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);
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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;
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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);
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return -1;
13
}
14
is->ic = ic;
15
if(avformat_find_stream_info(ic, NULL) < 0){
16
fprintf(stderr, "find stream info error\n");
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return -1;
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}
19
av_dump_format(ic, 0, is->filename, 0);
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for(i = 0; i < ic->nb_streams; i++){
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if(ic->streams[i])->codec->codec_type == AVMEDIA_TYPE_AUDIO && audio_index == -1){
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audio_index = i;
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break;
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}
25
}
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if(audio_index >= 0) {
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/* 所有设置SDL音频流信息的步骤都在这个函数里完成 */
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stream_component_open(is, audio_index);
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}
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if(is->audioStream < 0){
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fprintf(stderr, "could not open codecs for file: %s\n", is->filename);
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goto fail;
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}
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;
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/* 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 */
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if(is->audioq.size > MAX_AUDIO_SIZE){
39
SDL_Delay(10); // 毫秒
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continue;
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}
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ret = av_read_frame(is->ic, packet);
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if(ret < 0){
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if(ret == AVERROR_EOF || url_feof(is->ic->pb)) break;
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if(is->ic->pb && is->ic->pb->error) break;
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contiue;
47
}
48
if(packet->stream_index == is->audioStream){
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packet_queue_put(&is->audioq, packet);
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} else{
51
av_free_packet(packet);
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}
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}
54
while(!is->quit) SDL_Delay(100);
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fail: {
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SDL_Event event;
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event.type = FF_QUIT_EVENT;
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event.user.data1 = is;
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SDL_PushEvent(&event);
60
}
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return 0;
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}
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);
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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);
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/* 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 */
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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;
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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
