narrowband FM vs. FM

[baddandy23]

what is the difference between narrownband FM and just FM??? My 96 does only FM but my new 246T does narrowband also... is there any bennefit with the narrowband? thanks. :?

 

[pro92b]

There is a lot of confusion in how the modes are named. NFM as Uniden calls it is for use with the new split channel assignments on VHF and any new licenses which will use 2.5 kHz deviation instead of the old 5 kHz. Stations that have been on the same channel for more than a year probably use the old 5 kHz deviation equipment. Those that have changed frequencies or updated their equipment will use the new 2.5 kHz deviation.

FCC license records show which type of system is in use. Check the emission type - if it starts with 11 it is NFM and 16 is FM. That's from memory and if I have it backwards someone correct me.

The PRO-96 has only one mode and they call it FM. However the IF filter is narrow so this actually is the same as Uniden's NFM. The older PRO-92/93/95 have wide filters but the mode for these radios is also called FM even though it corresponds to Uniden's FM mode, not NFM. Confused yet?

The only time you should use NFM in a Uniden scanner that has FM and NFM capability is:

1. If the station is 2.5 kHz deviation
2. If it cuts out an interfering station on an adjacent frequency
3. If the audio on the station is unusually low

The gain of the audio path in the scanner for NFM mode is twice that of the FM mode. Setting the modes properly will help to even out the volume if you scan FM and NFM stations together. Setting the modes wrong will cause large volume differences. FM (5kHz deviation) stations received in NFM mode will sound louder than normal due to the extra gain of the NFM setting.

 

[INDY72]

Hope this doesn't confuse you even more.....

There are more than just that, you have FM broad which is like your FM broadcast radio stations-93.5, 93.7 etc.
Then there is NFM (Narrowband FM), which has been used for many many years. This is the older freqs you used to see issued by the FCC for LMR-155.6100, 155.6250, etc.
Then there is the newer SNFM (Super Narrowband FM), which is narrowing it even tighter in the prelude to , and beginning of the phase in of APCO P-25 digital-155.6175, 155.8575, etc.

This "narrowbanding" allows for more stations in a tighter bandwidth than was possible previously, and with the new digital modulation comming into play, it's gonna get even more narrow before long.

The techy side would say its moving from NFM which was 5 KHz, to 12.5 KHz spacing, down to 7.5, and 6.25 KHz spacing etc.....

Most of the older scanners do not properly support SNFM, and round it to the closest NFM freqs,... The newest are beginning to allow for the SNFM spacing, but the Radio Shack scanners do not actually call it NFM, much less the right name which is SNFM,.... Its just another of the minor faults in the scanning world, sorta like not supporting 700 MHz Trunking properly yet. With time comes improvements.

 

[fireant]

How do I know if I have a older Pro 95 or not ? does the newer pro 95 accept narrow if I understand you right? Just thought I would ask excellent information though.

fireant

 

[pro92b]

There is only one version of the PRO-95. The PRO-95 is older than the PRO-96 (an older design). The PRO-95 does not have NFM bandwidth.

 

[N_Jay]

Hope this doesn't confuse you even more.....

. . . . The newest are beginning to allow for the SNFM spacing, but the Radio Shack scanners do not actually call it NFM, much less the right name which is SNFM,.... . . . ..

SNFM is NOT the "right name".

It may be a name that is becoming common in the scanner world.

2.5 kHz Deviation FM is still NFM (or FM-N), just at a lower deviation and bandwidth.

And P25 is not FM at all.

------------------------------------------------
P.S. Not picking or MILF, just trying to help everyone keep this confusiong topic strait.

 

[INDY72]

The first steps to going to P-25 digital ready in many cases, including the Fed freqs.. is to go to SNFM spacing.... as i siad.. its all part of the big picture...

 

[N_Jay]

The first steps to going to P-25 digital ready in many cases, including the Fed freqs.. is to go to SNFM spacing.... as i siad.. its all part of the big picture...

Or, the first step is to implement P25 on the existing channels.

Like most of the State and Local agencies.

 

[colheli]

Narrow band FM really has nothing to do with APCO Project 25. P25 is a standard signalling "protocol" for digital radios, whereas NB is a function of bandwidth. They just happen to both be coming about at the same time. Eventually, ALL FM comms will be 12.5 Khz spacing, but not necessarily P25. Previously, the frequency assignments were spaced 25 Khz apart (151.000, 151.025, 151.050, 151.075, 151. 100, etc.) Narrowband allows two licensees per 25Khz bandwidth. (151.000, 151.0125, 151.025, 151.0375, 151.050, etc.) The FCC is mandating narrow band for ALL FM users, in the next 10-20 years. They are NOT mandating P25. Although many folks are under the impression that narrow banding is a first step to P25, it really is just a coincidence.

