Codecs like H.264 reduce bandwidth by only sending full frames every so often, mixing them with partial frames only capturing changes in between the full ones. They are called 'I' frames because they are the initial / full frames, followed by 'P', or predictive frames.*
Note: if you are not familiar with codecs, please read our Surveillance CODEC Guide before continuing.
I Frame Questions
Since I frames require much more bandwidth than P frames (frequently 10 or 20x more), some will argue that reducing the rate of I frames will reduce overall bandwidth significantly. For instance, instead of having an I frame each second, reduce it to 1 every 5 seconds.
On the other hand, some will argue that reducing I frames can result in quality problems because it can be harder for the processor to continue to faithfully update and represent the image if it has changed significantly since the last I frame.
We seek to answer these two questions:
- How much bandwidth savings can you achieve by reducing the I frame interval?
- How much quality degradation can occur by reducing the I frame interval?
The Tests Conducted
In order to answer these questions, we used five 720p cameras at various price points and performance levels:
- Avigilon H3 1MP
- Axis M1114
- Axis Q1604
- Bosch NBN-733V
- Dahua HF3101
We aimed these cameras at a toy train set to create consistent motion, and varied I-frame levels from a default of one per second to as high as five and as low as one every four seconds.
*Some versions of H.264 also support 'B' or bidirectionally predictive frames, but these are less common in surveillance cameras and therefore excluded from this study.