Saratoga County Public Safety Project 25 Phase I (From RR)

John_S

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The problem almost sounds like multipath in FM broadcast...or as if the different towers are out of phase or synch with each other. After looking at the map, I see that the closest tower is actually at city hall...and I'm about 1/4 mile away from it...way closer than any other tower. So hopefully this will minimize simulacast issue...especially if I use the stock antenna. That's encouraging at least.
 

k2hz

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The problem almost sounds like multipath in FM broadcast...
That is exactly the issue with a simulcast system. The signals from multiple towers will have different phase delays at a particular site and also be aggrevated by multipath reception. A P25 digital radio has DSP circuits that can deal with proper decoding. A scanner that is primarily an analog device, attempting to decode the digital signal after passing through an analog FM discriminator, can't handle the multipath distortion. So, the only hope for reasonable reception is indeed if you are close enough to a tower that the direct signal can overcome the out of phase signals from multiple transmitters and multipath reception.

That is also why a SDR, which is a digital device, can directly decode the digital signal with proper DSP software.
 

John_S

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Maybe Uniden should incorporate a multipath meter...just like an old school analog FM tuner from the 70's. I use a Kenwood KT-8300 that has one and it helps get the antenna pointed correctly.
 

k2hz

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Maybe Uniden should incorporate a multipath meter...just like an old school analog FM tuner from the 70's. I use a Kenwood KT-8300 that has one and it helps get the antenna pointed correctly.
Generally, a meter won't help. You will still get out of phase signals an analog based scanner can't handle. Some scanners do include a signal quality indicator you can use to optimize digital reception. But, unless you are very close to and in line of sight of a tower, reception will still be marginal. Directional antennas only help in rare circumstances where all the potential interfering signals are in the null pattern of the antenna.

The other frustrating thing is the multpath reception constantly changes with leaves on the trees, snow on the ground, local weather etc..
I helped a friend set up his TRX1 with a Yagi antenna in the summer and it worked great until winter when he lost reception almost completely on most days. He was fairly close to one tower but had other towers on all sides. Probably with the no leaves to attenuate the other signals and reflections from snow on the ground the Yagi directivity was insufficient to reduce the other signals.

Multipath can generally be dealt with on FM broadcast due to "capture effect" where a signal only a few db stronger will overcome the weaker signals. Attempting to decode a digital signal after it has gone through the FM limiter/discriminator detector circuits on a typical scanner is made worse by the fact that the detector compresses the amplitude of all the incoming digital data from the various sites or reflections to about the same level so it is very difficult for the digital decoder to sort out the mess. The same design that optimizes analog FM reception is detrimental to digital.

While I am on the subject, the FM capture effect works best with wide band FM like broadcast and earlier versions of mobile radio FM. As the FCC repeatedly lowered bandwidth, the capture effect was degraded. With the 2013 narrow banding, what is called the modulation index was decreased to the point that most of the advantages of FM such as signal to noise ratio and capture were severely degraded. This is one of the reasons that digital technology is now more attractive for mobile radio systems.

On a receiver capable of direct digital reception, the DSP circuits can easily sort out the best signal from the out of phase stuff from multiple sources.

Bottom line with an analog based scanner, reception of P25 simulcast depends on location and can vary with season, weather and other factors.
 
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kmontano

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Sorry to kick this old thread back in, but I've been seriously thinking about reviving my scanning interest here in Saratoga Springs, and looking at some hardware. But before jumping back into this, I'm wondering if any of the issues mentioned in the thread have been improved? I'm not so crazy about having to throw down that much money for an SDR radio...been looking at BCD996P2 or Home Patrol. I'm in a 3rd floor apartment in town, and even an outdoor antenna would only be a few feet away if needed. Thanks in advance.
I’ve had good luck receiving the P25 system on my BCD436HP using a rubber duck antennae inside the third floor of the Collamer Building downtown. I don’t think you’d have too much of an issue receiving the system downtown. Being a simulcast system, non-SDR scanners will always have issues with it and I’ve noticed mixed reception depending on where I am in Saratoga Springs.
 

chrismol1

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I use a $30 SDR dongle from nooelec with sdr trunk software, boatloads of features, simultaneous receive, talkgroup priority, recording, multiple digital formats and frequent updates. I'd recommend this to anyone hesitant to spend a decent penny on an SDR scanner
 

KA1RBI

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P25 "simulcast distortion"* does NOT result from the same sort of multipath issues that affect analog FM. Moreover, when we talk about signals from multiple P25 towers operating on the same frequencies that arrive at the receiver "out of phase" we are NOT talking about the phases of the various RF carriers (operating in the hundreds of MHz ranges). Instead, the element of the digital signal that we are talking about when we refer to the simulcast "out of phase" condition is the phasing of the symbol clock (with period 1/4800 sec. for phase I and 1/6000 second for phase II).

Attempting to decode a digital signal after it has gone through the FM limiter/discriminator detector circuits on a typical scanner is made worse by the fact that the detector compresses the amplitude
Attempts to decode a "conventional" C4FM/FSK4 P25 digital signal by passing it through an FM limiter/discriminator (or PLL) will work fine with no problems. Such signals are compatible with FM circuitry. The problem is that LSM and CQPSK are NOT FM signals; they fail the two primary tests that we would ask all FM signals to fulfill (viz., that we can transmit it using a class-C final amplifier and that we can cleanly receive it using an FM demodulator). The very definition of the LSM acronym includes the word Linear (i.e., not compatible with class-C amplifiers). Regular FM signals such as analog FM or FSK4/C4FM do not suffer any distortion when passed through a limiter circuit.

