The cheap versions will be analog devices and you will get a lot of interference on them. All the cameras use the 2.4 GHZ frequency, the same as your phone, garage door openers, door automatic openers in stores, etc. Analog is cheaper.
The best and more pricey version is digital which locks the signal from the camera to the monitor so you will not have interference.
When I bought mine analog was the only choice. I am considering changing mine out to digital.
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