NFM vs FM

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HogDriver

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On the Pro-197, when I switch between FM and NFM, I don't notice any difference in the audio. I'm asking because I'm wondering whether switching this will make the radio less susceptible to interference from strong signals like paging transmitters and other close, strong signals. Right now I just press the attenuation to squelch it
 

SquierStrat

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It may help, depending on how strong the interfering signals are. If one was to work better then the other for your situation, it would be NFM. certainly worth a try..
 

gmclam

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NFM

These days, depending on the band, I would select NFM if you don't hear a difference in the audio level. It seems like some radio models merely employ a different filter and don't change the discriminator "width".
 

thomasbillman1

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NFM if basicly used in the 162.000 to 174.000 range. with the digital radios that the feds use. I find that comms come in a lot better.
 

N8IAA

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On the Pro-197, when I switch between FM and NFM, I don't notice any difference in the audio. I'm asking because I'm wondering whether switching this will make the radio less susceptible to interference from strong signals like paging transmitters and other close, strong signals. Right now I just press the attenuation to squelch it

You may just wan to set it to AUTO mode, and see what comes up on the screen for those frequencies. When in question, that's what I do.
Larry
 

N1BHH

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You may just wan to set it to AUTO mode, and see what comes up on the screen for those frequencies. When in question, that's what I do.
Larry

The Pro-106/197 can't tell the difference on incoming signals if it were NFM, it would show as FM (or DG for digital signals). Using NFM will help when you are in frequency congested areas, such as major metropolitan areas where adjacent frequencies are most likely in use in nearby cities or towns.
 

Mike_G_D

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To clarify some points here:

For the purposes of this discussion understand that the Radio Shack Pro-197 is the same internally as the GRE PSR600.

"Auto" modulation mode in both Uniden and GRE models uses a pre-programmed database to decide what mode to use in terms of NFM(FMN) or FM. So it is dependent on two major things: 1) the time (date) during which the database was created which determines what band vs. mode settings were allowed by the FCC (in the US) AT THAT TIME; and, 2) the correct interpretation of those rules by the Uniden or GRE designers/programmers.

Because the date the pre-programmed NFM(FMN) vs. FM settings were included likely predates the US narrowband mandate it is really no longer valid. Also, even when it was valid there were issues due to how the Japanese engineers (presumably) interpreted the US FCC rules as written at the time it (the pre-programmed list) was created. There were also issues with special conditions and allowances such that many systems were using the "wrong" mode on frequencies the "auto" mode thought should be something else.

For the most part, now, in the US, the best bet is to use NFM(FMN) on all analog FM between 148MHz and 512MHz. The federal portions supposedly changed early on but the civilian sections are SUPPOSED to be all changed over now as well. In general, the narrow band mandate did not apply to low band (30MHz to 50MHz) systems and did not apply to 700/800MHz systems. It also does not apply to amateur radio (HAM), "Marine Band", GMRS, and MURS (though MURS already had/has three "narrowband" channels).

Based on my testing, which I have gone into in detail many times in the past on these forums and will, therefor not do so now unless prompted (and PM would probably be a better way to discuss that anyway), I note that Uniden and GRE handle the NFM(FMN) vs. FM modes differently. They each handle what I consider "half" of the true total of what they should do.

Uniden handles the audio processing part of the "narrow" FM equation presumably by either simply across the board loudness boosting when in FMN mode (Uniden calls it FMN while GRE calls it NFM) or by modifying the discriminator settings to "map" frequency deviations accurately to perceived loudness. As far as I can determine, that is all that they do - they do no changes in IF filter response for the FMN mode. The weakness of this is that when you encounter closely spaced FMN signals you may have severe "bleedover" adjacent channel issues such that in strong signal cases you may "hear two channels at once" effectively. I've encountered this issue myself with my Uniden unit and it was severe enough to make it unusable for my original intended purpose (federal and state fire monitoring) to such a degree that I only now use it for low band CHP monitoring (where the narrowband mandate does not apply).

GRE, apparently, deals with the "other half" of the "solution" by ignoring the audio processing part (as far as I can tell) but instead activating a narrower IF filter bandwidth in some fashion (either by switching in a narrower IF filter or...some have suggested a software approach which I am skeptical of but cannot at this point completely rule out as I have no schematic and have not deemed it important enough yet to tear my radios apart and examine them closely). The advantage of this over the Uniden approach is better handling of adjacent signal issues (much, much, much better, in my experience!!) but the disadvantage is that there is no audio processing/compensation for the lower audio from the lesser deviated signals. You can offset this manually, to a limited extent, by using the GRE "audio boost" setting per channel to bump up the volume on said signals but this is really a "band aid" solution and not the best way to handle the issue.

