Simulcast Distortion

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Revision as of 18:03, 8 January 2018 by Darunimal (talk | contribs) (Rearranged and removed tone, cleaned up, detailed more info)

Current generation digital scanners, do a poor job of handling Simulcast Phase I & Phase II system transmissions. The problem is most evident when a scanner user is monitoring a true APCO Project 25 Simulcast system that use the CQPSK and H-DQPSK (Motorola LSM / Harris WCQPSK) modulation scheme, which is not FM modulation based but is either phase and/or amplitude modulation based. This is most often noticed when a strong transmission constantly breaks-up, especially when a scanner user is coming from monitoring Analog signals, where one would have an expectation - to receive the new digital voice to be crystal clear, throughout the transmission. In this article, we'll describe and explain the most common reason for this sort of problem, and suggest some ways you might alleviate some of the simulcast distortion your scanner experiences, as well as additional solutions that maybe non-scanner based altogether.


Multipath reception describes the situation where reception of digital simultaneously broadcast signal from one or more transmit towers, i.e. over paths of different lengths, from a Site's Tower(s). Generally, single-tower multipath is not a problem; the FM capture effect will pick the strongest signal - as multipath reflections will be of lower power, or the wrong polarization. Though, on a Site's towers that are broadcasting simultaneously on the same frequency, (i.e. digital trunk signals voice and data) there are multiple transmitters on separate towers, with the power from more than one of them is strong enough for your scanner's receiver to pick-up 2 or more of the signals.

Often, when receiving multipath signals from an analog system you may hear just a bit of wavering in the signal via a receiver and has no trouble detecting the correct signal, let alone your ear/brain focuses on the correct/strongest sound if two or more are present. Older antenna based analog TV's experienced this by ghosting. Though, shortwave signals suffer this and cause troubles in Single-sideband SSB, due to the nature of the way the signal is handled. Those of you, who have experienced listening to multipath SSB are well aware of how tiresome it is to listen to.

The traditional explanation for scanners to have simulcast distortion, in a received digital trunk radio systems, has often been blamed on multipath and/or inter-symbol interference, which are actual and real phenomenon. For some though, they believe the explanation is just a convenient and easy excuse, it's truely not the only cause of simulcast distortion. Scanners can continue to exhibit symptoms of simulcast distortion even after multipath signals have been minimized. It is most likely because the scanner (receiver) is not using an IQ demodulation technique. Furthermore, as long as there is sufficient overlap in the bit positions, a receiver designed to utilize a IQ demodulator can extract a digitally "Simulcast" System's Site's data & audio in the face of multipath. System's that use "Simulcast" Site(s) i.e. Phase I and II, are specifically designed to synchronize their tower's tranmissions, for their intended coverage area and current users (that are logged-on). So, timing issues are mostly minimized, in real-time by subscriber's transceivers sending data back and forth to the controllers - outputting corrections. Which, helps to constantly maintain the best synchronous information for the subscriber's radios. Though, when the slip is more than one bit time, it can result: in a broken transmission i.e. inter-symbol interference. This generally, occurs on the fringe of an intended coverage area - of a tower(s), where portable use is discouraged and higher output of mobiles are expected/required to be used. If multipath was a significant issue for actual APCO P25 Phase I and II System use, then a "Simulcast Systems" would not be acceptable for Public Safety use.


All Scanners Multipath Mitigation

Until the above happens, the only thing a scanner user can do is to attempt to mitigate the problem, depending highly on an individual Users situation. Many of the below Mitigating techniques are aimed at reducing the potential contributing factors of Multipath, to the receive and decode problems. Some, mitigation techniques, could be:

  • Scanner Settings
  • Scanner Firmware
  • Location
    • Antenna
    • Scanner Location
  • Some mitigation techniques are going to be everything but the scanner.

The variables list may be large, but that shouldn't deter anyone from at least trying to "tweak" specific settings first..:

  • Dwell or Hold Time increase, increasing the ability of the scanner to capture and decode all of a Systems voice transmissions & data parameters sent on the Control Channel.
  • Limit the amount of Systems - Scanned, or just scan one Site without additional Conventional, Priorities, or Wx, additional Sites or Systems
  • Digital AGC if its ON try it OFF, because it may be be making Voice transmissions less intelligible, especially if there are multiple errors already in the decode. While it can also, clip loud audio or over-drive quite audio.
  • Keep your Firmware updated in your scanner. Some users report that Simulcast is the cause of all their headaches, when in reality the scanner doesn't have a perfectly written algorithm to decode a particular type of System, Voice, or Band Plan, YET. Sometimes it may seem that the most current version of Firmware takes a step backwards, but that's due to the fact that each system type is unique. It may well be what worked better on system 'A' actually causes system 'B' users to notice a backward step. For some, rolling back to a previous firmware in many models isn't difficult and should be done with recent problems corollary with the newest firmware, but be assured a System's control channel frequency hasn't changed. Also, don't re-write your programming at the same time as a firmware upgrade because you are creating too many variables to objectively deduce where a new problem arose.

Remember only one setting or thing should be changed at a time, and always archive and save your beginning settings, whether that be in different folders, or on other media like an additional thumb drive, even snapping pictures or taking video or just on good ole scratch paper. Thanks and Goodluck and happy monitoring...


  • 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 in Auto, switch the P25 Decode Threshold for that system to Manual and 6. While 6 isn't a panacea, it's at least a beginning numeric to try starting with, typically up-to 9 in Manual mode will make the biggest difference. Not all scanners allow this ability System/Site specific/individually.


