FFMPEG + SDL音频播放分析
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目录 [hide]
- 1 抽象流程：
- 2 关键实现：
- 2.1 main()函数
- 2.2 decode_thread()读取文件信息和音频包
- 2.3 stream_component_open()：设置音频参数和打开设备
- 2.4 audio_callback(): 回调函数，向SDL缓冲区填充数据
- 2.5 audio_decode_frame()：解码音频
- 3 FFMPEG结构体
- 3.1 channel_layout_map
- 4 FFMPEG宏定义
- 4.1 Audio channel convenience macros
- 5 SDL宏定义
- 5.1 SDL_AudioSpec format
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
int main(int argc, char **argv){ SDL_Event event; //SDL事件变量 VideoState *is; // 纪录视频及解码器等信息的大结构体 is = (VideoState*) av_mallocz(sizeof(VideoState)); if(argc < 2){ fprintf(stderr, "Usage: play <file>\n"); exit(1); } av_register_all(); //注册所有ffmpeg的解码器 /* 初始化SDL，这里只实用了AUDIO，如果有视频，好需要SDL_INIT_VIDEO等等 */ if(SDL_Init(SDL_INIT_AUDIO)){ fprintf(stderr, "Count not initialize SDL - %s\n", SDL_GetError()); exit(1); } is_strlcpy(is->filename, argv[1], sizeof(is->filename)); /* 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 */ is->parse_tid = SDL_CreateThread(decode_thread, is); if(!is->parse_tid){ SDL_WaitEvent(&event); switch(event.type){ case FF_QUIT_EVENT: case SDL_QUIT: is->quit = 1; SDL_Quit(); exit(0); break; default: break; } } return 0;}decode_thread()读取文件信息和音频包
static int decode_thread(void *arg){ VideoState *is = (VideoState*)arg; AVFormatContext *ic = NULL; AVPacket pkt1, *packet = &pkt1; int ret, i, audio_index = -1; is->audioStream = -1; global_video_state = is; /* 使用ffmpeg打开视频，解码器等 常规工作 */ if(avFormat_open_input(&ic, is->filename, NULL, NULL) != 0) { fprintf(stderr, "open file error: %s\n", is->filename); return -1; } is->ic = ic; if(avformat_find_stream_info(ic, NULL) < 0){ fprintf(stderr, "find stream info error\n"); return -1; } av_dump_format(ic, 0, is->filename, 0); for(i = 0; i < ic->nb_streams; i++){ if(ic->streams[i])->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index == -1){ audio_index = i; break; } } if(audio_index >= 0) { /* 所有设置SDL音频流信息的步骤都在这个函数里完成 */ stream_component_open(is, audio_index); } if(is->audioStream < 0){ fprintf(stderr, "could not open codecs for file: %s\n", is->filename); goto fail; } /* 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）*/ for(;;){ if(is->quit) break; /* 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 */ if(is->audioq.size > MAX_AUDIO_SIZE){ SDL_Delay(10); // 毫秒 continue; } ret = av_read_frame(is->ic, packet); if(ret < 0){ if(ret == AVERROR_EOF || url_feof(is->ic->pb)) break; if(is->ic->pb && is->ic->pb->error) break; contiue; } if(packet->stream_index == is->audioStream){ packet_queue_put(&is->audioq, packet); } else{ av_free_packet(packet); } } while(!is->quit) SDL_Delay(100);fail: { SDL_Event event; event.type = FF_QUIT_EVENT; event.user.data1 = is; SDL_PushEvent(&event); } return 0;}stream_component_open()：设置音频参数和打开设备
int stream_component_open(videoState *is, int stream_index){ AVFormatContext *ic = is->ic; AVCodecContext *codecCtx; AVCodec *codec; /* 在用SDL_OpenAudio()打开音频设备的时候需要这两个参数*/ /* wanted_spec是我们期望设置的属性，spec是系统最终接受的参数 */ /* 我们需要检查系统接受的参数是否正确 */ SDL_AudioSpec wanted_spec, spec; int64_t wanted_channel_layout = 0; // 声道布局(SDL中的具体定义见“FFMPEG结构体”部分) int wanted_nb_channels; // 声道数 /* SDL支持的声道数为 1, 2, 4, 6 */ /* 后面我们会使用这个数组来纠正不支持的声道数目 */ const int next_nb_channels[] = { 0, 0, 1, 6, 2, 6, 4, 6 }; if(stream_index < 0 || stream_index >= ic->nb_streams) return -1; codecCtx = ic->streams[stream_index]->codec; wanted_nb_channels = codecCtx->channels; if(!wanted_channel_layout || wanted_nb_channels != av_get_channel_layout_nb_channels(wanted_channel_layout)) { wanted_channel_layout = av_get_default_channel_lauout(wanted_channel_nb_channels); wanted_channel_layout &= ~AV_CH_LAYOUT_STEREO_DOWNMIX; } wanted_spec.channels = av_get_channels_layout_nb_channels(wanted_channel_layout); wanted_spec.freq = codecCtx->sample_rate; if(wanted_spec.freq <= 0 || wanted_spec.channels <=0){ fprintf(stderr, "Invaild sample rate or channel count!