Hmmm...ok, let's see, how do I parse this all out...
Firstly, I would be surprised if self contained consumer scanner receivers designed to decode P25 CAI voice do not use any form of error correction! In all of my professional experience, having no error correction at all on even hard wired systems, let alone much more channel quality variable wireless systems, in real finalized working in-the-field systems would be irresponsible and problematic at best! I would think that these scanners (just so we're clear, we are talking about the current crop of fully self contained non-SDR consumer units like the GRE PSR500/600/800 and Uniden BC996XT,BC396XT, HP-1) have at least SOME form of error correction once the bits have been recovered by the hardware. I REALLY would be surprised if this was not the case!!
So I am going to go out on a limb and say that they do have some error correction but how good it is is open to discussion.
Once again, once you've recovered the bits and stuffed them into a DSP or FPGA or whatever, and start working software and/or DSP firmware magic on them, it is beyond my comfortable knowledge base. Essentially, I turn into a fairly useless idiot. So I defer to others to discuss the details and ramifications of actions within that realm.
Up to that point, based on what I know, there are various pieces of hardware that become important to various qualities in the received signals. In terms of simulcast distortion the quality and nature of the demodulator block(s) would have quite a large impact on the demodulated signal quality. The rest of the RF and IF chain would certainly be of value BUT would affect the reception of all other types of signals also - in other words, if your IMD is poor all modes would be adversely affected, as would also be the case with IF adjacent channel rejection, 1/2 IF rejection, etc. So, for the purposes of the primary discussion at hand, from a hardware standpoint alone, the demodulator quality and design is of paramount importance (with some impact possibly from the final IF filter response and quality, group delay, etc. - we probably need to keep that in mind once the impact of the demodulator is fully understood). Given this, I know that a true I/Q demodulator is the best way to handle more complex digitally modulated signals such as PI/4DQPSK and, presumably (this is where there is some uncertainty in this thread) CQPSK.
I had forgotten about that thread you linked to, rak313. I do remember following it way back when, along with many other similar ones, but, as I have found with many hardware based threads of a similar technical bent, they seem to come to an abrupt end and leave me hanging. I recall that KA1RBI was one of my favorite posters to follow regarding this but he seems to no longer frequent these forums much (though I don't scour every section). I think that what happens is that a lot of these folks are very involved in the USRP and GNU radio efforts and bring the meat of the discussions back over to sites devoted to those technologies. When it comes to sticking with more traditional superhet receivers and consumer self contained units a big problem is the lack of information - those that do know do not tell (largely because, I imagine, they are bound by company confidential information policy) and those of us who do not are left to speculate, as we are doing, using partial and incomplete information, though the speculation might be "fun" it can also be frustrating and form frequent "recursive loops" in the resulting discussions.
Now, I looked at the 455KHz IF I/Q link and did not see where the Pro-2006 was used, but I did not scour the article due to time - I saw a Collins rig being used (if I understand it, he was using the Collins rig to sniff the IF off of another receiver which may have been the 2006. I'm not sure why the Collins was used rather than simply hard tapping into the final IF of the VHF/UHF receiver - maybe because he wanted to use the Collin's superior filtering? But then the Collins is downconverting the IF to audio which seems an unnecessary extra step). Anyway, I need to read it over again in more detail. What you should know is that the 2006 is an older analog only scanner that, I am pretty sure, used a basic FM discriminator switched with an AM detector when using that mode. It has not been in production for many years (ten or so, I think, at least?). So I wouldn't use it as an indicator of current technology in consumer radio scanner design for demodulating P25 CAI voice. Also, if he is just sniffing the final IF, that is prior to the demodulator of the scanner and so completely negates that circuit (and replaces it with the Collin's IF and audio plus the external circuit).
Now I am going to ask what will be to many on this board a really stupid question. As I have said many times, I am really ignorant when the bits have been recovered and are being played with inside a DSP or FPGA, etc. My question is, can we now fully implement a software analog of an I/Q demodulator - that is, in software alone, presumably using DSP magic, can we "make" an I/Q demodulator that performs just as well and just as reliably as a hardware I/Q demodulator? Now I can see maybe two ways to go about this given my current lack of knowledge of the subject - using an high level FPGA that is software programmable so as to write code that makes the FPGA implement an I/Q demodulator using the digital gate structure of the FPGA or actually performing DSP magic on the digitized IF bits within a DSP that performs the same function as what a hardware I/Q demodulator would do. The former method implies that, in the end, digital hardware is used to perform analog processing, I am guessing through very fast switching and sampling, and is configured through software while the latter method literally performs the "I/Q demodulation" function entirely using digital signal processing functions. I am ashamed to say that my knowledge has become so rusty that I must admit uncertainty regarding the former method but the latter is completely outside my knowledge base.
The point of my question is that, if the I/Q hardware is no longer needed and can be implemented using software, then the receiver design simply needs to downconvert to an IF for A/D conversion and taken from there - no hardware demodulator is used at all. As I understand it, this is considered a form of hybrid SDR. Now, how easy and cheap is this now? And how effective is it under all RF channel conditions relative to what you get with a good quality hardware I/Q demodulator? Is it is now so cheap, easy, and reliable so as to simply use the RF and IF to do the downconverting only and omit any further dedicated analog hardware for demodulation and signal processing and perform all other tasks within DSP engines and/or FPGA's or whatever (after first doing A/D, of course, but that could also be integrated on a large scale SOC)? In that case, if so, then the types of consumer scanners we are talking about here could use (and maybe do use?) this method of post IF SDR which would mean that all of the heavy lifting regarding handling of complex signals (like LSM CQPSK) would come down to written software code - in that case, firmware updates might then have hope for fixing all simulcast distortion ills.
-Mike
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