Baby monitors may have a visible signal as well as repeating the sound. This is often in the form of a set of lights to indicate the noise level, allowing the device to be used when it is inappropriate or impractical for the receiver to play the sound. Other monitors have a vibrating alert on the receiver making it particularly useful for people with hearing difficulties.
The features we focused on were those we thought either increased the performance of the monitor or made it more user-friendly for parents and increased the odds of getting good quality sleep. We looked for monitors that have sound activation that keeps the parent unit quiet when the baby isn't crying, so parents can potentially fall asleep faster because they don't have to listen to white noise. Some of the monitors were so loud, even at low volumes, that the white noise might keep light sleeping parents awake; this defeats the purpose of having a video product to begin with. We also liked the models with screens that automatically "wake" and/or go to sleep.