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Revision as of 12:59, 28 November 2007 by Loumaag (New page about digital distortion issue in regard to simulcast sites)
Problems receiving simulcast digital audio
The current 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 believe to be crystal clear.
This problem is due to the scanner receiving multipath signals and not being able to properly correct the digital signal so that the audio can be recovered. 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. 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 IMBE 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 comes when this multipath signal is just not exactly the same (due to time/distance issues) when received by the scanner and the vocoder has trouble correcting the slightly out of sync signal it is receiving. This 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 real radios (the ones actually used by the subscribers), then P25 CAI (indeed any digital signaling) would not be acceptable for a public safety use. It should be pointed out that this multipath distortion is much less prevalent in real radios but in fact this does happen a little to real radios. You may hear some user actually comment about it when asking someone to repeat something that "broke up" when you heard the requested repeated transmission just fine. In addition, the system engineers have the advantage that they can "tweak" the actual system's timing values until the problem is all but eliminated.
Since we, the scanner hobbyist, are not paying really big bucks for a system that must work properly, the error correction algorithms used in the scanner to deal with this problem are not nearly as strong or as good as those used in real radios. This is not particularly the manufacturer's fault but rather a situational problem. To properly come up with a better algorithm for error correction, the software engineer must be able to "tweak" the code by repeated trial and error. And once it works for system 'A', will it work as well for system 'B'? Also, if the specific system that you are listening to has not been "tweaked" to the exact right specifications, this will just exacerbate the problem for the scanner user. Remember, actual systems can be adjusted properly or nearly properly and that one variable can cause us problems.
There is no "one" solution to this problem; however, there are things that you the scanner user can do to mitigate the problem depending on your situation.
- 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.
- Keep your firmware updated in your scanner. The scanner manufacturers are indeed addressing this problem as more and more of these simulcast digital systems come on line. 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.
--Lou Maag 17:59, 28 November 2007 (UTC)