\n"); return -1; } wanted_spec.format = AUDIO_S16SYS; // 具体含义请查看“SDL宏定义”部分 wanted_spec.silence = 0; // 0指示静音 wanted_spec.samples = SDL_AUDIO_BUFFER_SIZE; // 自定义SDL缓冲区大小 wanted_spec.callback = audio_callback; // 音频解码的关键回调函数 wanted_spec.userdata = is; // 传给上面回调函数的外带数据 /* 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 */ /* next_nb_channels数组指定）直到成功打开，或者全部失败 */ while(SDL_OpenAudio(&wanted_spec, &spec) < 0){ fprintf(stderr, "SDL_OpenAudio(%d channels): %s\n", wanted_spec.channels, SDL_GetError()); wanted_spec.channels = next_nb_channels[FFMIN(7, wanted_spec.channels)]; // FFMIN()由ffmpeg定义的宏，返回较小的数 if(!wanted_spec.channels){ fprintf(stderr, "No more channel to try\n"); return -1; } wanted_channel_layout = av_get_default_channel_layout(wanted_spec.channels); } /* 检查实际使用的配置（保存在spec,由SDL_OpenAudio()填充） */ if(spec.format != AUDIO_S16SYS){ fprintf(stderr, "SDL advised audio format %d is not supported\n", spec.format); return -1; } if(spec.channels != wanted_spec.channels) { wanted_channel_layout = av_get_default_channel_layout(spec.channels); if(!wanted_channel_layout){ fprintf(stderr, "SDL advised channel count %d is not support\n", spec.channels); return -1; } } /* 把设置好的参数保存到大结构中 */ is->audio_src_fmt = is->audio_tgt_fmt = AV_SAMPLE_FMT_S16; is->audio_src_freq = is->audio_tgt_freq = spec.freq; is->audio_src_channel_layout = is->audio_tgt_layout = wanted_channel_layout; is->audio_src_channels = is->audio_tat_channels = spec.channels; codec = avcodec_find_decoder(codecCtx>codec_id); if(!codec || (avcodec_open2(codecCtx, codec, NULL) < 0)){ fprintf(stderr, "Unsupported codec!\n"); return -1; } ic->streams[stream_index]->discard = AVDISCARD_DEFAULT; //具体含义请查看“FFMPEG宏定义”部分 is->audioStream = stream_index; is->audio_st = ic->streams[stream_index]; is->audio_buf_size = 0; is->audio_buf_index = 0; memset(&is->audio_pkt, 0, sizeof(is->audio_pkt)); packet_queue_init(&is->audioq); SDL_PauseAudio(0); // 开始播放静音}audio_callback(): 回调函数，向SDL缓冲区填充数据
void audio_callback(void *userdata, Uint8 *stream, int len){ VideoState *is = (VideoState*)userdata; int len1, audio_data_size; /* 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 */ while(len > 0){ /* 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，*/ /* 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*/ /* 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 /* 多的桢数据 */ if(is->audio_buf_index >= is->audio_buf_size){ audio_data_size = audio_decode_frame(is); /* audio_data_size < 0 标示没能解码出数据，我们默认播放静音 */ is(audio_data_size < 0){ is->audio_buf_size = 1024; /* 清零，静音 */ memset(is->audio_buf, 0, is->audio_buf_size); } else{ is->audio_buf_size = audio_data_size; } is->audio_buf_index = 0; } /* 查看stream可用空间，决定一次copy多少数据，剩下的下次继续copy */ len1 = is->audio_buf_size - is->audio_buf_index; if(len1 > len) len1 = len; memcpy(stream, (uint8_t*)is->audio_buf + is->audio_buf_index, len1); len -= len1; stream += len1; is->audio_buf_index += len1; }}audio_decode_frame()：解码音频
