What is a police scanner and how does it work?
A police scanner is a radio receiver that moves quickly between many frequencies, stopping whenever it finds a conversation. That is the whole idea: instead of sitting on one channel and waiting, it sweeps a list and lets you hear whichever one is active.
Why it is called scanning
Ordinary radios sit on one frequency. Public safety agencies use dozens: separate channels for dispatch, for units talking to each other, for fire ground operations, for events, for different districts. Listening to one of those means missing the rest.
A scanner cycles through a list of channels several times a second. When it detects activity it stops and plays the audio, then resumes cycling when the transmission ends. You hear a stitched together picture of everything happening across the whole system.
What you can hear
- Police dispatch. Calls being assigned, units responding, status checks. The most listened to traffic by far.
- Fire and EMS. Often more structured than police traffic, with clear dispatch tones and incident updates.
- Aviation. Control tower and approach frequencies at airports.
- Marine. Harbour and coastguard traffic.
- Rail. Dispatchers and crews on freight and passenger lines.
- Weather. Continuous forecast broadcasts and storm spotter nets.
Analogue, digital and trunked systems
Three technical differences decide how easy a system is to follow.
Analogue systems are the oldest and simplest. One frequency carries one conversation, and any receiver tuned to it hears the audio.
Digital systems send voice as data. The audio is clearer at the edge of coverage, but a receiver has to understand the specific format, commonly P25 in North America or DMR elsewhere.
Trunked systems are the biggest change. Rather than giving each department a permanent frequency, a trunked system holds a pool of frequencies and assigns one for each transmission, controlled by a data channel. Two consecutive replies in the same conversation can land on different frequencies. A scanner that does not follow the control channel hears disconnected fragments, which is why an old scanner sounds broken on a modern system even though nothing is wrong with it.
Hardware scanners versus listening online
A hardware scanner receives radio directly. It works without internet, hears everything within range, and needs programming with the right frequencies or system data for your area.
Listening online means someone else runs the receiver and streams the audio. You get access to any covered area in the world rather than only your own, no programming, and no equipment. The trade is a delay of a few seconds and dependence on that person keeping the feed running.
Our guide to listening online covers the practical differences.
Why some feeds sound poor
Scanner audio is usually low bitrate mono, often 16 kbps, because it is speech and volunteers pay for the bandwidth. Add radio noise, handset compression and background engine noise, and quiet transmissions can be hard to follow. Filtering tuned to the voice band helps more than turning the volume up, which is what the Clarity Boost feature in Police Scanner Pro does.
Is any of this private?
Traffic you can hear on a scanner is unencrypted radio, broadcast in the open. That is why it has always been listenable. What is changing is how much of it stays unencrypted, which is covered in why scanners are going silent.
Whether listening is legal where you are is a separate question, and it varies. See are police scanners legal.
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