The impact of GOP and I frame intervals on image quality and bandwidth
Explain the role of the GOP structure and I/P frames, and explain why video dragging and positioning are slow and the picture is occasionally mosaic related to this.
Frame type basics
- I frame (key frame): Completely records an entire picture, can be decoded independently, and has the largest volume.
- P frame (predicted frame): Only records the difference from the previous frame, small size, relies on previous frame decoding.
- B frame (bidirectional prediction frame): Refers to the previous and next frames. It has the highest compression rate but large delay. It is usually not used in real-time monitoring.
What is the GOP
GOP (Group of Pictures) is the interval between two I frames. For example, GOP=50 means there is an I frame every 50 frames. Approximately one keyframe every 2 seconds at 25fps.
Impact
| GOP length | Image quality / bit rate | Drag positioning | Error tolerance |
|---|---|---|---|
| Short (GOP=25) | Higher code rate, slightly better picture quality | Fast positioning | Fast recovery after packet loss |
| Medium (GOP=50) | Balanced | Fast | Better |
| Long (GOP=100+) | The lowest code rate | Drag obviously slows down | The screen is blurred for a long time after packet loss |
FAQ
- How many seconds does it take for the playback drag to appear?: The GOP is too long and the player needs to find the nearest I frame before it can start decoding.
- When the network jitters, the screen is mosaic: The I-frame interval is too long, and it takes a long time to recover after losing key frames.
- Save storage but poor playback experience: excessive pursuit of low bit rate lengthens the GOP, which is not worth the loss.
Setting suggestions
- Normal monitoring: GOP is set to 1~2 times the frame rate (GOP=25~50 at 25fps).
- Channels (cash registers, counters) that require detailed playback of evidence collection: The GOP should be smaller to ensure positioning and image quality.
- When smart encoding is enabled, the GOP will be dynamically adjusted by the algorithm and does not need to be set manually.
- Points with poor network conditions: shorten the GOP appropriately to improve the ability to resist packet loss.
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