issue #3993: fix av off sync for mp4 dvr.

trunk/src/kernel/srs_kernel_mp4.cpp

@@ -4820,6 +4820,10 @@ uint32_t SrsMp4Sample::pts_ms()
 
 SrsMp4SampleManager::SrsMp4SampleManager()
 {
+    video_start_dts_ = 0;
+    audio_start_dts_ = 0;
+    has_first_video_ = false;
+    has_first_audio_ = false;
 }
 
 SrsMp4SampleManager::~SrsMp4SampleManager()
@@ -4900,6 +4904,16 @@ SrsMp4Sample* SrsMp4SampleManager::at(uint32_t index)
 
 void SrsMp4SampleManager::append(SrsMp4Sample* sample)
 {
+    if (!has_first_audio_ && sample->type == SrsFrameTypeAudio) {
+        has_first_audio_ = true;
+        audio_start_dts_ = sample->dts;
+    }
+
+    if (!has_first_video_ && sample->type == SrsFrameTypeVideo) {
+        has_first_video_ = true;
+        video_start_dts_ = sample->dts;
+    }
+    
     samples.push_back(sample);
 }
 
@@ -5077,6 +5091,11 @@ srs_error_t SrsMp4SampleManager::write_track(SrsFrameType track,
             } else {
                 // The first sample always in the STTS table.
                 stts_entry.sample_count++;
+                if (track == SrsFrameTypeVideo) {
+                    stts_entry.sample_delta = video_start_dts_ > audio_start_dts_ ? video_start_dts_ - audio_start_dts_ : 0;
+                } else if (track == SrsFrameTypeAudio) {
+                    stts_entry.sample_delta = audio_start_dts_ > video_start_dts_ ? audio_start_dts_ - video_start_dts_ : 0;
+                }
             }
         }
         

trunk/src/kernel/srs_kernel_mp4.hpp

@@ -1914,6 +1914,11 @@ public:
 //      The keyframe is specified by stss.
 class SrsMp4SampleManager
 {
+private:
+    uint64_t video_start_dts_;
+    uint64_t audio_start_dts_;
+    bool has_first_video_;
+    bool has_first_audio_;
 public:
     std::vector<SrsMp4Sample*> samples;
 public: