For instance, the recorder is 720P HD and supports 4 channels, which are channel 1, channel 2, channel 3, channel 4.
Channel 1 and channel 2 are a pair, so do channel 3 and channel 4.
1. If 2 ordinary analog cameras with different resolutions such as 1000TVL and 1200TVL are plugged into channel 1 and channel 2, they work.
2. If 2 HD cameras with different resolutions such as 720P and 960H are plugged into channel 1 and 2, only one can work and the other one can't.
3. If one ordinary analog camera such as 1000TVL and one HD camera 720P are plugged into channel 1 and 2, there is no doubt the 1000TVL camera won't work and 720P camera will work fine.
In conclusion, the 2 cameras plugged into a pair channel such as channel 1 and 2(or channel 3 and 4) should be in same resolution if they are HD cameras. If they are not HD cameras, it doesn't matter.