Why Dongle Decodes P25 Better Than Discriminator Scanner?

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With a 2-dongle setup on DSD+ on a P25 system, I get little or no errors:



But if I tune the same control channel on a BCT15X scanner with a discriminator mod, I get a lot of errors, even though the scanner is getting a full strength signal:



How can I fix this? Are there some sound settings I can adjust? This is on Windows 7, 64-bit. The dongle gets a strong signal too:

 

RFI-EMI-GUY

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Are you decoding a simulcast system? I believe, and I am sure some experts will weigh in, that the linear simulcast modulation (LSM) mode has an amplitude component than defines the precise timing interval for the symbols.

An FM discriminator by design removes the amplitude modulation.

A discriminator tap only captures the phase or frequency modulation component of the symbol. The P25 data is captured with a discriminator tap, however there is timing slippage and uncertainty of the symbol width that isn't correctly recovered.

To fix this, you could tap the receiver at the IF stage and feed the IF signal into the dongle. Then tune the dongle to the IF frequency. The IF bandwidth of the BCT15X needs to be confirmed and this is simplistic, but the direction you could take.
 
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I suspect it is Linear Simulcast Modulation (LSM) because of the size of the county that must be covered. Is there an exact way to tell if a system is LSM? I used to have a P25 scanner that did poorly decoding the audio, so I think it is.
 

RFI-EMI-GUY

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I suspect it is Linear Simulcast Modulation (LSM) because of the size of the county that must be covered. Is there an exact way to tell if a system is LSM? I used to have a P25 scanner that did poorly decoding the audio, so I think it is.
Check the FCC database for the license of the agency you are monitoring . If the control channels are the same at two or more sites it is a simulcast system, and will be LSM in most cases (Motorola and Harris).
 
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Then that would prolly be a yes as the control frequency has 3-4 different locations and different callsigns in the county. So basically, dongles normally handle simulcasting on P25 better than a discriminator scanner?
 

RFI-EMI-GUY

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Then that would prolly be a yes as the control frequency has 3-4 different locations and different callsigns in the county. So basically, dongles normally handle simulcasting on P25 better than a discriminator scanner?
Yup;

The dongles have a DSP that handles the amplitude and phase simultaneously using, who would have guessed? mathematics! (wished my high school algebra teacher would have explained this practical application!)

A Motorola or Harris radio receiver has DSP. The discriminator tap is the cheap and dirty solution even the expensive scanners like my new BCD536HP has. So you will get errors and hiccups even when the receiver is stationary. The signals from the towers arrive at different times and phase at your antenna and even that relationship changes over time due to reflections and atmospheric refraction. So the brainiacs that designed LSM introduced an amplitude component that better defines the timing (edges) of each symbol. There is a guy on this board who has a site called LSM101 that goes into detail on the proprietary schemes. My take away, I could be wrong, is that the amplitude energy sharply goes to zero and that is the information that sets the timing of the decoder. The discriminator tap method can only approximate the symbol timing on the average duration of the received symbol.
 
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slicerwizard

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With a 2-dongle setup on DSD+ on a P25 system, I get little or no errors:
Because you're using FMP24/DSD+, which handle CQPSK signals properly.


But if I tune the same control channel on a BCT15X scanner with a discriminator mod, I get a lot of errors, even though the scanner is getting a full strength signal:
Because the scanner is using FM demodulation and you haven't told DSD+ to use its PSK optimizations (-mp)


How can I fix this?
Use dongles. Or use the -mp option with your scanner.


I suspect it is Linear Simulcast Modulation (LSM) because of the size of the county that must be covered. Is there an exact way to tell if a system is LSM?
Just look at the waveform that you posted in the Unitrunker thread.


The dongles have a DSP that handles the amplitude and phase simultaneously using, who would have guessed? mathematics!
No. Dongles merely pass the raw I/Q stream to your PC. All DSP'ing is done in the PC software.
 
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Thanks. Never knew about -mp. It does seem to reduce the number of errors, but there are still some errors while the dongle has almost none. The scanner is a lot easier and more fun to search around with. But I'll use the dongle.

But as for the waveform, how do I tell just by looking at it? What is it in the waveform that tells you if its simulcast or not?
 
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Does DSD+ have any kind of a Health indicator or percentage that tells you how good of a decode you're getting?
 

slicerwizard

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But as for the waveform, how do I tell just by looking at it? What is it in the waveform that tells you if its simulcast or not?
Compare that particular waveform to any other 4 level signal (like DMR)


Does DSD+ have any kind of a Health indicator or percentage that tells you how good of a decode you're getting?
Not that I'm aware of. I just look at the console screen and see if there are any ERR indications.
 
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Here's a shot of a local DMR ham repeater:



And, from the other thread, here's a shot of P25 control channel:



So comparing these 2, how do we determine if there is any simulcasting?
 
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Do a search for "DSDTune".
Everytime I try to run DSDTune, I get this screen for like 1-2 seconds and then it just quits and closes itself out without doing anything:



I have a .bat file named "462.30000.bat" that has the following in it:

@echo off
REM : batch file created with dsdtune utility on Mon Jun 06 19:59:31 2016

dsdplus.exe -i2 -v2 -wsl2.1 -wss87.300 -wel2.1 -wes693.1022 -wch20 -wcl4.1 -wch20
 

GTR8000

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DSDTune no longer works with DSD+ 2.29 or later because the -dr and -dh command line options were removed from DSD+.
 

KD8DVR

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DSDTune no longer works with DSD+ 2.29 or later because the -dr and -dh command line options were removed from DSD+.
I'm sure the dsdtune author will wait for a free official release before he fixes it. The issue has been reported and acknowledged on the dsdtune site.

AntiSquid Disclaimer: All posted content is personal opinion only and may not imply fact or accusation.
 
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And we can only hope that "critical" release will let us listen to a "certain" kind of P25 audio (hint, hint).
 
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