How can I ensure proper synchronization between audio and video when processing raw YUV video and PCM audio using the Zoom Meeting SDK on Linux? I am experiencing issues such as the YUV video playing faster than the audio, persistent audio/video drift, and challenges with maintaining sync when streaming media files or capturing raw data. What are the recommended strategies for handling timestamps, encoding, and muxing to achieve accurate A/V synchronization?

Answer

To achieve accurate audio and video synchronization when using the Zoom Meeting SDK on Linux, it is crucial to follow a set of best practices that address common issues related to timing, encoding, and muxing. Here’s a comprehensive guide:

Utilize SDK Timestamps:

Always rely on the timestamps provided by the SDK for both audio and video frames (e.g., AudioRawData::GetTimeStamp and the timestamps from raw data callbacks). This ensures that you are working with the correct timing information rather than relying on wall-clock time or the order of callback arrivals.

Real-Time Encoding and Muxing:

Instead of writing raw YUV and PCM data to disk and processing them later, encode and mux the audio and video streams in real time as they are received. This approach maintains the correct timing and prevents issues with playback speed and synchronization.
Use libraries like FFmpeg or GStreamer to handle the encoding and muxing. For example, you can set up a GStreamer pipeline that normalizes both audio and video streams and outputs them with synchronized timestamps.

Avoid Hardcoding Frame Rates:

If you need to specify a frame rate for the encoder, ensure it matches the actual delivery rate of the SDK callbacks. It is often better to let the encoder handle timing based on incoming timestamps rather than forcing a fixed frame rate.

Implement a Master Clock:

Choose a master clock for synchronization, typically using audio as the reference. Implement a jitter buffer (150–300 ms) to absorb variability in frame delivery before composition/playback.
Calculate target presentation timestamps (PTS) for video frames based on the master clock and desired output frame rate, adjusting by dropping or duplicating frames as necessary to maintain sync.

Handle Drift Explicitly:

If you notice drift between audio and video, consider resampling audio slightly to stay aligned with the master clock. Ensure that you are consistently writing PTS into your encoding pipeline based on SDK timestamps.

Streaming Media Files:

When streaming media files, build a GStreamer pipeline that demuxes and decodes the media, normalizes the video to a fixed size and format (e.g., I420 at 1280x720 @ 30fps), and ensures audio is in PCM format. Use appsink elements with sync enabled to maintain A/V synchronization.

Auto-Start Recording: - For automatic recording without host intervention, obtain a meeting's join token for local recording. This allows your application to start recording as soon as it joins the meeting.

By following these strategies, you can significantly improve the synchronization of audio and video streams when using the Zoom Meeting SDK on Linux.

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Written By:

Aydin Schwartz

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