Many Madison parents are unknowingly broadcasting private conversations over unsecured baby monitors.

NBC 15 found homes across the city using analog monitors that share the microphone over short-range FM radio signals. Anyone with a similar device or programmable radio receiver can listen to not just the baby, but phone calls and other sound throughout the home.

"Last night I heard a different voice. It's a very thick voice I heard him, it's not my kids," said Ngawang Gyaltsen, a Madison father whose baby we overheard using a cheap device from a second-hand store from down the street. In the same neighborhood we also overheard phone calls and adults chatting outside babies' bedrooms.

"We tell folks 'Don't use them, just throw them away,'" said Adam Vogt from J&K Security Solutions. The company installs high-end cameras and other security and automation systems. Vogt says eavesdroppers have been known to go "monitor driving" for unsuspecting homes, and he says the risks are real.

"They're talking on the phone to a credit card company and they're giving their social security number, address, birthday, you basically have all the tools to take over their identity."

The affected monitors are among the cheapest options at local big box stores like Target and Toys "R" Us. Both locations also offer digital models that are harder to hack.

Satara Home in Madison sells several of the more secure digital video devices, and staff are so confident in the encryption they even use one as their in-store security monitor.

To avert eavesdropping, avoid models broadcasting at 49 MHz, an indication usually included clearly on the packaging. We found many of these devices locally are sold by Safety 1st and Graco. Look instead for "digital" alternatives.