Analog Simulcast issues? or something else?

[AJAT]

I been listening to the following system for a while on the west simulcast site with no issues: Snohomish County Public Safety (SERS) Trunking System, Everett, Washington - Scanner Frequencies I recently moved about 5 minutes to the East and can no longer receive anything on the west simulcast site without doing some unique things. I found I get perfect reception if I turn on the attenuator, and also use a JIM M75 preamp turned up to +10 dB with a standard rubber duckie antenna. I tried all different configurations, this one works perfect. I live right smack in the middle of the reception area. After reading the RR article on simulcast issues it seems that it is only an issue on digital system not analog. Does anyone think this is a simulcast issue or some other issue. There is a cell tower nearby from my new location could that be an issue? I really don't want to by 6 preamps for all my other scanners. I am still settling in my house and have not had a chance to put up any outside antennas or anything, just been using the rubber duckies, back of set antennas.

 

[marksmith]

If it was a P25 system I would say you moved from a place where only one tower site could be received (no simulcast issues) to a place where more than one tower site can be received, resulting in simulcast distortion.

By turning on attenuation you are reducing impact of the additional towers to reduce the distortion.

Motorola systems are not typically simulcast unless they are P25 phase 1 or 2.

The system is using APCO 25 interface meaning it is using digital over analog. In essence it is a digital system on those talkgroups where APCO is used. My guess is you have some talkgroups that are analog like Fire and other services but police are using APCO digital so that certain channels can be encrypted. You can tell which are APCO because the signal is digital and is much quieter than the analog channels.

Not sure you are dealing specifically with simulcast because of the system type. But the fact that attenuation helps means you are reducing signal either from that system or an interfering source.

 

[ofd8001]

Whenever simulcast distortion is suspected and it is practical to do so, I suggest moving/taking the scanner to a location about 1/4 mile away from a site. If the audio clears up, that strongly suggests simulcast distortion.

 

[RFI-EMI-GUY]

If it was a P25 system I would say you moved from a place where only one tower site could be received (no simulcast issues) to a place where more than one tower site can be received, resulting in simulcast distortion.

By turning on attenuation you are reducing impact of the additional towers to reduce the distortion.

Motorola systems are not typically simulcast unless they are P25 phase 1 or 2.

The system is using APCO 25 interface meaning it is using digital over analog. In essence it is a digital system on those talkgroups where APCO is used. My guess is you have some talkgroups that are analog like Fire and other services but police are using APCO digital so that certain channels can be encrypted. You can tell which are APCO because the signal is digital and is much quieter than the analog channels.

Not sure you are dealing specifically with simulcast because of the system type. But the fact that attenuation helps means you are reducing signal either from that system or an interfering source.

Motorola has had simulcast systems since the 90's. APCO 16 Smartnet, Smartnet II, SmartZone, Mixed analog/P25 CAI . I think this system is Mixed analog/P25 CAI so it won't have the linear simulcast, but it will be simulcast.

 

[slicerwizard]

Motorola systems are not typically simulcast unless they are P25 phase 1 or 2.

As already noted, not true. Public safety trunking systems that need good in-building coverage are frequently simulcast. How else are you going to cover a large city? How do you think those areas were sufficiently covered by the Motorola 3600 bps systems that were later replaced by simulcast P25 systems?

 

[AJAT]

Everything points to simulcast distortion. I am no means a radio expert and I know by reading one article does not make me an expert. To sum up this article. Simulcast Distortion - The RadioReference Wiki It seems distortion from simulcast has to do with phase shift modulation not being able to demodulate 2 strong signals together. Although this is a mixed analog/digital, I would assume the control channel is all analog with FM modulation, and only 3 Talk groups would be digital on the voice system according to the data base. I am using a 346XT (analog only) scanner and it is working with the way I have it set up, so I would assume I am receiving all FM modulation. I also have the same results with the 396xt and Pro-106. Using a SDR radio with 2 dongles both with rubber ducky antennas,everything works perfect as designed. I would come to the conclusion that for some reason even FM analog system can suffer from simulcast distortion. At least a $160.00 preamp to solve the issue is better then a $650.00 SDS100.

Edit: after re-reading more closely this might be why it can’t properly decode the control channel? : “ In a simulcast system, multiple transmitters on separate towers simultaneously broadcasting on the same frequency, which is strong enough to negate the FM capture effect.”

 

[RFI-EMI-GUY]

Everything points to simulcast distortion. I am no means a radio expert and I know by reading one article does not make me an expert. To sum up this article. Simulcast Distortion - The RadioReference Wiki It seems distortion from simulcast has to do with phase shift modulation not being able to demodulate 2 strong signals together. Although this is a mixed analog/digital, I would assume the control channel is all analog with FM modulation, and only 3 Talk groups would be digital on the voice system according to the data base. I am using a 346XT (analog only) scanner and it is working with the way I have it set up, so I would assume I am receiving all FM modulation. I also have the same results with the 396xt and Pro-106. Using a SDR radio with 2 dongles both with rubber ducky antennas,everything works perfect as designed. I would come to the conclusion that for some reason even FM analog system can suffer from simulcast distortion. At least a $160.00 preamp to solve the issue is better then a $650.00 SDS100.

