Trunked scanning method

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W8wer

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I’m curious how my BCD996XT performs scanning of a trunked system. The hardware specifications give a scanning speed of 100 channels per second in conventional mode. That means only 10 milliseconds for each channel. When scanning a trunked system, it seems to spend much more time than that on each site, even based on 10 milliseconds per frequency per site.

My guess is that it scans all the control channels at each site I programmed just as it would in conventional mode. If it hears any signal though, it has to pause and determine if the signal is from a talkgroup I have programmed. That makes the scan speed variable and dependant on the number of signals heard. On a system with many users, like Ohio MARCS, it might have to pause on each frequency at each site, so scanning speed could be comparatively slow.

Is any of this right?

Bill
 

UPMan

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Not quite. It only uses the control channel on a trunked system for "scanning." It sits on the single control channel frequency for about 1-2 seconds. It listens to the data on that control channel to see if any talk groups of interest are active, then jumps to the assigned frequency for any such group.

There is no "scanning speed" for a trunked system...since the scanner isn't actually scanning (switching between multiple frequencies) in order to catch channels.
 

W8wer

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Okay, but doesn't it step through each control frequency at each site? Isn't that "scanning" the control frequencies? Also, it appears it does not wait 1-2 seconds on control frequencies where it doesn't hear anything. I have a couple of sites programmed with the same number of control frequencies. One site is out of range of where I am now, and it spends only a fraction of a second on that site, but a couple of seconds on the site that is in range.
 

UPMan

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The very first time it hits a site, it will scan to the control channel (to the first one it can receive). I believe subsequent "scans" of the site it starts on the last-used control channel frequency and, as long as the control channel data is still present, does not scan any other frequency on that site.

If you are not in range of a site, then there is no control channel for the scanner to dwell on for that site (it only knows a frequency is a control channel because the data is there). Since control channels are continuous, if the scanner doesn't find control data in a single pass of programmed frequencies, it moves on to the next site.

The couple of seconds you see on sites that are in range are, for all practical purposes, spent entirely sitting on the control channel.
 
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W8wer

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There's what I didn't know, the control channel transmits continuously. I programmed the control channel for a nearby site as a conventional system, and I could hear the continuous digital data.

To take it a little farther then, I assume that after it processes (I'll use that instead of scans) the trunked system, it moves to the next system, whether trunked or conventional. I have one trunked system and several conventional systems programmed. It displays only the trunked sites while scanning but does receive the conventional systems when there is traffic.

It would also seem that the time it takes to process a trunked system could result in missed traffic on other systems even if there is no traffic on the trunked system.
 

UPMan

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Yep, that is the way it works. The scanner will dwell on the trunked control channel for the same amount of time, whether 0 channels are active or 19 channels are active (the max number of channels that can be active on a site is # site frequencies minus one).
 
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