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Old 11-06-2009, 11:52 PM
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Question Why do I hear this sound?

I just got a new PRO-197. Everything is digital here; P25 locally. Now and then, with no definitive pattern, some of the transmissions are "broken, chopped, distorted" sounding voice transmissions. They are obviously part of the conversation. It is as if the digital transmission is not fully decoded. It's a tough one to describe without hearing it.
Any ideas? Is it some sort of encryption users can select?
Maybe I'll try to make a wav file of it tomorrow.
Joe
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Old 11-06-2009, 11:56 PM
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More than likely it's just as you described, the P25 audio is not fully being decoded.
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Old 11-07-2009, 12:09 AM
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You didn't mention what agencies you were listening to that use P25. But I'll start off by giving you a few ideas...

When you are monitoring an obvious P25 digital signal but it is only decoding partially, some of the likely problems could be:

1. Signal strength

It may be that the signal is marginal into your scanner, and so it cannot get a decent decode of the signal all of the time. When it can't get a decent decode, you will hear the P25 data sound interspersed with momentary patches of decoded P25 audio.

2. Interference

If the P25 signal is strong but you are still having difficulty decoding it, it may be that the signal is being interfered with by another strong signal close by in the band.

If what you are experiencing occurs again, immediately after the transmission stops you should make note of whether you hear anything else on the frequency or on a nearby frequency possibly bleeding over. It could be another transmitter on the same frequency interfering (less likely), it could be a signal on an adjacent channel (just up or down in frequency from the frequency you are monitoring), or it could be overload of your receiver by a strong signal somewhere else in the band.

3. Simulcast distortion

Digital signals are sometimes rough if you are attempting to monitor them from several signal sources (multiple transmitters) at the same time.

Simulcast systems are those systems that have transmitters in multiple locations to provide adequate coverage for the agencies that use the system... those transmitters all activate at nearly [but not necessarily exactly] the same time. When you have a transmitter from your north that is 2 miles away, one from your west that is 5 miles away, one from your south that is 8 miles away, all transmitting a digital signal at the same time, the time difference between when the data reaches your scanner from each of those sites, although nearly imperceptible to the naked ear, can be enough of a time difference that the digital data is not received [or not recieved in sequence] at the same exact time, so the scanner has difficulty in putting those packets of data back together in order to decode them and produce your audio.

Do a search on these forums for "simulcast distortion" and you'll find a lot of people discussing it. It is common on P25 trunked systems that are simulcast. Not all digital trunked systems are simulcast - some have completely separate sites that are only activated for radios affiliated with that particular site, rather than all sites transmitting each time a transmission is made into the system.

If you are experiencing simulcast distortion, the best way that most people seem to achieve satisfactory decode is to get a directional antenna that you can point in the direction of the strongest tower (perhaps the one closest to you) or that you can orient in such a way that most of the sites are "nulled" off the sides of the directional antenna.

If you are experiencing interference from on the particular frequency that you are listening to, it could be interference generated internally in the scanner, interference from nearby electronic equipment, etc.

The list above is not a comprehensive list of reasons why you would have difficulty decoding, but they are by far the most popular reasons.

Also, you should make sure you have the latest firmware uploaded to your scanner... Most digital scanners have, over time, numerous versions of firmware provided by the maker. And usually each new version provides some improvement over the previous version. So, check your scanner's firmware and make sure it is the latest. If it isn't, follow the manufacturers instructions for installing the latest.

It will definitely help others to help you if you let everyone know what type of scanner you have, what version of firmware you have, and what P25 signals you are attempting to monitor.

Ok.. this is just a starter for you. I'm sure a lot of people will chime in with useful advice [and perhaps some not so useful advice]. If you have questions, continue to ask them and somebody will respond to them [maybe me, likely somebody else].

Mike
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Old 11-07-2009, 12:21 AM
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Here is the WAV file...
http://www.qsl.net/n3elm/p25_sample.wav (3 transmissions).
Dispatch is always in the clear. You'll hear dispatch as the second transmission.
Signal strength is very good. Latest firmware (v1.7).

Last edited by wc4r; 11-07-2009 at 12:27 AM..
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Old 11-07-2009, 1:24 AM
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Yep that is indeed what it sounds like when the signal is not fully decoded.

