This article first appeared in the September 2001 issue of Monitoring Times.


As public safety agencies continue to field new digital trunked radio systems, many listeners question whether to purchase a new scanner soon or wait until a digital-capable unit becomes available.

I'll try to be succinct. I am in a quandary about making a communications receiver purchase, and I really need the advice of someone who has a broader and more knowledgeable view than I do about what might transpire over the next three years or so regarding 30+Mhz communications.

I bought a Radio Shack Pro-91 scanner - my first - for the bargain price of $170 back in 1999. It does a good job (it's easy pickings in the Phoenix metro area where I live), but I know I'm missing a lot that's out there; besides, I would love to have a handheld unit capable of pulling in everything: AM broadcast, FM, shortwave, air, etc. But these units (I have my eye on the new Alinco) cost a considerable amount of money. My worry is this: that the whole scanning arena is going to be put upside down by the advent of digital. For example, there is already talk of the Phoenix police department going digital. I would hate to spend a lot of money on a radio whose capabilities would be greatly diminished in a couple of years.

What do you think? Buy now or wait for digital? I heard that Uniden or Motorola or somebody is already working on just such a scanner.

Your help would be greatly appreciated.

Paul in Scottsdale, Arizona

Well, Paul, my crystal ball is no better than anyone else's, but I'll give you some background information that might help in your decision.

There are several different types of digital voice systems in use now in the United States, so let me summarize them in order of importance.

Project 25

The Association of Public Safety Communications Officials, International (APCO) Project 25 is by far the most popular digital system being fielded in the United States today. Previous Tracking the Trunks columns have detailed the specifics of Project 25 as they relate to scanner listeners.

Since most cities are upgrading older radio systems, the vast majority of Project 25 systems coming on-line use the old-style Motorola control channel rather than the fast channel described in the Project 25 standards. What this means is that current scanners that can trunk-track are able to follow Project 25 systems, although they can't decode the audio.

Project 25 uses a voice encoder called IMBE, or Improved Multi-Band Excitation. IMBE is patented by Digital Voice Systems, Inc. (DVSI) and requires a license agreement in order to legally use it.


A completely different system is being installed in the State of Pennsylvania, called OpenSky. This system uses the popular Internet Protocol (IP) and is rather unique in that all radio traffic is digital from one end of the system to the other. OpenSky uses a different DVSI vocoder known as AMBE (Advanced Multi-Band Excitation), and as you might expect it is not compatible with Project 25.

On the positive side, Pennsylvania has indicated the possibility of making available pre-programmed OpenSky scanners, although there's no firm information on availability or pricing.

Tyco International, the same company that purchased Com-Net/Ericsson in April, owns OpenSky. With both systems under one roof it will be interesting to see how many Ericsson systems eventually migrate to OpenSky. Already Tyco has announced that OpenSky would be installed in a number of cities in Palm Beach County, Florida for public safety use.


The IMBE vocoder is also used in Ericsson's ProVoice, but ProVoice is not compatible with Project 25, so a Project 25 scanner will not automatically be able to follow ProVoice systems.


Older Motorola ASTRO systems may use a different vocoder referred to as VSELP, which stands for Vector Sum Excited Linear Prediction. As with any non-IMBE vocoder, VSELP is not compatible with Project 25, so again a Project 25 scanner would not be able to decode the audio on an ASTRO VSELP system.


Ericsson has an old product called AEGIS that uses yet another type of vocoder, and of course it's not compatible with Project 25.

Digital Decoders

Regarding the rumors you've heard, there are at least two different Project 25 scanner products that have been announced.

In April, Uniden's product planning manager revealed that they were working on a scanner that can monitor Project 25 voice transmissions, and expected to be manufacturing it in a year or so.

A year ago in Boston at the annual APCO convention, ScannerMaster demonstrated a digital decoder board connected to a Bearcat 780XLT. Although Rich Barnett denies the decoder board was monitoring ASTRO communications, there is expectation in the scanner community that such a board will be available soon that will "drop in" to a 780XLT and allow Project 25 transmissions to be heard. At the Dayton Hamfest in May ScannerMaster was advertising a drop-in board to be available in 2001.

Given these two efforts, there is a good chance that by this time next year a Project 25 scanner will available to the general public. Even though new product development always takes longer than expected, I'd still save some money and wait a few months to see what actually becomes available.

I am not aware of any independent, public efforts to decode the other digital systems, so I would not expect widespread monitoring of OpenSky, ProVoice, VSELP or AEGIS. However, I'd love to hear from anyone working on decoding these systems!

Selecting a Scanner

Digital considerations aside, choosing a scanner is a very subjective decision and a scanner that's right for one person may be completely wrong for another.

Do you have a long commute, or spend a lot of time in your car? Perhaps an installed mobile unit is the way to go. There are no batteries to check and it's always available. Be careful in Michigan, though, since you'll need a permit and a permanent installation is easy to see. Other localities may also have restrictions on scanner use in a vehicle.