I guess you can blame the economics of consumer scanner manufacturing and sales for the lack of desire on both GRE's and Uniden's part to fully handle the "narrowband" FM issue. I can, to some extent, see why Uniden omitted the narrower IF filter for expense reasons (though I fervently wish they would have simply used a better common narrowband FM [we're talking the sub+/-5KHz deviation versus the wideband FM and analog TV greater than +/-25KHz deviation] filter with steeper skirts as my Wouxun HT apparently has done) but I really cannot see why GRE didn't properly handle the audio processing issue when they already went through the expense and complexity of including a narrower IF filter setting given the relative cheapness of the audio processing!

Oh well...

-Mike
 
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gmclam

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GRE & NFM

Mike_G_D; Thank you for the detailed explanation. I have not used a Uniden with a NFM channel, so I don't know about them. But I have used all of the GREs and guessed they did not do it correctly. I can see them adding a filter then implementing their "audio boost" on the channel. Uh huh.

I can't say I've monitored a single signal that changes when I apply FM vs NFM though. BUT, if the signal is narrow, then the filter would have nothing to remove. And for whatever reason there's no audio boost.

 

KB7MIB

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The PSR-500 in Auto mode does not tell you if a signal is FM or NFM. It only switches between AM or FM based on a preprogrammed scheme, which isn't always accurate, such as in the 138-144 & 148-150.7 bands, where there's a mix of AM & FM useage.
 

Mike_G_D

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Mike_G_D; Thank you for the detailed explanation. I have not used a Uniden with a NFM channel, so I don't know about them. But I have used all of the GREs and guessed they did not do it correctly. I can see them adding a filter then implementing their "audio boost" on the channel. Uh huh.

I can't say I've monitored a single signal that changes when I apply FM vs NFM though. BUT, if the signal is narrow, then the filter would have nothing to remove. And for whatever reason there's no audio boost.


Not sure if you intended it but your post is showing up as in all italics for me - just FYI.

Anywho, I'm not sure I get what you're saying in your second paragraph; I can step you through a procedure to prove (or disprove) the actions (or inactions) of an IF filter change (or not as the case may be) if you like. This is without test equipment; if you have access to proper lab equipment then it's simpler and more exact, of course. I can do it either way but since I lack access to said equipment now I can only use the former (not using test equipment) method in practice at home. I've gone through it many times on these forums but I will again if you so desire or we can do so via PM.

Insofar as what the filter will or will not do versus the bandwidth of an FM signal - regardless of the deviation it will certainly have an effect. We "decide" that a certain bandwidth is acceptable for the proper reception of an FM signal at some chosen deviation; technically or rather theoretically, the bandwidth of the transmission goes on for quite some spacing (theoretically infinite) outside of Fcenter (ideally symmetrically about the center). But those sidebands become less and less significant the further out from Fc that you go and can be filtered out without serious degradation to the received and demodulated audio. So, for narrowband FM with deviations close to or under the maximum audio frequency response (FM deviation is directly related to loudness, of course, but is also tied to audio frequency range via the modulation index equation as deltaF/highest frequency modulating signal component) we can use receiver IF bandwidths in the 10KHz to 25KHz range. Generally, we take a value of much less than 1 as being narrowband FM while values much greater than 1 are considered wideband FM (like analog TV audio and analog broadcast FM). Anyway, the idea in terms of a receiver IF filter is that we determine at some point beyond which, bandwidth-wise around Fc, we wish the filter to "cut off" or seriously attenuate any energy. This is determined by the required bandwidth for proper reception (for analog systems Carson's rule works ok so we can use that as a general guide for what filter bandwidth to design for; about 11KHz for 2.5KHz deviated systems and about 16KHz for 5KHz deviated systems) and by the required channelizing scheme being used (for example, spacings of 15KHz or 7.5KHz, etc.). Many times it's, as in all things engineering, a compromise. In any case, whatever filter is being used in a receiver it will have an effect on the received signals both desired and undesired.

Just because all signals present in the RF passband prior to the IF stage of a FM receiver are all "narrow" in the sense that they are using the newer "narrower" narrowband FM deviation of +/-2.5KHz does not mean that they will not have complex undesired affects to the desired signal. Restricting ourselves to only adjacent and alternate channel effects (what we mean by receiver "selectivity" in the lab), let's say your receiver uses an IF filter with 6dB down at about +/-18KHz and 50dB down at +/-50KHz. With two or more 2.5KHz deviated signals located within 7.5KHz of each other as in the North American civilian VHF High band channelizing scheme and with sufficient power you will certainly experience issues trying to separate those signals. Even if they are spaced 15Khz from each other, with such a filter, strong or moderate signals even at 2.5KHz deviation can have an effect on each other. The effects would certainly be worse if one or more of the undesired signals were using wider deviation, of course, but with sufficient strength even the 2.5KHz deviated signals will cause problems even at 15KHz away from the desired. Now, if you change that filter to a narrower one with, say 6dB down at 11KHz and 60dB down at 18KHz you will, of course, see a difference, again, in either deviation case with moderate to strong levels of adjacent channel activity.

