Two-way radio voice transmissions have a very important characteristic that makes trunked radio work so well. The average transmission is less than 10 seconds long. With this short transmission time, the trunked system controller can almost always find a frequency that is open for a transmission.

Figure 4 represents the individual transmissions on a typical small trunked radio system. The top line represents the trunked control channel. The other lines represent available voice channels that are assigned by the trunked system controller. The colored blocks on the voice channels represent several types of transmissions; six different talk groups, a private conversation , and a telephone interconnect. Notice that most transmissions are short and that the trunked system controller distributes the individual radio transmissions among the available frequencies. The exception is the telephone interconnect which ties up a radio channel for the entire duration of the call.

The disadvantage

Trunked radio systems have one major disadvantage—they cannot be monitored and recorded very easily.
During normal operation, each individual radio receives and de-trunks the transmissions in his talk group. 

The method is for each individual radio to assignment instructions of the trunked system controller and "follow" the transmissions as they move from one frequency to another. 

While it is possible to monitor and record the individual radio channels on a multi-track or multi channel recorder, the only thing you will capture are the individual transmissions that are portions of many separate conversations. It should be obvious that it would take considerable effort to assemble the scattered transmissions into a meaningful reconstruction of a conversation. Fortunately, there is a solution, and it can be found in three different ways: de-trunking prior to recording; de-trunking after recording using an adapted multi-channel recorder; and de-trunking after recording using a digital recording system for trunked radio.

Solutions

There are three ways to record a trunked radio system. 

1. De-trunk prior to recording—multi-channel This method uses available multi-track or multi-channel recorders and a de-trunking device or trunked radio for each talk group to be recorded. The de-trunking devices or trunked radios follow the conversations in a pre-set talk group and the subsequent audio is recorded on a designated track of the multi-track recorder.

But, de-trunking prior to recording is inefficient because very little of what is recorded is ever recovered (estimated to average less than 1/2 of 1per cent); the recorder typically records everything, including silence; a trunked radio (approximately $2000 each) is required for each talk group; and dedicated track or channel is required for each talk group. (Several recorders may be required).

2. De-trunk after recording using an adapted multi -channel recorder (patent pending). This method involves using a standard "off-the-shelf" multi-track recorder and a proprietary "pre processor" and "post processor."

Conventional audio from either standard (non-trunked) radio receivers or raw audio from other audio sources are connected to the standard multi-track audio recorder so that each discrete radio channel is connected to a track of the audio recorder.

A pre-processing device monitors the trunked radio control channel, interprets the channel instructions, and outputs playback control instructions in a modem-like digital format to the audio recorder. The pre-processor output is recorded on several tracks (for redundancy) that parallel the audio recordings of the various radio channels.