Which One Of These Is The Better Wave Form?

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This is a control channel on a P25 Phase I & II system. Both are on a solid strong local signal. #1 is DSD Plus with a BDC996P2 discriminator tap and #2 is with a dongle. Which wave form is considered the better form for the best decoding? And why would I get a different-looking wave form on a dongle than with a discriminator output on the same signal? Thanks.

1.


2.
 
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Markb

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The first one looks right.

The second looks like the waveform is clipped. Maybe lower your gain setting?

The biggest question is how well is DSDPlus decoding both of these. It may look ugly, but if it's working......

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EricCottrell

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Hello,

Both maybe going into clipping. They also represent two different waveform types. I would guess #1 is a simulcast site and #2 is not.

73 Eric
 

slicerwizard

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Number one is FM demodulated CQPSK. Max has been pretty vocal about FM demodulation not being the best way to handle CQPSK signals.

Number two is the waveform that DSD+ displays when FMPA/FMP24 and DSD+ are TCP linked and processing CQPSK signals.

Number one bad. Number two good.
 
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The first one looks right.

The second looks like the waveform is clipped. Maybe lower your gain setting?

The biggest question is how well is DSDPlus decoding both of these. It may look ugly, but if it's working......

Sent from my SM-G935V using Tapatalk
For the Hillsborough County system, they both seem to decode the same. I've tried a lot of different settings and cannot get both waveforms to looks the same. They both seem to have a different shape altogether.

But I did have a strange thing happen recently. I was in Orlando for a work trip and ran DSD+ on the Orange County P25 control channel (Site 5, which is the biggest & busiest). The waveforms also looked different on the discriminator & dongle (same as the screenshots above), but the dongle seem to decode a lot more activity. The discriminator showed a lot fewer voice calls and was slower to identify all the frequencies and system ID / WACN. For some reason, the dongle appeared to be the better decoder on Orange County. But on the Hillsborough County (which has a lot less activity), they're both decoding about the same.
 
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Hello,

Both maybe going into clipping. They also represent two different waveform types. I would guess #1 is a simulcast site and #2 is not.

73 Eric
Both screenshots were from the same control frequency on the same system. It is P25 simulcast.
 
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Number one is FM demodulated CQPSK. Max has been pretty vocal about FM demodulation not being the best way to handle CQPSK signals.

Number two is the waveform that DSD+ displays when FMPA/FMP24 and DSD+ are TCP linked and processing CQPSK signals.

Number one bad. Number two good.
So who is this "Max" person? Does this mean using a discriminator scanner will never work well no matter you do with the settings or how you do the discriminator mod in the scanner?

It looks like we have one vote saying #1 is better and your vote saying #1 is bad. So now I need a tiebreaker.
 

mtindor

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So who is this "Max" person? Does this mean using a discriminator scanner will never work well no matter you do with the settings or how you do the discriminator mod in the scanner?

It looks like we have one vote saying #1 is better and your vote saying #1 is bad. So now I need a tiebreaker.
Max = KA1RBI = OP25 developer. You can read the very long OP25 thread, which has links in many places to much of what Max has documented.

Mike
 

Spitfire8520

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For the Hillsborough County system, they both seem to decode the same. I've tried a lot of different settings and cannot get both waveforms to looks the same. They both seem to have a different shape altogether.
That is because they are different waveforms. Slicerwizard basically said that in his post.

The exact details of the difference is located in the "Notes.txt" file. When you use FMP24/FMPA with DSD+, they switch to a different (better) method to demodulate the CQPSK. When this happens, "DSD+ generates a pseudo-C4FM audio waveform and displays it in the source audio waveform window." In other words, DSD+ essentially translates the modulation from CQPSK to C4FM for the waveform display.
 
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slicerwizard

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Does this mean using a discriminator scanner will never work well no matter you do with the settings or how you do the discriminator mod in the scanner?
A tapped scanner will not produce clean audio from weaker CQPSK signals and DSD+ will barf on it. Meanwhile, C4FM signals with similar strength will decode ok. Hell, even with strong FM-demodulated CQPSK signals, I can't get much out of DSD+, even with the -mp switch. Only thing that works well is FMPA or FMP24, TCP linked to DSD+. That's when waveform #2 is displayed and that's what works.


It looks like we have one vote saying #1 is better and your vote saying #1 is bad. So now I need a tiebreaker.
Well, I vote that #1 is bad, so now it's two votes to one. Easy peasy.
 

mtindor

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It looks like we have one vote saying #1 is better and your vote saying #1 is bad. So now I need a tiebreaker.
I'm vote #3 for 1=bad, 2=good. FMPA/FMP24+DSDplus (TCP-linked) gives me very nice decodes on any simulcast system I've tested, far better than any scanner has -- and far better than SDR#-->VAC-->DSDPlus has.

Mike
 

KA1RBI

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A tapped scanner will not produce clean audio from weaker CQPSK signals and DSD+ will barf on it.
depends on the exact point in the scanner that is tapped. Most users tap at the output of the discriminator, which will not produce a clean signal if the station is LSM (and at least in my experience, not to mention the experience of many hapless scanner users, this is true regardless of whether the LSM signal is weak or strong). The holder of the WCQPSK (Harris) simulcast patent has claimed this isn't the case for that variant, but I have no independent verification of that.

If on the other hand you tap prior to the discriminator (i.e., tap at one of the IFs, typically the final IF stage) you will get a clean signal, but there are still potential issues: 1) you will need additional signal processing before this signal can be fed to most sound cards; 2) the signal is IF, not baseband - that is, it's in a format directly supported by none of the PC decoding apps (other than OP25); and 3) the oscillators in the scanner must have reasonable phase noise levels....

73

Max
 

EricCottrell

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Hello,

The traditional C4FM decoder has a moving average filter after the FM discriminator that will tend to smooth out waveform one into waveform two. It is mentioned in the P25 Phase 1 specification.

I had to remove filtering caps on the packet output of my Icom R2500 to be able to decode P25. I assume stock works fine for 9600 baud ham packet, but it destroys a P25 waveform.

I get the impression that commercial radios are using processor chips, like the TI OMAP, that have DSP processors in them. Demodulation of Analog FM and Digital is done in the DSP using a digitized IF stream in IQ format. The same demodulation is done by software on the PC when you use a RTL-SDR stick.

73 Eric
 

maus92

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CQPSK is used for simulcast and most Phase II. C4FM is used for non-simulcast Phase I.
I've been scouting the Maryland FiRST statewide system, a P25P2 system currently under construction, with three of it five regions accepted and operation. It uses a combination of county based simulcast cells and standalone sites that transmit on a FDMA control channel. It appears that the control channel on the standalone sites use C4FM, and their simulcast sites use CQPSK (judging by the waveforms output by UT2's scope function .) Does this make sense for a new Phase 2 system? The voice channels primarily operates in TDMA, but have the ability to revert to FDMA for interoperability (the data channels appear to be FDMA most of the time.) I was under the impression that Phase 2 systems use H-DQPSK and H-CPM for modulation - does that only apply to voice channels? Can anyone link a resource that could provide more detail how this system might work?
 
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