Despite all the silence you hear while listening to police and fire department radio systems, the public safety spectrum in many locations is quite full. While efforts are underway to allocate new frequency bands, the Federal Communications Commission (FCC) has also been pushing to fit more users in the same amount of space. Their basic plan is to slice up the existing channels into smaller pieces and require users to operate within those pieces. This is a difficult requirement for the older analog radios, but the new digital systems -- including APCO Project 25 -- are prepared for this eventuality.
The original APCO (Association of Public-Safety Communications Officials) Project 25 specifications, now a decade old, spelled out two phases for radio operation. Phase I operates in channels that are 12.5 kHz wide, which is the current FCC requirement. However, because the FCC wants to pack more users into the limited spectrum available, they'd like to eventually have everyone using radio channels that are 6.25 kHz wide, or half as much as before. To meet this requirement, Project 25 defined Phase II to operate within 6.25 kHz wide channels.
APCO-25 uses a modulation process called QPSK-c, which stands for Quadrature Phase Shift Keying, continuous. Modulation is just a fancy word for the process of carrying information content over some kind of carrier signal. The transmitter superimposes the information onto the carrier, and the receiver removes the carrier and reproduces the information. Everyday AM (Amplitude Modulation) and FM (Frequency Modulation) radios tune to a carrier signal and retrieve the audio information sent by the radio station.
For Phase I systems, the version of QPSK-c used is called Compatible 4-Level Frequency Modulation (C4FM). When the basic Project 25 specifications were being worked out a decade ago, C4FM was chosen primarily because it provides relatively good efficiency without requiring equipment manufacturers to produce complex and expensive radios. C4FM is designed to operate within a radio channel that is 12.5 kHz wide.
Under the Phase II plan of Project 25, another type of QPSK-c modulation called CQPSK (Compatible Quadrature Phase Shift Keying) is used. It's not all that different from C4FM, but requires a different transmitter and a little more work on the receive side to make things function correctly. The advantage is that it takes up less bandwidth than C4FM, allowing two users to fit where only one did before.
Because C4FM and CQPSK are so similar, the intent was that same basic receiver hardware could properly handle both Phase I (12.5 kHz) and Phase II (6.25 kHz) channels. This means that with modern digital signal processing (DSP) technology, the same scanner hardware should be able to handle both types of modulation.
As the new APCO-25 scanners make their way into hobbyist's hands, there are reports that they don't work correctly while monitoring some simulcast systems. (Simulcast just means that the same information is transmitted from more than one repeater at the same time, allowing users across a wide geographic area to all hear the same messages.) The symptoms are mainly the inability hear an entire transmission. The first second or two of voice is heard, which then trails off to silence.
To add to your list of acronyms, there is another type of modulation scheme that's used with some multi-site Project 25 systems. Linear Simulcast Modulation (LSM) is a trademarked term for a form of CQPSK that provides a way for receivers to properly handle multiple identical transmissions. It's just different enough that the regular C4FM processing doesn't work correctly.
Because no Phase II systems are currently in operation, the developers at Uniden and GRE didn't expect an immediate need to handle this type of modulation. However, several municipalities are using LSM/CQPSK modulation for simulcast, including Phoenix and Mesa in Arizona; the greater Twin Cities area of Minnesota; Hamilton County in southwest Ohio; and Austin and Travis County in Texas.
If you live near any of these areas, or monitor systems that are "pure" APCO-25 with simulcasting, you may want to wait until Radio Shack has an upgrade for the PRO-96.
As described in the August Tracking the Trunks column, the Radio Shack PRO-96 (built by GRE) was designed to accept updates to the "DSP Application" portion of the scanner through the use of flash upgrades. This kind of flexibility allows production problems and bugs to be corrected without the need to buy a new scanner or replace circuit boards. In this case, an upgrade can also add new features and capabilities.
Radio Shack is expected to provide a firmware update that will give the scanner the ability to process LSM transmissions. As of this writing there's no release date for such an update, or whether there will be a cost involved. There may be a way to have the upgrade done at your local Radio Shack store, or to download the upgrade from Radio Shack's web site. We'll keep you posted as we get more details.
