_CH_LAYOUT_6POINT0 }, 21 { "6.0(front)", 6, AV_CH_LAYOUT_6POINT0_FRONT }, 22 { "hexagonal", 6, AV_CH_LAYOUT_HEXAGONAL }, 23 { "6.1", 7, AV_CH_LAYOUT_6POINT1 }, 24 { "6.1", 7, AV_CH_LAYOUT_6POINT1_BACK }, 25 { "6.1(front)", 7, AV_CH_LAYOUT_6POINT1_FRONT }, 26 { "7.0", 7, AV_CH_LAYOUT_7POINT0 }, 27 { "7.0(front)", 7, AV_CH_LAYOUT_7POINT0_FRONT }, 28 { "7.1", 8, AV_CH_LAYOUT_7POINT1 }, 29 { "7.1(wide)", 8, AV_CH_LAYOUT_7POINT1_WIDE }, 30 { "octagonal", 8, AV_CH_LAYOUT_OCTAGONAL }, 31 { "downmix", 2, AV_CH_LAYOUT_STEREO_DOWNMIX, }, 32 };

FFMPEG宏定义

Audio channel convenience macros

1 #define AV_CH_LAYOUT_MONO (AV_CH_FRONT_CENTER) 2 #define AV_CH_LAYOUT_STEREO (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT) 3 #define AV_CH_LAYOUT_2POINT1 (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY) 4 #define AV_CH_LAYOUT_2_1 (AV_CH_LAYOUT_STEREO|AV_CH_BACK_CENTER) 5 #define AV_CH_LAYOUT_SURROUND (AV_CH_LAYOUT_STEREO|AV_CH_FRONT_CENTER) 6 #define AV_CH_LAYOUT_3POINT1 (AV_CH_LAYOUT_SURROUND|AV_CH_LOW_FREQUENCY) 7 #define AV_CH_LAYOUT_4POINT0 (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_CENTER) 8 #define AV_CH_LAYOUT_4POINT1 (AV_CH_LAYOUT_4POINT0|AV_CH_LOW_FREQUENCY) 9 #define AV_CH_LAYOUT_2_2 (AV_CH_LAYOUT_STEREO|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT) 10 #define AV_CH_LAYOUT_QUAD (AV_CH_LAYOUT_STEREO|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) 11 #define AV_CH_LAYOUT_5POINT0 (AV_CH_LAYOUT_SURROUND|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT) 12 #define AV_CH_LAYOUT_5POINT1 (AV_CH_LAYOUT_5POINT0|AV_CH_LOW_FREQUENCY) 13 #define AV_CH_LAYOUT_5POINT0_BACK (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) 14 #define AV_CH_LAYOUT_5POINT1_BACK (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_LOW_FREQUENCY) 15 #define AV_CH_LAYOUT_6POINT0 (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_CENTER) 16 #define AV_CH_LAYOUT_6POINT0_FRONT (AV_CH_LAYOUT_2_2|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER) 17 #define AV_CH_LAYOUT_HEXAGONAL (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_BACK_CENTER) 18 #define AV_CH_LAYOUT_6POINT1 (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_CENTER) 19 #define AV_CH_LAYOUT_6POINT1_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_BACK_CENTER) 20 #define AV_CH_LAYOUT_6POINT1_FRONT (AV_CH_LAYOUT_6POINT0_FRONT|AV_CH_LOW_FREQUENCY) 21 #define AV_CH_LAYOUT_7POINT0 (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) 22 #define AV_CH_LAYOUT_7POINT0_FRONT (AV_CH_LAYOUT_5POINT0|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER) 23 #define AV_CH_LAYOUT_7POINT1 (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) 24

#define AV_CH_LAYOUT_7POINT1_WIDE      (AV_CH_LAYOUT_5POINT1|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)

25

#define AV_CH_LAYOUT_7POINT1_WIDE_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)

26 #define AV_CH_LAYOUT_OCTAGONAL (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_CENTER|AV_CH_BACK_RIGHT) 27 #define AV_CH_LAYOUT_STEREO_DOWNMIX (AV_CH_STEREO_LEFT|AV_CH_STEREO_RIGHT)

SDL宏定义

SDL_AudioSpec format

查看源代码打印帮助 1 AUDIO_U8 Unsigned 8-bit samples 2 AUDIO_S8 Signed 8-bit samples 3 AUDIO_U16LSB Unsigned 16-bit samples, in little-endian byte order 4 AUDIO_S16LSB Signed 16-bit samples, in little-endian byte order 5 AUDIO_U16MSB Unsigned 16-bit samples, in big-endian byte order 6 AUDIO_S16MSB Signed 16-bit samples, in big-endian byte order 7 AUDIO_U16 same as AUDIO_U16LSB (for backwards compatability probably) 8 AUDIO_S16 same as AUDIO_S16LSB (for backwards compatability probably) 9 AUDIO_U16SYS Unsigned 16-bit samples, in system byte order 10 AUDIO_S16SYS Signed 16-bit samples, in system byte order

git clone https://github.com/lnmcc/musicPlayer.git

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