http://www.cnblogs.com/lidabo/p/3701074.html
FFMPEG + SDL音频播放分析
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
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
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