No word yet from Uniden on a fix for their scanners, although they have a new pair of scanners in the works to compete with the PRO-96.
New Uniden Scanner
There's a rule of thumb in the software business: "Never buy revision 1.0," meaning don't buy a product when it's first produced, since it's likely to have bugs. It takes time to iron out bugs and integrate new features. This happened to the Pontiac Fiero in the 1980's, which was famous for production problems early in its life. On the other hand, if no one ever bought revision 1.0 there would never be revision 1.1.
In any case, Uniden is circulating pre-release information about a pair of enhanced scanners to correct some of the shortcomings of the current 250D and 785D digital scanners. The new 296D (handheld) and 796D (base/mobile) scanners are very similar to the 250D and 785D, but will have the ability to track digital trunked systems that use a 9600-baud control channel. Also, a digital decoder card will be included -- current scanners require the purchase of a separate card (the BCi25D) in order to handle APCO-25 systems.
So far there is no exact release date, although it's expected to be available in early 2004. Price is rumored to be around a thousand dollars, although no official list price has been forthcoming.
Rapides Parish, Louisiana
I was curious as to whether the Pro-96 will be able to receive the city of Cleveland digital radio system, which is not APCO-25 compliant. Any information would be appreciated. Thank You.
Like Memphis, Tennessee, the city of Cleveland uses Motorola digital radios -- but they do not follow the APCO Project 25 standard. The digital voice is done through an older vocoder (voice encoder/decoder) called VSELP (Vector Sum Excited Linear Prediction) instead of the IMBE (Improved Multi-Band Excitation) vocoder specified in the APCO-25 standard. Since there is currently no consumer scanner that can process VSELP, you won't be able to hear digital transmissions on the PRO-96.
Cleveland's system uses the following frequencies: 851.0125, 851.1375, 851.1875, 851.2375, 851.2875, 851.3375, 852.0125, 852.1375, 852.1875, 852.2375, 852.2875, 852.3375, 853.0125, 853.1375, 853.1875, 853.2375, 853.2875, 853.3375, 854.1375, 854.1875, 854.2375, 854.2875, 854.3375, 855.1375, 855.1875, 855.2375, 855.2875 and 855.3375 MHz.
Some Cleveland talkgroups:
48 003 Citywide 1
80 005 Citywide 2
144 009 Public Safety Common
208 00D Fireground Ops 1
240 00F Fireground Ops 2
272 011 Fireground Ops 3
304 013 Fireground Ops 4
336 015 Fireground Ops 5
368 017 Fireground Ops 6
400 019 Fire Prevention Bureau
592 025 Fire Alerts
688 026 Fire Dispatch
3472 0D9 Airport Fire/Rescue 1
3504 0DB Airport Fire/Rescue 2
3536 0DD Airport Fire/Rescue 3
4976 137 Police
5104 13F Police 1st District
5136 141 Police 2nd District
5168 143 Police 3rd District
5200 145 Police 4th District
5232 147 Police 5th District
5264 149 Police 6th District
Even though your scanner won't work with the VSELP transmissions, if you have a sufficiently fast connection to the Internet you can listen to a live "web feed" by following the links at http://www.cleveland.com/policescanner/.
Pelham, New Hampshire
Pelham is a town of about 11,000 people in Hillsborough County, on the Southern edge of New Hampshire just across the border from Massachusetts. For many years Pelham has used VHF frequencies for public safety; police transmission on 154.770 MHz from a tower on Marsh Road and fire dispatches on 158.745 MHz from Jeremy Hill Road.
As far as my records go, Pelham continues to use these frequencies. However, the nearby city of Nashua, New Hampshire is using a Motorola ASTRO system with analog and digital traffic on the following frequencies: 866.0500, 866.6000, 866.7750, 866.9750, 867.3625, 867.5500, 867.7500, 868.2625, 868.4500 and 868.5125 MHz.
If any readers have more information about the Pelham radio system, please drop me a line!
That's all I have room for this month. Please e-mail questions, comments, and frequency lists to email@example.com, and you can find more information on my web site at www.signalharbor.com. Until next month, happy monitoring!
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