int audio_decode_frame(VideoState *is){ int len1, len2, decoded_data_size; AVPacket *pkt = &is->audio_pkt; int got_frame = 0; int64_t dec_channel_layout; int wanted_nb_samples, resampled_data_size; for(;;){ while(is->audio_pkt_size > 0){ if(!is->audio_frame){ if(!(is->audio_frame = avacodec_alloc_frame())){ return AVERROR(ENOMEM); } } else avcodec_get_frame_defaults(is->audio_frame); len1 = avcodec_decode_audio4(is->audio_st_codec, is->audio_frame, got_frame, pkt); /* 解码错误，跳过整个包 */ if(len1 < 0){ is->audio_pkt_size = 0; break; } is->audio_pkt_data += len1; is->audio_pkt_size -= len1; if(!got_frame) continue; /* 计算解码出来的桢需要的缓冲大小 */ decoded_data_size = av_samples_get_buffer_size(NULL, is->audio_frame_channels, is->audio_frame_nb_samples, is->audio_frame_format, 1); dec_channel_layout = (is->audio_frame->channel_layout && is->audio_frame->channels == av_get_channel_layout_nb_channels(is->audio_frame->channel_layout)) ? is->audio_frame->channel_layout : av_get_default_channel_layout(is->audio_frame->channels); wanted_nb_samples = is->audio_frame->nb_samples; if (is->audio_frame->format != is->audio_src_fmt || dec_channel_layout != is->audio_src_channel_layout || is->audio_frame->sample_rate != is->audio_src_freq || (wanted_nb_samples != is->audio_frame->nb_samples && !is->swr_ctx)) { if (is->swr_ctx) swr_free(&is->swr_ctx); is->swr_ctx = swr_alloc_set_opts(NULL, is->audio_tgt_channel_layout, is->audio_tgt_fmt, is->audio_tgt_freq, dec_channel_layout, is->audio_frame->format, is->audio_frame->sample_rate, 0, NULL); if (!is->swr_ctx || swr_init(is->swr_ctx) < 0) { fprintf(stderr, "swr_init() failed\n"); break; } is->audio_src_channel_layout = dec_channel_layout; is->audio_src_channels = is->audio_st->codec->channels; is->audio_src_freq = is->audio_st->codec->sample_rate; is->audio_src_fmt = is->audio_st->codec->sample_fmt; } /* 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 */ if (is->swr_ctx) { const uint8_t **in = (const uint8_t **)is->audio_frame->extended_data; uint8_t *out[] = { is->audio_buf2 }; if (wanted_nb_samples != is->audio_frame->nb_samples) { if(swr_set_compensation(is->swr_ctx, (wanted_nb_samples - is->audio_frame->nb_samples)*is->audio_tgt_freq/is->audio_frame->sample_rate, wanted_nb_samples * is->audio_tgt_freq/is->audio_frame->sample_rate) < 0) { fprintf(stderr, "swr_set_compensation() failed\n"); break; } } len2 = swr_convert(is->swr_ctx, out, sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt), in, is->audio_frame->nb_samples); if (len2 < 0) { fprintf(stderr, "swr_convert() failed\n"); break; } if(len2 == sizeof(is->audio_buf2)/is->audio_tgt_channels/av_get_bytes_per_sample(is->audio_tgt_fmt)) { fprintf(stderr, "warning: audio buffer is probably too small\n"); swr_init(is->swr_ctx); } is->audio_buf = is->audio_buf2; resampled_data_size = len2*is->audio_tgt_channels*av_get_bytes_per_sample(is->audio_tgt_fmt); } else { resampled_data_size = decoded_data_size; is->audio_buf = is->audio_frame->data[0]; } /* 返回得到的数据 */ return resampled_data_size; } if (pkt->data) av_free_packet(pkt); memset(pkt, 0, sizeof(*pkt)); if (is->quit) return -1; if (packet_queue_get(&is->audioq, pkt, 1) < 0) return -1; is->audio_pkt_data = pkt->data; is->audio_pkt_size = pkt->size; }}FFMPEG结构体
channel_layout_map
static const struct {const char *name;int nb_channels;uint64_t layout;} channel_layout_map[] = {{ "mono", 1, AV_CH_LAYOUT_MONO },{ "stereo", 2, AV_CH_LAYOUT_STEREO },{ "2.1", 3, AV_CH_LAYOUT_2POINT1 },{ "3.0", 3, AV_CH_LAYOUT_SURROUND },{ "3.0(back)", 3, AV_CH_LAYOUT_2_1 },{ "4.0", 4, AV_CH_LAYOUT_4POINT0 },{ "quad", 4, AV_CH_LAYOUT_QUAD },{ "quad(side)", 4, AV_CH_LAYOUT_2_2 },{ "3.1", 4, AV_CH_LAYOUT_3POINT1 },{ "5.0", 5, AV_CH_LAYOUT_5POINT0_BACK },{ "5.0(side)", 5, AV_CH_LAYOUT_5POINT0 },{ "4.1", 5, AV_CH_LAYOUT_4POINT1 },{ "5.1", 6, AV_CH_LAYOUT_5POINT1_BACK },{ "5.1(side)", 6, AV_CH_LAYOUT_5POINT1 },{ "6.0", 6, AV_CH_LAYOUT_6POINT0 },{ "6.0(front)", 6, AV_CH_LAYOUT_6POINT0_FRONT },{ "hexagonal", 6, AV_CH_LAYOUT_HEXAGONAL },{ "6.1", 7, AV_CH_LAYOUT_6POINT1 },{ "6.1", 7, AV_CH_LAYOUT_6POINT1_BACK },{ "6.1(front)", 7, AV_CH_LAYOUT_6POINT1_FRONT },{ "7.0", 7, AV_CH_LAYOUT_7POINT0 },{ "7.0(front)", 7, AV_CH_LAYOUT_7POINT0_FRONT },{ "7.1", 8, AV_CH_LAYOUT_7POINT1 },{ "7.1(wide)", 8, AV_CH_LAYOUT_7POINT1_WIDE },{ "octagonal", 8, AV_CH_LAYOUT_OCTAGONAL },{ "downmix", 2, AV_CH_LAYOUT_STEREO_DOWNMIX, },};FFMPEG宏定义