The same design that optimizes analog FM reception is detrimental to digital.
This simply isn't true, in general. Conventional P25 FDMA (C4FM/FSK4), NXDN, DMR, YSF -- all work fine using a regular FM receiver, tapped right at the discriminator, and followed by a DSP.

It's only really true for CQPSK - which must be demodulated as ***PSK***, not as FM.

* by "simulcast distortion" I mean the usual lousy reception that occurs when attempting to receive PSK P25 on any FM receiver designed for C4FM/FSK4.

73

Max
 

k2hz

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Digital signals other than LSM/CQPSK can indeed be received through a discriminator tap IF they are not simulcast. I should have said "The same design that optimizes analog FM reception is detrimental to digital SIMULCAST".

I agree that LSM itself suffers if demodulated as FM. Even without the simulcast/multipath issues the demodulation by a standard scanner is less than optimum. My point that adding signals received from multiple paths/sources results in additional distortion since the amplitude of the interfering signals delivered to the digital decoder circuit in the scanner is likely brought to the same amplitude as the desired signal by the limiter/discriminator detector.

So, both conditions are true - The LSM signal itself is not FM and is distorted by an FM detector and the distortion is multiplied in the presence of multiple signals arriving with slightly different delays due to path distance.

I generally do not bring up the fact as you emphasize that LSM itself needs a truly linear transmission and reception circuits since I had posted that I believed that to be true many years ago on a forum on another site and was told by an experienced technician for an LSM system that standard limiter/discriminator FM demodulation of the LSM signal would not cause any degradation. I do not believe this but he was joined in agreement by many others with the argument that their scanner can receive LSM fine as long as they are near a tower. So, I don't restart that discussion.

Incidentally, I do also see significant problems in using a discriminator tap vs direct SDR demodulation of simulcast conventional P25 using DSD+. The effects of multipath intersymbol interference can be seen on a scope on the discriminator output coincident with breakup of the decoded audio. My personal experience with early digital microwave systems and conversion of the a path from analog to digital was that digital was more sensitive to multipath effects due to intersymbol interference on digital than the fading effects on analog.
 

John_S

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I’ve had good luck receiving the P25 system on my BCD436HP using a rubber duck antennae inside the third floor of the Collamer Building downtown. I don’t think you’d have too much of an issue receiving the system downtown. Being a simulcast system, non-SDR scanners will always have issues with it and I’ve noticed mixed reception depending on where I am in Saratoga Springs.
Collamer building huh?...about as good as it gets in town for reception...I'm jealous.
 

John_S

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I use a $30 SDR dongle from nooelec with sdr trunk software, boatloads of features, simultaneous receive, talkgroup priority, recording, multiple digital formats and frequent updates. I'd recommend this to anyone hesitant to spend a decent penny on an SDR scanner
That sounds like a good route to take, but I'm thinking more around a stand alone unit...probably 996P2 or 536HP...something in that range. I've been away from scanning for a while...and dealing with trunking even longer. So something easier to program and run would be better to get going with.
 

vannossc

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I recently installed the PulsePoint app on my iPhone. It has Saratoga Country coverage and I'm now curious if this app gets its data and radio traffic directly from the county proper or is there some other provider (civilian?) that feeds this county coverage to this service. It clearly let's me know how much traffic I'm missing without mounting my new antenna on the roof! This must be the ever elusive simulcast traffic...

Just curious about that app - still have my 436HP on and listening to Schenectady and Albany counties.
 

kmontano

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I recently installed the PulsePoint app on my iPhone. It has Saratoga Country coverage and I'm now curious if this app gets its data and radio traffic directly from the county proper or is there some other provider (civilian?) that feeds this county coverage to this service. It clearly let's me know how much traffic I'm missing without mounting my new antenna on the roof! This must be the ever elusive simulcast traffic...

Just curious about that app - still have my 436HP on and listening to Schenectady and Albany counties.
PulsePoint is tied directly into the county’s CAD system. If you monitor Saratoga County’s VHF Fire/EMS alert frequency you’ll notice calls appear in PulsePoint about 30-60 seconds before the call is put out over the air. Not sure on the exact source of the radio feed in PulsePoint but I believe it’s sourced from Broadcastify.
 

John_S

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I’ve had good luck receiving the P25 system on my BCD436HP using a rubber duck antennae inside the third floor of the Collamer Building downtown. I don’t think you’d have too much of an issue receiving the system downtown. Being a simulcast system, non-SDR scanners will always have issues with it and I’ve noticed mixed reception depending on where I am in Saratoga Springs.
Just out of curiosity...and due to the fact that I'm seriously looking at the BCD536...I'm wondering if your 436 seemed to need much tweaking to enable reception of Saratoga County...or was it fairly good with the default settings?
 

John_S

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Just a quick note to mention that I'm getting clear reception here in town....1/4 mile away from the City Hall tower. Just had a BCD536HP arrive and got it up and running with stock antenna. Plugged in a Motorola external speaker, which helps a little...but not having any issues with distorted reception. Had to do a bit of paring down, using System key, to let it do a better job of coverage, but this radio works very nicely here. Now trying to decide whether it's worth the work to make a custom grouping for Favorites.
 

bob550

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Pick them up just fine in Glenmont with a G5!
Seriously? I've never had any luck from my home near the Beth. Town Garage. The range map for the South Simulcast shows the signal barely touching the northern tip of the county. Are you using the stock antenna, something else, or just lucky?
 

jgorman21

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Seriously? I've never had any luck from my home near the Beth. Town Garage. The range map for the South Simulcast shows the signal barely touching the northern tip of the county. Are you using the stock antenna, something else, or just lucky?
Yes. Stock antenna. Usually minimum of 2-3 bars. the dealer told me that I probably would not get them here. I really have idea why. Also Troy PD that used to be weak comes in too. Not as well as Saratoga though.
 
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