Edit: after re-reading more closely this might be why it can’t properly decode the control channel? : “ In a simulcast system, multiple transmitters on separate towers simultaneously broadcasting on the same frequency, which is strong enough to negate the FM capture effect.”

1) Yes analog simulcast can experience distortion. It is commonly called time domain/differential interference (TDI). It is like multipath but generally a larger scale.
2) A 3600 baud control channel will experience TDI distortion to about same extent as the analog channels. The control channel is actually digital binary two-level frequency shift keying. It has roughly the same period as an 1800 hz square wave.
3) Have you tried decoding the channels directly in conventional mode without using the control channel?
4) A preamplifier by itself will not solve simulcast distortion.
5) I cannot comment on the SDS100 and whether it has any advantage on 3600 control channel decoding.
6) Without getting into specifics as to site separation, it is important to minimize the time delay between the nearest tower and any interfering towers. This cannot be "tuned out" or "programmed out" by deleting sites in the scanner. You can only use a yagi or other directional antenna to point toward the sites that yield the best results.
7) If you can move your receiver to a location well within the intended coverage of the system or quite close to a tower, you can verify that all your receiver programming is correct. Then work to improve your antenna configuration.

 

[ofd8001]

I'm familiar with two different analog simulcast systems, both on VHF and both are tone/voice for responder alerting. If things are "out of whack", there is bad audio.

The sound from that situation is a different sound than digital simulcast distortion.

 

[AJAT]

3) Have you tried decoding the channels directly in conventional mode without using the control channel?

Thanks for the good info. I have not tried the conventional mode yet.

4) A preamplifier by itself will not solve simulcast distortion.

With the attenuation setting turned “on” in the scanner settings and the pre-amp turned up it works perfectly. From what I read the attenuation setting attenuated the signal about -20dBs, with the preamp set at about +10 or 11dB it would be attenuating the signal about -10dBs which I guess is the sweet spot. Running along side the SDR software it decodes everything and very little if any static on the voice channels.

 

[RFI-EMI-GUY]

Thanks for the good info. I have not tried the conventional mode yet.


With the attenuation setting turned “on” in the scanner settings and the pre-amp turned up it works perfectly. From what I read the attenuation setting attenuated the signal about -20dBs, with the preamp set at about +10 or 11dB it would be attenuating the signal about -10dBs which I guess is the sweet spot. Running along side the SDR software it decodes everything and very little if any static on the voice channels.

What model scanner do you have?
Tell me more about the preamp and any filtering.

 

[MTS2000des]

Motorola systems are not typically simulcast unless they are P25 phase 1 or 2.

I managed and cared for a Smartnet II analog 9 site simulcast 800MHz system that went online in 1989 and we took it offline December 29, 2015. It certainly had it's share of issues (had to do mod comps almost monthly towards the end). A transmitter launch time being off will certainly cause audio distortion/garble/heterodynes, or a receiver/subscriber operated outside the coverage area (say, on the perimeter) will produce unwanted results.
FYI our old system was one of the first simulcast 800MHz systems in the USA. All MSF5000 based hardware, MTC3600 controllers, Astro TAC comparators. Old school design with a CEB that took up half of a room.

 

[RFI-EMI-GUY]

Dekalb County Georgia was probably the very first 800 MHz Smartnet I simulcast. If I recall, 10 sites, MICOR radios, analog DSB microwave backbone. Made my ears bleed.

 

[AJAT]

What model scanner do you have?
Tell me more about the preamp and any filtering.

346XT, Pro-106,396XT all with the same results. The preamp is a Jim M75. It is set to 225-1500Mhz Jim M75 Pre-Amplifier | Scanner Master

 

[RFI-EMI-GUY]

346XT, Pro-106,396XT all with the same results. The preamp is a Jim M75. It is set to 225-1500Mhz Jim M75 Pre-Amplifier | Scanner Master

The reason I ask is that I have a BCD536HP that I use with the standard rear whip antenna and I have noticed, since I am quite close to the towers, that even when removing the antenna I still receive a lot of signal. probably via the power connector and USB WIFI dongle. This ingress of signal getting around the antenna connector is disturbing.

Those signals coming into the power and USB connectors are likely mixing with noisy internal circuits and creating TDI locally.

Recently I moved the DC power wall wart away from the radio and wrapped the power and USB into ferrite donuts. I have noticed an improvement in that I have not heard garbled signals for a while.

Your selecting 20 dB attenuate and pumping more signal into the antenna jack may be "fixing" an inherent problem with these receivers.

I intend to do more testing.