Simulcast digital distortion
Current generation digital scanners, even upgraded to the most recent firmware (where applicable), do a poor job of handling simulcast transmissions on any system using P25 CAI. This is most often noticed by a breaking up of a fairly strong transmission that one would normally expect to be crystal clear. This problem is most evident when monitoring a true Project 25 simulcast system (9600 baud control channel) using the CQPSK/LSM and H-DQPSK modulation scheme. In this article, we'll describe and explain the most common reason for this sort of problem, and suggest some ways you might alleviate simulcast digital distortion.
This problem is rooted in the hardware design of all scanners. In scanners, the received signal is the passed through a FM demodulator before being processed by the digital signal processor (DSP) where the digital information is recovered. This design is cheap to implement, but also strips the critical components of an incoming simulcast signal that is needed to guarantee a successful decode. In other words, part of the signal information lost with no way to recover it. The end result is the significantly degraded ability for scanners to successfully decode a simulcast signal.
The traditional explanation for simulcast digital distortion has often been blamed on multipath. While this explanation is convenient and easier to understand, it is not the root cause of simulcast digital distortion in scanners. Scanners can continue to exhibit symptoms of simulcast digital distortion even after multipath has been eliminated.
Multipath reception describes the situation where your receiver receives the signal from one or more transmit sites, i.e. over paths of different lengths. Generally, single-site multipath isn't a problem; the FM capture effect will pick the strongest signal, as multipath reflections will be of lower power, or the wrong polarization. But on Sites that are Simulcast trunking, there are multiple transmitters on separate towers, and the power from more than one of them is strong enough for your receiver to pick them both up. When receiving multipath signals from an analog system you may hear just a bit of wavering in the signal and your ear/brain has no trouble detecting the correct sounds coming from the speaker. (Older antenna based TV's experienced this by ghosting)(or)(Shortwave signals that suffer this cause more trouble in single sideband (SSB) due to the nature of the way the signal is handled and those of you who have experienced listening to multipath SSB are well aware of how tiresome it is to listen to.) In a digital signal, the vocoder in the scanner must detect every bit (0 or 1) that is coming in, assemble these bits into a packet, then, translate this stream into an audio signal that is understandable by us. The problem we're discussing occurs because the signals which come in from separate towers are not in time synchronization when they get to your scanner, because the paths are different lengths. As long as there is sufficient overlap in the bit positions, a good receiver can extract the audio in the face of this multipath interference, but when the slip is more than one bit time, it results in a broken transmission, usually resulting in what a lot of folks call a "pixelated" signal. It usually manifests itself as a readable signal followed by a broken signal then back to a readable one, etc.
If the above situation was present in commercial radios (the ones actually used by the subscribers), then P25 CAI would not be acceptable for a public safety use. Commercial radios greatly benefit from utilizing properly designed hardware where the signal is processed prior to any FM demodulation stage. This design allows the radio to process the signal without any loss of information, unlike the cheaper designs used in scanners.
Mitigation for Scanners
The solution requires the manufacturers to redesign their hardware so that they can process the signal before the FM demodulation stage of the scanner. Doing so would eliminate the loss of critical signal information required to successfully demodulate a simulcast signal and allow for scanners to perform much more closely to a commercial radio. As this problem is the result of the physical hardware design within a scanner, there is little hope for any sort of firmware solution.
Until the above happens, the only thing a scanner user can do is to attempt to mitigate the problem depending on the situation. Many of these mitigations are aimed at reducing the potential contribution of multipath to the problem. Some mitigations could be in the scanner settings or scanner type, some mitigations are going to be everything but the scanners settings. The variables list may be large, but that shouldn't deter anyone from at least trying to "tweak" specific settings first such as P25 Threshold and/or AGC Digital settings.
- See this post by UPMan: Loose/Open Squelch Improves LSM Simulcast Reception
- AGC Digital if its ON try it OFF
- P25 Threshold option can be changed after monitoring the Site with Auto to see the best setting for that System to be be decoded on, so if decode is best at 6, switch the P25 decode threshold for that system to Manual and 6. Not all scanners allow this ability System/Site specific/individually.
- DSP can be changed in some models and can vary the rate at which the colliding signals are dealt with, a lower number can quicken the digital signal processing, to low could be good or bad and to high could be good or bad.
- ADC Gain and DAC could be lowered or raised to help mitigate multipathing signals as well, typically a Negative 2 has been used by some users to help the issue.
- If you are closer to one site than another, you may not experience this problem at all. This explains why in discussions about the problem on specific systems, some people find it intolerable while others hardly notice it at all. This leads to one solution for base station monitors; a directional antenna pointed at the site you want to monitor. If you are receiving all of the signal from only one site, there is no multipath distortion to deal with. This of course does no good for people who are mobile.
- Attenuation of all the signals sometimes helps. This is of course due to the fact that if you attenuate the signals you possibly lose the ability to hear the interfering signal from the multipath source.
- In lieu of attenuation, remember that the *less* gain your antenna system has, the better off you'll be; reducing the overall signal at the receiver input will (generally) increase the signal-to-signal ratio, so capture effect will ameliorate the problem for you.
- Keep your firmware updated in your scanner. Of course sometimes it may seem that the most current version of firmware takes a step backwards, but due to the fact that each system is unique, it may well be that worked better on system 'A' actually causes system 'B' users to notice a backward step, so rolling back to a previous firmware in many models isn't difficult and should be done with recent problems corollary with the newest firmware .
- Attenuation of the secondary signal(s) causing interference via all antenna's nulling properties, A Yagi has 2 null spots 130-140 degrees from where it's being pointed, a omni-directional is at its tip (and its bottom)(so if you have 3 towers and if 1 and 3 form a straight line you may be able to most reliably monitor tower 2 with the antenna laid horizontally in the the line between 1 and 3.
Unication  has developed several models of pagers that have some limited scanner-like features. Many hobbyists have obtained these receive only devices in order to monitor simulcast systems. These receivers are intended for commercial use, therefore having proper hardware design, and works without any "tweaks" needed with scanners. To be clear, these units are commercial receivers designed for public safety users and are not scanners!
As of July 2017, these units are limited to Phase 1 capabilities. However, it is reported that a Phase 2 firmware upgrade will be available at some point in the future.
Related Wiki Articles
See also the Cliff effect article on
Discussions on RR Forums
- This article about HDTV has some additional information about multipath and other distortion / interference problems. The discussion "maps" well to public safety multipath issues, as well.
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