如何同步视频到音频

同步视频到音频涉及3个方面：

1.计算音频时钟
2.计算视频时钟
3.据音频时钟纠正视频时钟

// 计算音频的已解码数据需要播放的时间
int audio_decode_frame(VideoState *is, uint8_t *audio_buf, int buf_size, double *pts_ptr)
{
......
/* if update, update the audio clock w/pts */
is->audio_clock += (double)data_size /bytes_per_sec;
......
}
// 当前音频时钟 = 音频的已解码数据需要播放的时间 - 音频缓存中剩余数据需要播放的时间
double get_audio_clock(VideoState *is) {
......
int hw_buf_size = is->audio_buf_size - is->audio_buf_index;
pts -= (double)hw_buf_size / bytes_per_sec;
......
}

void video_refresh_timer(void *userdata)
{

// 计算前后2帧的延时
delay = vp->pts - is->frame_last_pts; /* the pts from last time */

/* update delay to sync to audio */
ref_clock = get_audio_clock(is);

// 计算音频时钟和视频时钟的差值
diff = vp->pts - ref_clock;

/* Skip or repeat the frame. Take delay into account
FFPlay still doesn't "know if this is the best guess." */
sync_threshold = (delay > AV_SYNC_THRESHOLD) ? delay : AV_SYNC_THRESHOLD;
if(fabs(diff) < AV_NOSYNC_THRESHOLD)
{
// 据音频时钟纠正视频帧的延时, 太晚了则Skip, 太早了则加大延时
if(diff <= -sync_threshold)
{
delay = 0;
}
else if(diff >= sync_threshold)
{
delay = 2 * delay;
}
}

// 据系统时钟纠正视频帧的延时
is->frame_timer += delay;
/* computer the REAL delay */
actual_delay = is->frame_timer - (av_gettime() / 1000000.0);
if(actual_delay < 0.010)
{
/* Really it should skip the picture instead */
actual_delay = 0.010;
}

}