If you're like me, portability is important. A small, easy-to-carry package brings the scanner along wherever you go. A handheld scanner in a backpack or fanny pack with a small earpiece is a low-visibility way to monitor, despite the Secret Service-type look. If you'd rather fit in with a different crowd, use a Walkman-type pair of headphones with a stereo-to-mono mini-plug so observers will assume you have a CD player rather than a scanner.

Some scanners have the ability, or can be modified, to accept commands from an external frequency counter like the OptoElectronics Scout. This feature is referred to as "reaction tune" and is very handy when you're near a tower or other transmitter but you're not sure what frequency it is using.

Since this is a column about trunked radio I won't go on about the myriad of non-trunking features that might influence your decision, but some other areas to consider include instant weather information, priority scan channels, and the ability to interface the scanner to a personal computer or personal digital assistant (PDA).

Instead, I've included a table of existing trunking scanners below, along with their date of introduction, any systems they can track in addition to Motorola Type I and II, and the number of channels the scanner memory can hold.

TrunkTracking scanners

Uniden and Radio Shack are the two main suppliers of scanners that can track trunked radio systems. Each model listed, at a minimum, can track Motorola Type I and Type II systems.

Bearcat BC235XLT 1997 300
Bearcat BC245XLT 1999 EDACS 300
Radio Shack PRO-90 1997 300
Radio Shack PRO-91 1998 150
Radio Shack PRO-92 1999 EDACS, LTR 500
Radio Shack PRO-94 1999 EDACS 1000


PRO-2066 1998 150

PRO-2067 2000 EDACS, LTR 500


Bearcat BC780XLT 2000 EDACS, LTR 500
Bearcat BC895XLT 1997 300
Radio Shack PRO-2050 1997 300
Radio Shack PRO-2052 1999 EDACS 1000

Phoenix, Arizona

The last update I received indicated that the Phoenix police department is still using analog on conventional frequencies in the 150 and 450 MHz bands, and have not yet gone digital. There is a five-channel digital trunked test system operating from the Thompson Peak site in the McDowell Mountains north of Mesa, but I don't have any confirmed frequencies or talkgroups. This site is slated to eventually provide support for Phoenix and Mesa air operations in the East Valley.

Can any readers provide an update on the Phoenix digital system?

Sawyer County, Wisconsin

I am a reader of your Monitoring Times column on Tracking the Trunks. I was wondering if you could publish frequency lists and talkgroups and system types for my local area. I live in Sawyer County, Wisconsin, and information any trunked radio systems that are close by would be welcomed.


I checked the Federal Communications Commission (FCC) database at for trunked radio systems operating in Sawyer County, Wisconsin. There are several frequencies assigned to Nextel, however you will not be able to hear any voice traffic on these frequencies since the transmissions are in digital format.

The only other entry for trunked radio was a license holder listed as "Air Communications of Wisconsin'' with the following frequencies: 451.9875, 452.2125, 452.3875, 452.7375 and 453.0125 MHz. The FCC database also indicates that the tower for these frequencies is located near the corner of Highway B and Tower Road. I'd be interested in learning what you or other readers might find out about this system.

Arkansas State Police


The Arkansas State Police is using the following voice frequencies: 856.3125, 856.4125, 856.4625, 856.7625, 856.8125, 856.8875, 856.9375, 856.9875, 857.3125, 857.4125, 857.4625, 857.7625, 857.8125, 857.8875, 857.9375, 857.9875, 858.3125, 858.4125, 858.4625, 858.7625, 858.8125, 858.8875, 858.9375, 858.9875, 859.3125, 859.4125, 859.4625, 859.7625, 859.8125, 859.8875, 859.9375 and 859.9875 MHz.

Data channels are usually found on 860.3125, 860.4125, 860.4625, 860.7625, 860.8125, 860.8875, 860.9375 and 860.9875 MHz.

The 32 voice channels listed are re-used throughout the state. The state is split in 12 troops. The number of repeaters in each troop depends on its size. Some troops only have three repeaters, some have all eight, and most troops only have four or five repeaters in the troop. Each repeater is able to carry four different conversations at the same time, thanks to the four voice channels. Motorola Type I signaling is being switched over to type II signaling. I don't know how that change will affect the fleet maps.


Can anyone confirm that the Arkansas Highway Police is still using 150.995 MHz?

PRO 91 Update

Apparently there are now two versions of the Radio Shack PRO-91 trunked scanner. The new model, number 20-521A, has added a "Disconnect Tone Detect" feature, which will cause the scanner to automatically return to the data channel when a disconnect tone is received. Motorola trunked systems generate a sub-audible disconnect tone when a transmission is complete, but interference may cause the scanner to either miss the disconnect tone or to falsely believe that a disconnect tone has been sent.

From the updated manual:

Disconnect Tone Detect - The scanner automatically tunes to the trunking data channel when it receives a disconnect transmission. You can turn this off, to let you continuously monitor a channel with a weak transmission where conversations are often disconnected.

Most other trunk tracking scanners have this feature as well.

That's all for this month. More information is available on my website at, and I welcome your electronic mail sent to Until next month, happy monitoring!

Comments to Dan Veeneman

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