Connecting a Simplex Radio to a Trunked Radio System

The Problem

Unlike simplex radio communications, trunked radio communications is not instantaneous. After a user keys his or her radio, the trunked system may take up to a few hundred milliseconds to grant the user access to a channel on the system.  This is referred to as “latency.”

Under normal conditions, after the user has keyed a radio, he or she will count “1-2-3”, then begin speaking, to prevent the loss of the initial transmission.

A solution

To compensate for this latency, without requiring the user wait before speaking, we have built an “audio buffer” into the ICRI. Out of all of the logic and audio circuitry that exist within the ICRI, the buffer is one of the most important (and sometimes most “mysterious”). The audio buffer delay allows a user to key their radio, and immediately being speaking, without losing those initial words, when transmitting over a trunked radio system.

ICRI - Audio Buffer Delay

ICRI – Audio Buffer Delay

The photograph to the left, depicts a 1 second delay. The yellow channel on the top of the screen, is the incoming audio from the handset port, while the blue channel on the bottom of the screen is the outgoing audio. There is a 1 second delay between the input audio and the output audio, as a result of the buffer.  If the trunked radio system had a 1 second latency, for example, then this audio would be dropped without a the audio buffer delay. 

Applications: Simplex to Trunked Radios

With an ICRI, a user can transmit from a simplex radio, through the ICRI, over a trunked system to dispatch or an EOC, and know that their entire transmission will reach the listener, without having to wait before speaking. This process occurs without the need for user input or adjustments. Many users are unaware, when the delay occurs.

At C-AT, when we know the ICRI will be used to connect a simplex radio to a trunked radio, we pre-set the audio buffer delay to at least 250ms.

Users in the Field

The audio buffer delay is adjustable in the field, via jumpers. A state agency recently adjusted the delay on over 20 units, increasing it to 450ms, after they switched to a new trunked system that has a longer latency.  They made this simple adjustment in the field, then redistributed the ICRIs to their personnel, knowing that no additional changes would need to be made for the ICRIs to operate with the new trunked system.

Additional Info

Some satellites can have a latency of up to 3 seconds. With an external cable, the ICRI can compensate for this delay, as well.


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