The impact of GOP and I frame intervals on image quality and bandwidth

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

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.