Where 1990 scanning is exactly the same in 2016 – RF Scanning 101

Because “Where 1990 scanning is exactly the same in 2016 – AND – where it’s completely different” was too long of a title.

I’ll start building off my previous post.  As a reminder, in 1990, your programmed RF Radio scanner looked like this:

Channel 1 – 460.025
Channel 2 – 460.050
Channel 3 – 460.075
Channel 4 – 460.100

Modern day radio towers are lower to the ground than 1990 radio towers.
Modern day radio towers are lower to the ground than 1990 radio towers.  There are also more of them.  1 radio tower in 1990 is usually split into 2 or 3 regional towers in modern day systems since the tx/rx of higher frequency radio waves require it.

Fast forward to 2016 where your receiving scanner (trunked under the hood) looks like this:

Frequency 1 – 460.025 (control channel; aka computer)
Frequency 2 – 460.050
Frequency 3 – 460.075
Frequency 4 – 460.100

Notice the addition of the computer, or control channel, in 2016.  Modern repeaters, known as digital infrastructure radio systems, are computer software driven.  To simplify the functionality, the computer (freq 1) is constantly listening to all computer programmed frequencies to see if someone wants to talk.  The computer knows who wants to talk by the pressing of the push-to-talk (PTT) button on the radio.

To better understand how the radio system functions, I’ll play out the following case scenario:

Freq 1:
Reminder, the computer is constantly waiting for someone to talk then temporarily assign them to one of the available freqs.  Freq 1 is listening on this system and is using freqs 2, 3, and 4 as available freqs.

Freq 2:
When Mayberry police officer 1 presses the talk button on her two-way radio, the control channel tells her portable radio to use the selected frequency (we’ll say freq 2 since nobody is using it).

Freq 3:
At the same moment freq 2 is assigned by the computer, Mayberry fireman 1 starts using his truck radio to call the dispatcher.  The computer searches then tells the fireman’s truck radio to use freq 3.

Freq 4:
Assume the officer on freq 2 is still talking but her partner is on a foot chase and presses a mayday emergency button on his portable radio.  The mayday signal (a HELP alert) is sent to the computer.  The computer is smart enough to override the police officer on freq 2 and tell the dispatcher that he (the officer now assigned to freq 4) needs help.  The computer is programmed to know the mayday call is a higher priority than regular communication so the dispatcher is now in direct communication with Mayberry officer 2 even though officer 1 was originally talking.

So this scenario is as follows before freq 4 in use:

pic credit: policemag.com
pic credit: policemag.com

Frequency 1 – 460.025 (control channel; aka computer)
Frequency 2 – 460.050 (Mayberry police officer 1)
Frequency 3 – 460.075 (Mayberry fireman 1)
Frequency 4 – 460.100 (available for use)

THEN, after Mayberry police offer 2 pushes the HELP emergency button:

Frequency 1 – 460.025 (control channel; aka computer)
Frequency 2 – 460.050 (Mayberry police officer 1)
Frequency 3 – 460.075 (Mayberry fireman 1)
Frequency 4 – 460.100 (Mayberry police officer 2 – HELP)

Isn’t this stuff great!

So we have learned that your scanner in 2016 can scan the exact same freqs as in 1990, but the modern scanner has morphed in functionality and is basically a mini-computer programmed through software to understand it should constantly listen to the radio tower computer (control channel).  The control channel computer does exactly what software programmers have told it to do, and your scanner is constantly listening to the control channel for the same reason – so the scanner will know what freq to choose and listen to at any moment.

Because this is all software driven, any changes to the system software may require you, the listener, to modify your scanner settings to match the system software.  You may also notice a delay in how fast your trunked scanner receives audio from the tower making it sound as if you joined the middle of a conversation.  That’s partly because scanners are searching wide ranges of freqs – they’re not tuned exactly to your local tower.  You can finely tune the programming of your local system since pre-programmed frequencies usually scan a full range (A – Z) rather than specific freqs (just G instead of A – Z).  But that’s getting a little too complex for this post since a site like RadioReference.com usually updates their frequencies by the time the listener realizes there is a problem.

Next RF Scanning 101 post > The control channel plays air traffic controller.