Fixes#3993 - WebRTC streams recorded to MP4 via DVR exhibit audio/video
synchronization issues, with audio typically ahead of video. **Note:
This issue is specific to MP4 format; FLV recordings are not affected.**
When WebRTC streams are converted to RTMP and then muxed to MP4, the
audio and video tracks may start at different timestamps. The MP4 muxer
was not accounting for this timing offset between the first audio and
video samples in the STTS (Sample Time-to-Sample) table, causing the
tracks to be misaligned in the final MP4 file.
Introduces `SrsMp4DvrJitter` class specifically for MP4 audio/video
synchronization:
- **Timestamp Tracking**: Records the DTS of the first audio and video
samples
- **Offset Calculation**: Computes the timing difference between track
start times
- **MP4 STTS Correction**: Sets appropriate `sample_delta` values in the
MP4 STTS table to maintain proper A/V sync
- Added `SrsMp4DvrJitter` class in `srs_kernel_mp4.hpp/cpp`
- Integrated jitter correction into `SrsMp4SampleManager::write_track()`
for MP4 format only
- Added comprehensive unit tests covering various timing scenarios
- **Scope**: Changes are isolated to MP4 kernel code and do not affect
FLV processing
This fix ensures that MP4 DVR recordings from WebRTC streams maintain
proper audio/video synchronization regardless of the relative timing of
the first audio and video frames, while leaving FLV format processing
unchanged.
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Co-authored-by: Haibo Chen <495810242@qq.com>
Co-authored-by: john <hondaxiao@tencent.com>
Co-authored-by: winlin <winlinvip@gmail.com>
Co-authored-by: OSSRS-AI <winlinam@gmail.com>
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