 

[pro92b]

VHF-HI channels were allocated with 15 kHz spacing. The new split channels are placed between these, 7.5 kHz away.

UHF is spaced at 12.5 kHz intervals.

 

[N_Jay]

Old VHF; 30 kHz channels at 15kHz spacing with separtation requirments and no defined repeater pairs

New VHF; 7.5 kHz Channels at 7.5 kHz spacing with no separation requirments.

Old UHF; 25 kHz channels at 25 kHz spacing, with defined 5MHz repeater offset. "Interstitial" or "Splinter" channels assigned at 12.5 kHz off main channels with various requirments. (bandwith, power, geographic separation, etc.)

New UHF; 6.25 kHz channels at 6.25 kHz spacing with allowance for pairing to create 12.5 kHz channels for 2.5 kHz deviation FM (SNFM, as some :wink: like to miss-name it), or 11 kHz bandwidth digital modulation.


Also P-25 is more thna just a "protocol", the standard also includes a modulation specification which is designed for 12.5 kHz channels.

Narrowbanding is not just a coincedence, P25 was designed with narrowbanding in mind.

Analog 12.5 kHz, is (in general) not just a step to P25. They can be implemented independantly. They are two different ways to implement a narrowband channel.

I think I hit all the obvious misconceptions. :wink:

 

[Voyager]

what is the difference between narrownband FM and just FM??? My 96 does only FM but my new 246T does narrowband also... is there any bennefit with the narrowband? thanks. :?

To cut to the chase, you will notice that SNFM (and official term or not, you have to distinguish the differences somehow) users will sound low in audio compared to NBFM users. On the PRO-96, there is nothing you can do about this, and the IF on the 96 is the same as any other radio - it is NOT significantly more narrow than previous scanners. I know this because I get just as much interference on a SNFM channel from the adjacent users as I do on a NBFM land mobile radio. The receiver may be designed better, but it is not more narrow than a typical NBFM receiver. The PRO-96 supports the STEPS for SNFM, but not the mode. (I.E. 15X.XXX5 frequencies)

On the BC246T, however, the receiver IS tightened up in NFM mode to bring up the audio on SNFM users to a level common to that of NBFM (FM) users.

To summarize the differences:
WFM (typically 75 kHz deviation - 200 kHz bandwidth)
FM (typically 15 kHz deviation - 40 kHz bandwidth - more or less)
NBFM (The most common - typically 5 kHz deviation - 16 kHz bandwidth) [Uniden calls this mode FM]
SNFM (typically 2.5 kHz deviation - 11 kHz bandwidth) [Uniden calls this mode NFM]

FM is nearly extinct (but not completely, yet), and not supported on any scanner made in the last 30 years (that I know of) although those that receive WFM will receive FM fairly well.

Joe M.

 

[pro92b]

the IF on the 96 is the same as any other radio - it is NOT significantly more narrow than previous scanners

Wrong - I know this from the service manual. It is as I said.

 

[N_Jay]

the IF on the 96 is the same as any other radio - it is NOT significantly more narrow than previous scanners

Wrong - I know this from the service manual. It is as I said.

OK, now you have me confused.

Are you saying the 96 DOES have a mode designed for 2.5 kHz deviation FM that switches to a narrower IF and/or more audio gain?

Or,

Are you saying it does not have such a mode, and only has the new channel center frequencies?

 

[Spleen]

This may help...this is from the spec page for the wideband radio I use currently (non-mobile):

About the only thing Wide-FM reception capability gets you around these parts are FM-Radio and Television Audio reception...aside from the occasional high-end cordless phone or baby monitor...you can tell someone is using a Wide-FM phone or monitor because the audio will sound incomplete or chopped up until you switch to WFM mode...

If your scanner doesn't support WFM, you can try to listen to TV audio, but it will sound like crap, unless you tune to the IFB frequencies, which are Narrow-FM.

 

[Voyager]

the IF on the 96 is the same as any other radio - it is NOT significantly more narrow than previous scanners

Wrong - I know this from the service manual. It is as I said.

Specifics, please???

If you go too narrow, NBFM signals will exceed the passband of the receiver and chop out. Most NBFM receivers have a passband of about 20-22 kHz, so when the signal get above 7 or 8 kHz, it will chop out.

