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Distant P25 Signals Coming In Garbled; Can It Ever Be Fixed; Firmware Update?

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hotdjdave

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On my BCD396T, when I receive a P25 transmission that is distant or low powered (e.g. simplex), the signal comes in, but it is garbled (digital unintelligible sounding). The signal is coming in, but the processor is not able to decode the P25 for some reason.

I have fine tuned my P25 settings to optimal levels. This is occuring on LAPD's system.

When this happens, many times I can hear the response of other units (assumably closer to my position) with no problem. The scanner is able to pick up non-P25 low power or distant transmissions without this problem (not being a problem with the scanner's sensitivity).

Is this something that can be fixed or addressed for a firmware update, or is it just the nature of the beast of trying to decode P25 transmissions?
 

rcvmo

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Romulus, Mi.
For one thing, do you have the proper 800 MHz antenna? What kind of antenna cable are you using? How many "jumpers" are in between?
Our experiment, not complete at this time, is taking the front end of a motorola 800 MHz maxtrac and feeding that into the IF input of a 2096. The scanner will be 800 MHz specific, but will reduce the garbled/ unintelligeable distant traffic.

rcvmo
 

hotdjdave

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rcvmo said:
For one thing, do you have the proper 800 MHz antenna? What kind of antenna cable are you using? How many "jumpers" are in between?
Our experiment, not complete at this time, is taking the front end of a motorola 800 MHz maxtrac and feeding that into the IF input of a 2096. The scanner will be 800 MHz specific, but will reduce the garbled/ unintelligeable distant traffic.

rcvmo
The system to which I am referring is in the UHF-T band (480 MHz - 512 MHz). I am using the OEM antenna, which I believe is adequate for this range. I also use a mobile antenna tuned as a tri-band antenna, covering the 140/450/800 MHz ranges, and still have the same problem. I assume that the distances are greater when using the mobile antenna, but are proportional.
 

yaesumofo

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Dave depending on your location the LAPD system should be prety clear. Receiving pacific simplex comms while you are in the valley will not happen unless you are high in the north side of the SF valley mountains.
I use a diamond discone in the LAX area and occasionally am able to receive the simplex communications in the valley. But I hear everything on the south side of the santa monica mountains.
It is all about antenns. If you really want improvement then get a commerical UHF antenna and put it as high as possible. Otherwise get a discone and this will be a major improvement over any ht antenna
That may help depending on your location.
Yaesumofo
BTW my radio is now 61% full I have several different versions of the lapd system and am testing them to see which is best for me. I love this radio.

hotdjdave said:
On my BCD396T, when I receive a P25 transmission that is distant or low powered (e.g. simplex), the signal comes in, but it is garbled (digital unintelligible sounding). The signal is coming in, but the processor is not able to decode the P25 for some reason.

I have fine tuned my P25 settings to optimal levels. This is occuring on LAPD's system.

When this happens, many times I can hear the response of other units (assumably closer to my position) with no problem. The scanner is able to pick up non-P25 low power or distant transmissions without this problem (not being a problem with the scanner's sensitivity).

Is this something that can be fixed or addressed for a firmware update, or is it just the nature of the beast of trying to decode P25 transmissions?
 

swest90

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Aug 8, 2004
Messages
170
Hey Dave, I have an extra Comtelco 480-500mhz 12db yagi. PM me if you are interested. I am using one out here in Pomona and I can pick up almost everything LAPD has repeated. No simplex with my distance of course.

Shawn
 

MetalCarnage

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Ajax Ontario
Ya, thats just what digital is like, i had the same prob with P25 out where i am, everything was coming in like that, garbled crap. I went out and bought a frequency specific antenna (140mhz in my case) and it was like night and day, my system went from 2-3 bars out of 6 in signal to 5-6 bars out of 6, and at the same time, no more garbled transmissions, all P25 came in crystal clear...just remember that with digital it has to decode data, not just simply receive audio like analog, if u have crap signal and are losing 20-30% of the digital then its obviously going to be hard for the radio to peice together something that makes for intelligent conversation
 
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N_Jay

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MetalCarnage said:
. . . . my system went from 2-3 bars out of 6 in signal to 5-6 bars out of 6, . . .
Does anyone know how many dB each bar represents?

Or is a "signal Quality" measure that is not in dB?
 

swest90

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N_Jay said:
Does anyone know how many dB each bar represents?

Or is a "signal Quality" measure that is not in dB?
You saying this just to bring up your arguement you were having in the other thread about signal strength meters and db? I thought you hashed this out over there... I would venture to guess many of us (including myself here) have no idea how many db each bar represents. Maybe UPman could fill you in, althought I thought you had it figured out in the other post.
 
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N_Jay

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swest90 said:
You saying this just to bring up your arguement you were having in the other thread about signal strength meters and db? I thought you hashed this out over there... I would venture to guess many of us (including myself here) have no idea how many db each bar represents. Maybe UPman could fill you in, althought I thought you had it figured out in the other post.

Not to bring up the argument, but to get another data point.
 

swest90

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Roger copy, My apologies. I do have one of those attennuation panels. I have never used it but I might have to give it a try tonight.
 

hotdjdave

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P25 Decoder Fine Tuning To Bring In Distant or Weak Signal

I do not expect to get LAPD Central, South, or West Bureau simplex in the Valley Bureau. I live in West Valley Division and sometimes I have this problem within WV Division. Sometimes I hear other nearby divisions on simplex when I am in WV.

This makes me wonder, do LAPD officers use other divisions' simplex mode? I have not ever heard it requested on the air; usually just a request to go simplex or to meet another unit on simplex (within the division). But maybe they do it without the request or knowledge through "Control" on their own.

Anyway, I am aware that using a better and higher mounted antenna will improve the quality in which I can pull in weak or distant transmissions. I just wondered if this issue could be resolved within the radio. Since I am hearing a transmission, just that it is scrambled. I thought maybe some more fine tuning in the radio's programming and APCO P25 decoder could resolve the issue.

I know that DTV (Digital Television) will capture a signal and if there is enough "good" signal, it will repoduce a perfect picture; however, if the signal does not have enough "good" signal, it will reproduce a "blue screen" or no picture - either perfect or nothing. I wondered if the scanner does not have enough signal to reproduce a clear audio transmission, then does it reject the signal. In my case, it appears that the radio is trying to reproduce the signal and then plays the audio, regardless of its quality. Seems to me to defeat the purpose of "digital" (P25) radio communications.

So this issue need be addressed and improvement or consession made.
 
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John_M

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Mar 8, 2005
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Rochester, N.Y.
Amplifiers won't help in this situation either. You need a good signal at the antenna. Amplifying a bad signal does just that, amplifies the bad signal. The decoder just can't decode a bad signal. I wonder if Motorola digital decoders are any better than the ones we have in our scanners?
 
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N_Jay

Guest
John_M said:
Amplifiers won't help in this situation either. You need a good signal at the antenna. Amplifying a bad signal does just that, amplifies the bad signal. The decoder just can't decode a bad signal. I wonder if Motorola digital decoders are any better than the ones we have in our scanners?

Yes the DSP software is better.

And amplifier CAN (A cautious CAN) help, but I would keep the noise as low as possible and the gain as low as practical, mount it at the antenna and protect it with a preselector.

This ain't magic, it's design.
 

brandon

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I'd like to know the answer to this question as well.
So far the best scanner for weak digital monitoring is the PRO-2096. Sometimes I cna hear Santa Monica PD digital from here...It's quite weak but the scanner can decode it while the Uniden cannot.
 
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