I've seen two ways to handle the IF filtering of 2.5KHz deviated analog FM signals versus 5KHz deviated signals - using one filter and using two (or somehow modifying the response of one filter as some claim is the case - for the sake of simplicity for this discussion we'll call that the same as effectively "two filters"). In other words, you use one typical IF filter for all "narrow" modulation modes and call it a day or you reduce your IF bandwidth for special "super narrow" modes like the new "narrower narrowband FM". I own two examples of the former method and three examples of the latter. The two of the former are my Uniden BCT15 and my Wouxun HT. The three of the latter are my three GRE made scanners, two Radio Shack desktop/mobile Pro-197's and one GRE PSR500 handheld. All three of the later units work similarly as one might expect - switching in the "NFM" filter reduces the IF bandwidth. The two of the former, however, while operating as one might expect (no IF bandwidth change) do behave differently relative to each other in the sense that the Wouxun HT has a more effective IF filter with, apparently, better stop band attenuation and maybe steeper skirts than the Uniden BCT15 (I would really need to put them on the bench with proper test equipment to fully characterize this, of course). In operation, it is obvious that the HT simply has a better IF filter than the Uniden scanner. So, while they may both compromise in the same way in the sense that they both only use one IF filter response to accommodate both 2.5KHz and 5KHz deviated analog FM signals the fact that the HT has a better overall filter means it works much better when dealing with closely spaced signals of either deviation than the Uniden. Again, I have proven this to myself in operation and can absolutely say that the Uniden has severe adjacent channel issues when confronted by multiple moderate to strong analog FM signals spaced 15KHz or less from each other regardless of deviation. The Wouxun HT simply works better because its one IF filter is better. The GRE made units in standard regular narrowband FM (what many now call "wideband" - ugh!) meaning the +/-5KHz deviation mode react similarly to the Uniden but when switched to "NFM" mode the IF passband is sharply reduced and is very evident when dealing with adjacent and alternate channel energy. When not in the presence of any such undesired signals you will likely not be able to tell any major difference in demodulated audio between the two modes as even the narrower setting seems wide enough for passage of most of the needed information bandwidth of a "wide" 5KHz deviated signal. Thus, even when listening to "wide" 5KHz deviated analog FM signals (such as Marine and amateur signals and those Part 90 units not yet changed over either with or without official FCC waivers) using the "NFM" mode can help when dealing with closely spaced interfering signals - a side benefit, of sorts, due to the GRE "laziness" of not changing discriminator response or otherwise compensating for the 2.5KHz deviation signal audio.

In short - you will likely not hear any change (or hear very little change) in audio in GRE made units when listening to a narrowband (either 2.5KHz or 5KHz deviation) analog FM signal when switching between NFM and FM modes but you will notice improvements in adjacent channel selectivity when engaging NFM mode over the FM mode. In Uniden units just the opposite will likely be experienced - you will notice audio changes when changing between FM and FMN modes but will see, or rather hear, no difference in adjacent channel selectivity; when no adjacent channel activity of sufficient strength is present, of course, it would be difficult to impossible to tell. At least, for the units I own, this has been my experience.

-Mike
 
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HogDriver

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Thanks mike. You answered my question to the dotted I and crossed T! As stated, I notice no difference in audio but can expect less interference from adjacent signals.
 

gmclam

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Thanks Mike.
Yes the post was in italics because of how I copied your name into the post. And the editor would not deselect italics. I could add bold, or change fonts; but italics was stuck on.
 

jfhtm350

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If the station is still wideband and you set it for narrowband you will notice the voice getting cut out. It just wont sound right at all. If the station is narrowband and you have it set on wideband you will notice a slight decrease in volume if that.
 

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That's how it *should* work, but apparantly that's not what the OP is hearing. He's not hearing any difference between FM and NFM.
 

gmclam

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Incomplete scanner functionality

jfhtm350 said:
If the station is still wideband and you set it for narrowband you will notice the voice getting cut out. It just wont sound right at all. If the station is narrowband and you have it set on wideband you will notice a slight decrease in volume if that.
That's how it *should* work, but apparantly that's not what the OP is hearing. He's not hearing any difference between FM and NFM.
Because these scanners cheat and do not presently perform ALL of the steps necessary to be in narrow FM mode.
 