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Old 11-07-2009, 1:53 AM
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Quote:
Originally Posted by wc4r View Post
Here is the WAV file...
http://www.qsl.net/n3elm/p25_sample.wav (3 transmissions).
Dispatch is always in the clear. You'll hear dispatch as the second transmission.
Signal strength is very good. Latest firmware (v1.7).
I'm guessing you are listening to the " James City County / York County / Williamsburg / Poquoson" system. if that's the case, it definitely is a simculast system.... and most likely you are experiencing the effects of simulcast distortion. Due to the time difference between when signal from each of the towers makes it to your scanner, your scanner cannot make sense of the bits of data in order to re-assemble them into sensible packets for decode.

Some would suggest turning on your attenuator... you can, but I doubt it will help. You might want to take a look at where all the towers are compared to your location and then attempt to provide some sort of shielding in the direction that will null out the most # of towers and still allow you to receive a strong signal from at leasete one of them.

If I were you, I'd try some cheap/free method in a pinch just a test... like grabbing some tin foil and holding up a sheet of it in a direction that faces the most # of towers, while holding your scanner on the other side of that tinfoil facing a strong tower... heck you could use a cookie sheet. You typically can get a "feel" for if this makes a difference or not. I know, it isn't a fix.. it isn't a solution. I'm just saying, in a pinch you could play around and see how blocking out signals from multiple sites can actually enhance your ability to decode a signal. Ideally you'll probably want to use a directional antenna ( "beam" - yagi, log periodic, etc.). A directional antenna will help null out some towers.

If the system above is the one you are trying to listen to, you can see where the towers are here:

FCC Callsign WQAW910 Details


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Old 11-07-2009, 8:36 AM
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You don't need to buy a directional antenna, and you don't even need to buy tin foil unless your family is planning on cooking a turkey for Thanksgiving...

The mobile unit is operating in secure mode. Most likely in software ADP encryption mode, instead of using costly hardware DES / AES encryption. Doesn't matter if dispatch or the other units are not currently in secure mode, as long as the proper key is installed in the radio, it will automatically switch during the transmission.

Now having said that, if you look closely at the description of the Williamsburg system. It states the following:
"The James City County/York County/Williamsburg system has sporadic encryption on all talkgroups. The encryption is sometimes up to 50% and may be from either side of a transmission (dispatch or mobile)."
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Old 11-07-2009, 10:01 AM
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You are correct in the service I was listening to (York/James City/Wbg). Attenuation did nothing. Shielding did nothing.
js_scan888- I think your analysis is correct. I noticed the part time encryption note also. I just thought the scanner was able to decode that mode as well. This is a bummer.
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Old 11-07-2009, 3:13 PM
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I think it's distortion.I hear the same thing in my area on 163.625 that I think is the border patrol.I once heard the distortion clear up as they where speaking.Untill then I thought I was hearing a partially encrypted singal.The dispatch comes in good while the mobiles are usally distorted.
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Old 11-12-2009, 4:12 PM
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The base is transmitting in the clear and the mobile is encrypted.
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Old 11-12-2009, 7:32 PM
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Quote:
Originally Posted by chgomonitor View Post
The base is transmitting in the clear and the mobile is encrypted.
That's what it sounds like to me.

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Old 11-12-2009, 7:39 PM
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That has been the pattern too, most mobiles are encrypted.
Is there no was to decode encryption? Kinda defeats the purpose of a digital scanner almost.
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Old 11-12-2009, 8:13 PM
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Quote:
Originally Posted by wc4r View Post
That has been the pattern too, most mobiles are encrypted.
Is there no was to decode encryption? Kinda defeats the purpose of a digital scanner almost.
It defeats eavesdropping and it makes no difference if it digital or analog.
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Old 11-12-2009, 8:17 PM
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"Is there no was to decode encryption?"
Taking your typo into account, not legally.

"Kinda defeats the purpose of a digital scanner almost."
That's the whole idea, secure communications.

Here's another clue for you all, the walrus is Paul.
Seriously, sooner or later they'll all be encrypted. Quite often there is little to no training and because to them it all sounds the same they don't know what the switch does but they figure it out eventually. That's what happened with my local PD, I heard them testing and getting more confused with every test. They operated mixed mode for months and then one day it happened, somebody figured it out and switched all the radios to secure. One other PD got it right on day one and another is still mixed, I'll not be the one to tell them.
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Old 12-16-2009, 2:14 PM
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I have a similar problem. Here in Tuscaloosa County our local sheriff's office recently switched to p25 on vhf. They have a simulcast system and sometimes they are in the clear and sometimes they are garbled (just like in the .wav file above). I think that in this case it is simulcast distortion because sometimes dispatch is in the clear and other times they aren't. I'm really thinking about getting a directional antenna and aiming it at one of the towers.
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