BNSFrailfan said:
Just not to long ago I bought myself a new Radio Shack Digital Scanner to monitor the Washington county law enforcement and I just don't monitor it anymore. Thier system is constantly breaking up. Changeing channels. I monitor it and then over a sudden the channel just breaks up big time. It's not my scanner. It's like the channel just goes off frequency all together. The DG goes to FM and so fourth. So I just gave up. I only use my Scanner only to monitor the Railroads now. It works great monitoring the Railroads. But as for Law enforcement? No!
“Thier system is constantly breaking up”
“It's like the channel just goes off frequency all together”
That, my friend is a bold statement. If you look at the Washington County TRS as a whole and examine the system from an objective perspective you will find that the facts just do not support the claim that the Trunking system is moving off frequency.
To explore that statement I observe the way that the system keeps itself stable. The Washington County Trunked radio system is a Motorola simulcast Astro-25 (edited) radio system. This means that voice and control data are being transmitted simultaneously from multiple tower sites on the same frequency. This is not a simple feat. To enable a receiver to demodulate voice and data over the air the Bit Error Rate (BER) of the inbound signal must remain within a very tight tolerance. In the case of two waves that are not in perfect sync this data will be horribly corrupted and un-usable.
Two factors primarily contribute to reducing the BER to an acceptable level. The first is frequency stability. Motorola meets this simulcast requirement by employing ultra stable external oscillators tied to Quantar repeater banks. Frequency stability requirements for this application are measured in single Hertz (not KHz) and the equipment used in the Washington County exceeds the requirements for frequency stability.
The second factor is timing. If you can picture two sine (or square) waves overlapping on an oscilloscope this will make more sense. If these two waves have the same data modulated on them and are on the same frequency and are not in perfect sync then as you can imagine the receiver on the user end of the link will have an enormous BER and not be able to demodulate the data and voice.
To achieve this ultra precise timing a clock much more accurate than your wristwatch is needed. For most applications a Rubidium clock is used with multiple layers of internal redundancy and a multi layer redundant and ultra clean power supply. To further increase the long and short term accuracy and ensure that multiple tower sites are in perfect sync a GPS time hack is continuously downloaded and used to “discipline” the rubidium clock. This enables all of the tower sites in the system to maintain an extremely tight tolerance for timing, which is necessary to properly employ the system.
Getting back to the original claim that the Washington County System is moving off of frequency we can see that for this to happen the following events must occur:
-The master 5 MHz reference oscillators must fail.
-The master 5 MHz reference oscillator redundancies must fail.
Or – Connectivity with the master reference must be lost.
For the Simulcast sync to fail the system must loose its timing supplied to it through the rubidium clock which is disciplined by our GPS satellites. These clocks are designed so that they are highly reliable and are capable of informing maintainers if any failure occurs so that a repair can be made before all precision timing capabilities have been lost.
As you can see a total failure of the Washington County TRS is highly unlikely in any circumstance other than a catastrophic event or disaster. Also you must consider that the whole system is being monitored 24/7 for the slightest anomalies by sophisticated equipment that is able to detect problems before major failures occur.
So then what phenomenon could be causing the scanner to not work on the system?
Your Radio Shack Pro scanner is a complicated device however it lacks many of the hardware and software features that a fully equipped system radio has thus it has a much narrower range of tolerance for data or environmental anomalies.
If we assume that the unit is programmed correctly then there are still a few phenomenons that could be causing this failure. The unit could be in an overlap area. That means that you are receiving from two or more sites at the same time using the afore mentioned simulcast technology. Any receiver that hopes to maintain acceptable BER while in an overlap area must have a very high ‘Q’. It needs to have an ultra stable receiver and error correcting data capability.
Distance between sites also plays a role. If you live in Blair then you are probably located within relative proximity to the Blair tower. If your receiver is still able to hear a tower site that is further away at the same time then that delay between the arrival time of a specific bit and the arrival time of that exact same bit that was simulcast from the distant site may become a factor. These bits arriving at your scanner at different times then expected may be causing audio degradation and exceed the internal software’s ability for error correction.
What I’m trying to say is that there are many reasons that your scanner may have trouble with Trunking the Washington County TRS but it is more likely to be either a programming error in the scanner or a simple environmental or physical factor relating to your position as compared to the transmitting towers than a failure of the system. After all, the subscriber units are not complaining and the dispatchers seem to be able to get through to the users.
After you re-verify your program as the above posters suggest I would try taking the scanner around the county and seeing if you can increase your receive signal quality with a geographical improvement. You may be surprised with what you find!
Have fun with it!
MR_O
