Hope this points you in the right direction. If you start at the beginning it will lead you to trunking.
Radio 101
AM - Amplitude Modulation- Picture a line graph with a spike in the middle. The voice to be sent causes the transmitter’s signal strength to rapidly shift up and down to send the information. For Scanners, AM is limited to CB radio (27MHz) and aircraft (108-136 MHz)
FM - Frequency Modulation- Picture the same graph. The voice or data shifts the transmitter’s frequency slightly left and right (lower and higher in frequency) to send the information. Most of the scanner world is FM. Wide and Narrow refers in part to how far the transmitter shifts the actual freqiuency away from the published center frequency.
Analog – in very simple terms, the microphone directly controls the modulation of the transmitter (AM or FM from above). All scanners work on analog.
Digital – in very simple terms, a program (codec) listens to the microphone and converts the voice to digital 1’s and 0’s. The transmitter sends the 1’s and 0’s by shift-hold-shift-holding the freqnency. Only digital receivers can detect the 1’s and 0’s and reassemble them back into a voice.
Apco P25 is the common public safety format. Digital formats also include EDACS, Trbo, NXDN, Tetra, Open Sky, and many others. Different digital formats are incompatible. Some formats can not be programmed into scanners.
Encryption - Scramble the voice or digital bits so scanners can’t decode the voice. Encryption varies from 'easy to bust' to 'forget about it'.
Simplex - My radio transmits directly to your radio. Your radio transmits directly to mine. We both transmit on and listen to the same frequency. If we are too far apart or there are hills, mountains or buildings in the way we can’t talk. Simplex can be analog or digital. Program the RR first column "Frequency" into your scanner. The second "Input" column will be blank. If a mobile transmitter is far away or blocked by things, a scanner won’t pick it up.
Repeater - Put a receiver on something tall. The tall receiver can then hear the mobiles over a wide area when they transmit on the uplink frequency. Hook a big transmitter to the tall receiver and have it simultaneously transmit everything it hears back out over a wide area, using a different downlink frequency.
Obstructions and distance between mobiles no longer matter. If the repeater can hear both of us and we can both hear the repeater, we can talk. Sometimes the downlink frequency will also be used simplex.
Repeaters can be analog or digital. Program the repeater downlink [RR first column "Frequency"] into your scanner. Normally the mobile uplink 'Input' frequency is useless for you. Usually, there will only be one agency or small town on a particular repeater.
Trunking - A system with 3 or more interlinked repeaters, possibly hundreds in multi-site systems. The small number of repeaters is shared by dozens to thousands of users and agencies. An automatic 'trunking controller' acts as traffic cop and controls all radios in the trunking system.
Instead of having a full-time dedicated “Apple Sheriff’s Repeater”, there is a virtual trunking channel called “Apple Sheriff’s Talkgroup”, or "TG". The mobile radios used on the system are allowed to only access specific talkgroups, and not access other talkgroups. When not actively on a temporarily assigned frequency using a talkgroup, receivers monitor the control channel for trunking controller instructions.
A college or casino may have a dozen talkgroups and 2 or 3 repeaters at a single site. A county trunking system can have hundreds of talkgroups, 6 repeaters at one site and 4 more at another site. Statewide multi-site systems can have thousands of talkgroups, and many interconnected sites with 6 to 8 repeaters each.
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Sharing works because everyone doesn’t talk at once. Sounds complicated, but it works very well.
Scanning trunked systems - Older trunking systems are analog. Most new systems are digital. For most trunking scanners, the actual voice frequencies are not programmed into a scanner. Instead, most scanners only need to be programmed to the type of system and the trunking system’s “Control Channel”. The RR database shows these in RED. Locate your local sites and try them all. Different scanners require different setup.
Just like the 'real' radios on the street, your scanner will receive instructions sent on the control channel by the trunking controller to automatically change to a voice frequency to follow a talkgroup. You can program your scanner to follow a list of allowed talkgroups (Apple PD TG, Fire TG and EMS TG) and ignore others (Dog Catcher TG, Park Mowers TG, Meter Readers TG). Or you can set it to listen for and follow all talkgroups.
On multi-site systems, only talkgroups with active (registersd) local users will be heard. Talkgroups that don't have registered users nearby will not.
