A baby monitor, also known as a baby alarm, is a radio system used to remotely listen to sounds made by an infant. An audio monitor consists of a transmitter unit, equipped with a microphone, placed near to the child. It transmits the sounds by radio waves to a receiver unit with a speaker carried by, or near to, the person caring for the infant. Some baby monitors provide two-way communication which allows the parent to speak back to the baby (parent talk-back). Some allow music to be played to the child. A monitor with a video camera and receiver is often called a baby cam.