  • DSP Level Adapt can be changed in some models and can vary the rate at which the DSP attempts to adjust varying P25 levels.
  • ADC Gain and DAC Gain could be lowered to help reduce bit error rates. Typically a -2 and -4, respectively, have been used by some users to help the lower the issues. See GRE/RS/Whistler based DSP ADC/DAC Adjustments for more information, as positive ADC setting can cause internal channel and component cross-talk.

Antenna and Reception

This section attempts to address the issues that might contribute to the multipath problem.

  • ATT - Attenuation of all the signals sometimes helps, typically the Attenuator in the most

scanners is via hardware at a standard of -20db. This is of course, due to the fact that if you attenuate all of the signals, your receiver possibly loses the ability to hear the interfering signal. Additional connectors can add low attentuation, as well as there are F-Connectors with fixed 3, 6, 12 decibel attenuators and many other varied attenuation techniques.

  • In lieu of Attenuation, remember that with Digital Signals: the *less* gain your antenna has, the better off you'll be; reducing the overall signal(s) at the receiver input will (generally) increase the signal-to-noise ratio, so the "capture effect" will ameliorate the problem for you.
  • Attenuation of the secondary signal(s) causing interference via all of an Antenna's nulling properties:
    • A Yagi has 2 null spots, typically 90 and 270 degrees from where it's being pointed. So off-center direction to a tower help negate a secondary tower at 40-75 degrees - beam width and attenuation can vary on the design
    • A Omni-directional, also, has 2 null spots at its tip and bottom. If you have 3 towers and if Tower 1 and Tower 3 form a straight line, then you may be able to most reliably monitor Tower 2 with the antenna laid horizontally in the the line between Towers 1 and 3.


    • Or adding a corner reflector - beam width and attenuation can also, vary on the design. Search for "cantenna."
  • A yagi 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 infer, that there is NOT a second or third Tower in front or slightly offset to the left or right the primary tower you want to monitor (or even a 4th or 5 tower behind the aimed yagi antenna.
      • One user actually pointed his Yagi at a tower, with a second tower lying 4-5 miles behind the first. He could never get proper decode, but continued to insist, since a yagi was from one small path, he couldn't be getting multipath; while true, he was still getting multiple mistimed signals (images) causing complete decode failure in his radio.
  • Scanner Location
    • Moving a scanner a few inches left or right and/or up or down can be a big factor, signals can have peaks and nulls, which creates areas of strong and weak signals, larger area's for lower frequencies and higher frequencies have smaller areas of peaks and nulls.
    • Moving a scanner can also help reduce local interfering signals as many electronics in the house or auto can create interference that overwhelm the front-end or any receiver.
  • Hairbrained solution get 'Joe' from the next county over to monitor your county and use your scanner to monitor Joe's county. You'll have to use a third-party solution to control and stream Joe's scanner and let Joe do the same with yours.

Scanner's real solution

The real solution: requires the manufacturers to redesign their hardware, so they implement I/Q demodulation. Doing so would: eliminate the possible loss of critical information required for successful reception and decoding of a Simulcast signal. Which, would allow for scanners to perform much more closely to commercial radios and pagers. As this problem is the result of the long-in-the-tooth Triple Heterodyne receiver with only AM and/or FM demodulator without Phase Modulation. Whereas, there seems to be very little hope for a firmware solution, APCO Phase II modulation uses CQPSK it is supposedly capable of backwards compatible with C4FM demodulators used in Phase I System equipment, and newer digital scanners do have DSPs that can preform some of the comparative and forward-error processing necessary in I/Q demodulation, the solution maybe under there noses or hoods if you will.

Software Defined Radios

Software defined radios are made up of a receiver USB device and a Computer to process the incoming System's signals - data and voice streams. Voice decode, can be superior to scanners, as the signal is processed via a I/Q demodulator, similar to how actual radios would process the incoming signal, less the narrower filtering and stability of commercial-quality oscillating crystals. Typically limited by only the CPU processing power, and filtering of local spurious signals.

To be honest, SDRs can be tedious to set-up, and Phase II cannot be done on a PC/Mac but more modes can be decoded than current scanners. Conceptional, but not currently, these set-ups are not really portable, nor can they decode multiple Systems at one time at least not easily like a scanner.

See APCO Project 25#Software Based Decoders for additional information.

Public Safety Pagers

Unication has developed a couple models of "Receive ONLY", P25 capable, Public Safety pagers. These pager/receivers are intended for "Public Safety Users." Therefore, they have use I/Q demodulators and narrower filtering for band specific needs. They work generally without any: "tweaks" that may be needed for a scanner, less better antenni for more reception outside an intend Site's coverage area.

To be clear, these units are Commercial-Grade receivers designed for Public Safety Users, not scanners. Programming as well as operation of these pagers are designed for Public Safety Users; designed to monitor a single trunk System's Site at a time, and possibly a conventional VHF or UHF Dispatch channel.

These units are currently limited to Phase 1 capabilities. It is reported that a Phase 2 upgrade (license) will be available for purchase at some point in 2018, with beta-testing confirmed Q1 of 2018.

Commercial Radios

Commercial radios greatly benefit from using hardware designed to process the signal via a I/Q demodulator and band specific filtering prior to any FM demodulation stage with much narrower and higher quality filters. This design allows the radio to process the signal with little loss of information and have constant forward error-correction processing algorithms written by the OEM to work nearly perfectly with the System they were intended to be used on. This solution is not be ideal for a typical scanner user, as it can be expensive and extremely complicated to setup correctly and cause can "Life Safety Hazards" for actual Public Safety users when they are set-up incorrectly.

Related Pages

Related Wiki Articles

See also the Cliff effect article on Wikipedia.

Discussions on RR Forums

Other Resources

  • 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.

External Links