Audio channel convenience macros
#define AV_CH_LAYOUT_MONO (AV_CH_FRONT_CENTER) #define AV_CH_LAYOUT_STEREO (AV_CH_FRONT_LEFT|AV_CH_FRONT_RIGHT) #define AV_CH_LAYOUT_2POINT1 (AV_CH_LAYOUT_STEREO|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_2_1 (AV_CH_LAYOUT_STEREO|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_SURROUND (AV_CH_LAYOUT_STEREO|AV_CH_FRONT_CENTER) #define AV_CH_LAYOUT_3POINT1 (AV_CH_LAYOUT_SURROUND|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_4POINT0 (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_4POINT1 (AV_CH_LAYOUT_4POINT0|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_2_2 (AV_CH_LAYOUT_STEREO|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT) #define AV_CH_LAYOUT_QUAD (AV_CH_LAYOUT_STEREO|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) #define AV_CH_LAYOUT_5POINT0 (AV_CH_LAYOUT_SURROUND|AV_CH_SIDE_LEFT|AV_CH_SIDE_RIGHT) #define AV_CH_LAYOUT_5POINT1 (AV_CH_LAYOUT_5POINT0|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_5POINT0_BACK (AV_CH_LAYOUT_SURROUND|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) #define AV_CH_LAYOUT_5POINT1_BACK (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_6POINT0 (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_6POINT0_FRONT (AV_CH_LAYOUT_2_2|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER) #define AV_CH_LAYOUT_HEXAGONAL (AV_CH_LAYOUT_5POINT0_BACK|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_6POINT1 (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_6POINT1_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_BACK_CENTER) #define AV_CH_LAYOUT_6POINT1_FRONT (AV_CH_LAYOUT_6POINT0_FRONT|AV_CH_LOW_FREQUENCY) #define AV_CH_LAYOUT_7POINT0 (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT) #define AV_CH_LAYOUT_7POINT0_FRONT (AV_CH_LAYOUT_5POINT0|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER) #define AV_CH_LAYOUT_7POINT1 (AV_CH_LAYOUT_5POINT1|AV_CH_BACK_LEFT|AV_CH_BACK_RIGHT)#define AV_CH_LAYOUT_7POINT1_WIDE (AV_CH_LAYOUT_5POINT1|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)#define AV_CH_LAYOUT_7POINT1_WIDE_BACK (AV_CH_LAYOUT_5POINT1_BACK|AV_CH_FRONT_LEFT_OF_CENTER|AV_CH_FRONT_RIGHT_OF_CENTER)#define AV_CH_LAYOUT_OCTAGONAL (AV_CH_LAYOUT_5POINT0|AV_CH_BACK_LEFT|AV_CH_BACK_CENTER|AV_CH_BACK_RIGHT)#define AV_CH_LAYOUT_STEREO_DOWNMIX (AV_CH_STEREO_LEFT|AV_CH_STEREO_RIGHT)
SDL宏定义
SDL_AudioSpec format
AUDIO_U8 Unsigned 8-bit samplesAUDIO_S8 Signed 8-bit samplesAUDIO_U16LSB Unsigned 16-bit samples, in little-endian byte orderAUDIO_S16LSB Signed 16-bit samples, in little-endian byte orderAUDIO_U16MSB Unsigned 16-bit samples, in big-endian byte orderAUDIO_S16MSB Signed 16-bit samples, in big-endian byte orderAUDIO_U16 same as AUDIO_U16LSB (for backwards compatability probably)AUDIO_S16 same as AUDIO_S16LSB (for backwards compatability probably)AUDIO_U16SYS Unsigned 16-bit samples, in system byte orderAUDIO_S16SYS Signed 16-bit samples, in system byte order
git clone https://github.com/lnmcc/musicPlayer.git
转帖：http://lnmcc.net/2013/03/28/ffmpeg-sdl%E9%9F%B3%E9%A2%91%E6%92%AD%E6%94%BE%E5%88%86%E6%9E%90/
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