IFrame

An IFrame is a complete image frame (known as an Intra Frame) in MPEG encoding that is coded without reference to other pictures. Compression is achieved by comparing frames following an IFrame and only sending the changes until the next IFrame is generated. The IFrame is the frame that is used as the primary reference point that is compared to the other frames in the stream. This method allows for frames that are much smaller in size because they are only pulling the changes in reference to the original IFrame.

It is commonly understood of H.264 compression that it only performs optimally at 20 - 30IPS because it will generate a key frame either at a specific number like one every 30 images (making the image distorted if transmitting/recording at a slower rate) or if there is significant change from one image to the next (making the file stream/image larger).

Most manufacturers today will allow you control the I frame interval (it's called different terms in the config but usually it's there). By default, the most that is used is 1 IFrame every 30 images. You can lower this down and there are some tradeoffs in doing so.

IFrames are large frames (approximately the same size as a MJPEG image). Its the p and b frames (when used), that are smaller and reduce total bandwidth consumption.

If you are streaming at only 5 fps and still have 1 IFrame per second, then the bandwidth savings will be significantly less than a 30fps stream. In this case lower frames is less optimal (with regards to bandwidth).

You should be able to control the IFrame interval independent of image rate on many cameras. On many cameras you can adjust the Inter-Frame Interval (IFI) independent of image rate independent of video quality (compression rate).

Click here to learn more about Compression for CCTV

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