Mike_G_D

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To be fair, using only one IF filter can be acceptable if that filter is tight enough and has decent pass band and stop band specs and steep skirts; my Wouxun HT, at least the one unit that I own, seems to be ok in this regard so if the Uniden had at least that good of a filter in its IF for narrowband signals it would be better.

To me, this means you can save some cost and use one decent narrowband filter for nearrowband FM signals with deviations at or below +/-5KHz (covers the "old" narrow FM [+/-5KHz deviation] as well as the "new" narrow FM [+/-2.5KHz deviation]) if you pick its characteristics carefully enough - at least for analog FM signals.

As in all things engineering, cost and other constraints come into play and it's all a compromise at the end of the day. I think, for analog FM signals, one IF filter could work as that is done by other companies besides Uniden and for a Part 90 type accepted transceiver, albeit a cheap one. I just think that the Uniden IF filter is not as sharp as it should be for this. In fact, now that most of the Part 90 US stuff is now (supposed to be) the "new" narrowband FM deviation it might be sufficient to skew towards optimizing for the narrower FM signals at the expense of the older wider ones (the ones most likely to notice problems with this, if the filter is so narrow as to cause distortion to the +/-5KHz signals, are those listening to Ham stations, marine traffic, and GMRS stations).

Using multiple filters adds to the complexity as the switching needs to be sound and low noise (and fast) for a scanner and the extra matching networks add more components as well. Then, too, because the passband attenuation may be different between the two different filters, sensitivity and noise figure may be affected differently and must be considered if the differences are severe enough. That is for actual hardware filters. I can't speak for SDR approaches; my knowledge of those is insufficient for me to make accurate comments about; SDR approaches may be a better and more cost effective approach with today's technology.

For standard hardware filters, there are also problems with choosing a tighter IF filter. One obvious one is the related need to tighten up the local oscillator and reduce drift as the center of the passband is now more critical. In "the old days" you could get away with wider filters as the crowding was less - this also meant that you could relax the local oscillator frequency tolerances as that wider filter gave you a lot of "slop" to play with. Even being off by as much as 5KHz would be ok if your filter was wide enough and you didn't have nearby adjacent channel signals to deal with. Now, with closer spaced signals and tighter filter widths the need for tighter frequency tolerance is much greater. This can be even more of a problem with digitally modulated signals. Unfortunately, for consumer scanner gear this is one area that could add considerable cost (at least it used to years ago - not sure now) as tightening up the frequency tolerance meant using at least a temperature compensated crystal oscillator (TCXO) rather than a basic crystal oscillator. These were common on professional gear but uncommon on consumer scanner gear and most ham equipment. Again, SDR approaches may alter this approach as user adjusted compensation might be more practical now.

Anyway, using the one IF filter with dual discriminator profiles (or some other post processing for audio loudness) may be the most cost effective approach if the choice of the one filter is done with sufficient care and attention. Digitally modulated signals add more complexity to this as the quality of the passband is even more important in some cases (such as those modes sensitive to group delay issues, etc.). I should note that the GRE's narrower filter setting DOES NOT work when in P25 mode - yes, you can engage it via the settings but they really have no effect and the wider filter is actually used when using P25 digital mode. This has been confirmed not just by me but by word of mouth from an "inside source" communicating to one of the admin's on this site. It's an old thread and I'd have to look for it but it's here somewhere. Anyway, I am guessing that they (GRE) did this because the narrower setting was more or less an afterthought in the design (it also cannot be used when in search modes, either pre-programmed service search or user defined) and too narrow a filter with possibly less than stable passband characteristics might adversely affect digital signals while analog FM is much more forgiving.

-Mike
 
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jhhays

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How to put Pro 197 scanner in nfm mode

How to put Pro 197 scanner in nfm mode. Im new to all of this. Live in Maricopa County AZ and I am trying to get a les choppy signal of p25 Mesa, AZ PD. people tell me to turn on Narrow. Could someone tell me how? THX
 

thomasbillman1

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NFM

Not going to work on p25 on NFM is more for narrow band such as forest service works well with the freequency range of 163.000 to 173.000. what soft ware are you using? Mesa is very choppy PD is the worst , mesa PD hates using it they use their cell phone a lot.Your ATT button works well with the p25.
 

jhhays

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Okay thanks Thomas

I am using the RadioShack software that came with the cord, I think I had to go online and pay maybe 30 go it. I will try to use att. In English, coul u tell me what it does? How do u think the active multiple Mesa pd feeds are so good?

Thanks again
 
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