Scan All Channels?

[desert-cheetah]

I am thinking about getting a GRE-500 in the near future because most of my local PD agencies are digital and one is going digital next month. I'll still be able to get state police and a smaller agency one town over on my PRO-95, but I imagine it'll get old real fast. Anyway, I don't know a whole lot about how the digital scanning works and I've heard about talk groups and V-File so I'm wondering...would I still be able to scan all channels like I can with my Pro-95 or would it be more like the kind of scanner where you chose a 'bank' of channels and can only scan one bank at a time. Obviously, spending that kind of money I want to make sure the scanner can do what I want it to and scan all channels.

Thank you and have a great day!

 

[mikey60]

I am thinking about getting a GRE-500 in the near future because most of my local PD agencies are digital and one is going digital next month. I'll still be able to get state police and a smaller agency one town over on my PRO-95, but I imagine it'll get old real fast. Anyway, I don't know a whole lot about how the digital scanning works and I've heard about talk groups and V-File so I'm wondering...would I still be able to scan all channels like I can with my Pro-95 or would it be more like the kind of scanner where you chose a 'bank' of channels and can only scan one bank at a time. Obviously, spending that kind of money I want to make sure the scanner can do what I want it to and scan all channels.

Thank you and have a great day!

You can scan any and all channels at the same time. The PSR-500 uses a different method of grouping channels or "Objects" together called scan lists. Scanlists are similar to banks in how they are turned on and off, but unlike banks, a scan list can be only one channel, or as many as 1852 channels, depending on what you want to monitor.

The main difference between the bank scanners and the PSR-500's scan lists asside from the number of channels that can be placed in it, is that each channel (conventional Object, Talkgroup Object, Search Object, or Sweeper object) may be a member of any or all scan lists. This means that if you have a conventional channel that you currently have in multiple banks, you can just enter that channel once, and assign it to multiple scan lists.

Another benifit to this, is that since any scanable object may be a member of any scan list, you can mix talkgroup objects from different systems into the same scan list and scan all of them at the same time. I have a scan list on my radio when I travel that scans the MPSCS here in Michigan, along with a UHF EDACS system, and an 800 MHz EDACS system, plus some conventional stuff.

Mike

 

[desert-cheetah]

Awesome. Thanks, Mike! And thank you for explaining all that.

 

[jackj]

Another way of looking at this is to consider each Talk Group as a channel. The agency can have as many mobiles as it wants in a Talk Group, just like a conventional analog channel. When a mobile transmits then every other mobile in that talk group will hear it but units that are members of another talk group won't hear it.

Digital systems can be as simple as simplex analog systems with one base station and a few mobiles to as complex as the Ohio MARCS state-wide system with hundreds of towers, each with several transmitters, and tens of thousands of mobiles.

You decide what talk group(s) you want to listen to and program in only those groups. Your radio will hear only those groups you have programed in.

 

[RKG]

There is a difference between what you can do and what you should do.

Let us start by explaining a bit about how scanning works, and the difference between scanning conventional channels and scanning trunked talkgroups.

A radio scanning conventional channels steps through each of the channels in the scan list sequentially. It first has to change the frequency of the local oscillator, then park on the new freq for a length of time to determine whether or not there is someone transmitting RF on the freq, then if there is RF on the freq, the radio has to listen a while longer to see if it is sending the PL or DPL tone for which the scanner is programmed. If yes to both questions, the radio then parks on that channel for as long as the freq and tone are valid, plus a "delay" timer. If the freq and tone disappear long enough for the "delay" timer to expire, the radio then resumes the process, going through each channel in the list and then starting over again at the top.

What this says is that, yes you can program a scan list in a PSR-500/-600 that has 1,000 conventional channels in it, but the time between sequential peeks at any given channel is so long that you're likely to miss a lot of traffic on that channel.

In a Motorola trunked system, on the other hand, the radio starts by listening to the data channel. It is looking at out-going commands to shift to a voice channel in response to a channel grant request by a user (this is known as an OSW, for "outbound service word"). If your radio sees an OSW for a talkgroup you have programmed in your scan list, it will switch to the freq of the voice channel declared in the OSW and wait for traffic. The radio will remain there until something happens, the something being either the issuance of a "disconnect tone" or the disappearance of valid freq and tone plus the expiration of a "delay" timer -- depending upon how the system and how the scanner are programmed.

What this means is that you could program a PSR-500/-600 with a scan list of 500 talkgroups. If the system is idle, the length of the scan list will not cause you to miss traffic on the first OSW pointing to one of your scan list members, but if there is any more than de minimis traffic on the system, you will miss stuff.

Now let's consider the case where a scan list mixes both conventional channels and trunked talkgroups. The radio will sequentially sample the conventional channels, and then it will change to the control channel of the trunked system waiting for an OSW that matches the scan list. It will stay on the trunked control channel until a "no traffic" timer expires, and that timer is reset every time the radio sees a valid OSW. While the radio is listening to the trunked system control channel, it cannot hear or scan any of the conventional channels in the list.

The foregoing does not address how a "priority" function works, but my fingers are getting tired.

There are two unwritten "rules" that are widely followed by system designers and technicians in the public safety world. Rules #1: keep scan lists short. Rule #2: do not mix trunked talkgroups and conventional channels in the same scan list. Hopefully, you will begin to see the rational for these "rules."