With a SNFM receiver, it will have a passband of about 15 kHz. With that kind of passband, a signal will chop out at about 4.5 kHz. Since most NBFM transmissions peak between 4.5 to 5.0 kHz, most would sound choppy since they would frequently exceed the passband. They do not.

So again, what is the passband of the PRO-96?

I can always hear an adjacent channel user that is about 20 miles away on the adjacent SNFM channel (specifically, hearing 155.505 MHz on 155.4975 MHz) on my PRO-96. I hear it the same as my NBFM Maxtrac, if not slightly worse on the PRO-96. As I said, it may be better than previous models, but still is about NBFM spec.

All I can get from the user's manual is a -6dB selectivity spec of 20 kHz bandwidth (+/- 10 kHz). That is IDENTICAL to the PRO-95 and IDENTICAL to the PRO-94. So, where is this new selectivity coming from?

In any case, you only have one mode so you cannot make up for the bandwidth difference between SNFM VS NBFM. I don't believe the AGC has any effect in analog mode.

Joe M.

 

[pro92b]

Are you saying the 96 DOES have a mode designed for 2.5 kHz deviation FM that switches to a narrower IF and/or more audio gain?

No, the PRO-96 does not have switchable selectivity in the IF. It uses a Murata CFULA455KF1Y filter. It has only four elements (economy type) and a -6 dB bandwidth of +/-6 kHz.

The PRO-93 and PRO-95 use a Murata CFU455D2 filter, also four elements but with a -6 dB bandwidth of +/-10 kHz. Most scanners use the +/- 10 kHz type filter. Those with selectable modes (NFM vs FM as Uniden calls it) will have 6 kHz and 10 kHz filters. The BC780 schematic shows the two filters selectable by a CMOS bilateral switch.

Professional radios will use better filters having more elements for superior stop band rejection.

I can post the filter datasheets if you are interested.

Addendum: The PRO-96 manual I downloaded from Radio Shack dated 6/03 show the selectivity as +/-10 kHz. The newer owners manual (7/03)included with the service manual lists the selectivity as +/-8 kHz. The parts list definitely shows a 6 kHz filter. This bandwidth is fixed for all FM mode reception. There is a different filter for AM.

 

[Voyager]

Are you saying the 96 DOES have a mode designed for 2.5 kHz deviation FM that switches to a narrower IF and/or more audio gain?

No, the PRO-96 does not have switchable selectivity in the IF. It uses a Murata CFULA455KF1Y filter. It has only four elements (economy type) and a -6 dB bandwidth of +/-6 kHz.

The PRO-93 and PRO-95 use a Murata CFU455D2 filter, also four elements but with a -6 dB bandwidth of +/-10 kHz. Most scanners use the +/- 10 kHz type filter. Those with selectable modes (NFM vs FM as Uniden calls it) will have 6 kHz and 10 kHz filters. The BC780 schematic shows the two filters selectable by a CMOS bilateral switch.

Professional radios will use better filters having more elements for superior stop band rejection.

I can post the filter datasheets if you are interested.

Forget the datasheets. I would like to see the service manual stating this filter is used. That means the user manual is wrong, and the radio should chop on NBFM transmissions. Has anyone actually confirmed that filter is actually used in the scanner?

For a bandwidth of 12 kHz, and a signal of 16 kHz, the radio should chop. Granted, that's only a -6 dB reference, but it doesn't make any sense. The radio should perform much better with adjacent channels if the narrow filter is used.

I see the stop bandwidth is 17.5 kHz. That's cutting things AWFULLY close with a 16 kHz bandwidth signal. That means that if anyone is even slightly overdeviating, they should cut out. I happen to have a system that is a little hot (it's ham band, so it's legal - it's also 440 so there are no 'neighbors') and I'm sure the peaks get very close to that 17.5 kHz bandwidth spec.

Of course, that still doesn't change the fact that only one mode is supported. The audio level difference will remain.

Joe M.

Addemdum to your addendum: Is it possible there was a production change and the earlier units DO have a wider filter?

 

[pro92b]

Looks like you missed the addendum. Yes, the owner's manual is wrong, at least the old one is. There isn't room on the filter for the whole part number but the selectivity is usually marked by a letter. F is +/-6 kHz and D is +/-10 kHz. I don't have a PRO-96 to look but the filter may be visible if you take the rear cover off.

 

[pro92b]

With the two manuals being only a month apart, it is unlikely that the PRO-96 ever had a wide filter. It was based on the PRO-95 design and the manual probably started out as a copy of the PRO-95 manual. I think they just missed the update on the selectivity spec initially.