The history of the police scanner clearly dates back to the early 1930s. The police would broadcast via a local AM radio station. Anyone with a radio could hear the calls. A conventional radio system is basically full of frequencies. Frequencies are the communication channels.
To listen to broadcasts, you must program certain channels into the scanner. The first type of police scanners were controlled by glass with about four channels. To listen, you would have to have a quartz crystal tuned to that frequency and connected to an internal slot. Each of those crystals would cost around $ 7.00 and would eventually become very expensive to maintain.
Then in the late 1970s, programmable scanners were invented. It had a keyboard with which you entered frequencies and the memory of the scanner stored all your frequencies. Then the trunked radio system came along and allowed you to track five to thirty channels or frequencies. A site controller is where users are assigned frequencies automatically. Like emergency departments would have one frequency or truckers would have another frequency. These guys are harder to monitor because they often switch channels even between conversations.
Today, it has digital link scanners. It is a computer that encodes a voice transmission into data streams and then the receiver will decode it so that an audible voice can be heard. They were first introduced in 2002 by Uniden and then in 2003, Radio Shack released their digital scanner. These are great to use because you can say things on the radio and not be heard, but only by those who can decode it, which would be a few people. The only people who can decode the codes are the people who are programmed for the channel.
It is good to have police scanners because they are something different from television and radio. You can sit in your backyard and understand why the police are coming up the street and you know it would be something to be alarmed about or something not to fry about. Police scanners are bought because a person belongs to one of the emergency departments or simply because of the interest of the community.
Often times, there are advertisements on a police scanner that will air first before television stations get the information. For example, you will first be notified from a police scanner about any landing tornadoes or if the weather becomes too severe before the news carries the information. This type of information is announced by the National Weather Service and is completely reliable. Police scanners can also be both entertaining and informative because of all the nonsense people will be saying over the air.