Can someone explain to me, in layman's terms, what the difference is between the new NB FM vs. the old WBFM in terms of listening on an older scanner? Is the modulation scheme different? I'm talking about an analog signal at this point.
Well, I would have to say, that given this clear and well written question, I would be confused if I tried to learn the answer from this thread. :wink:
So since you are asking about "The new NB FM" with regard to scanners, we will not need to go into any discussion on true wideband FM and used in broadcast FM and the audio channel of analog TV. Nor do we need to confuse the discussion with the technical definition of WBFM vs. NBFM as it is irrelevant to the question.
For this discussion we will consider WBFM as the mode used as the standard for the land mobile industry for most of the last 40 to 50 years.
We will likewise consider NBFM to be the mode used in the land mobile industry, starting in limited applications about 20 years ago, spreading to fairly common use over the last 10 years, and becoming the standard requirement over the next couple of years.
Since you asked specifically about analog, we will stay with FM as used by the land mobile industry and not add all the confusion added by trying to mix a discussion about digital into the answer.
So here is the simple answer to your clear question. It is not a simple subject, so please read though this answer at least twice carefully.
There are two interrelated pieces you need to understand. Sometimes it is easier to do visually than with a narrative, so grab some scrap paper and start drawing.
Channel spacing. Think of the spectrum as a lone and channel centers as tick marks on the line. The ticks are channel center frequencies, and the space between the ticks is the channel spacing.
Channel bandwidth is the size of the "Lump" of information placed on the channel.
WBFM typically used a channel spacing of 25 to 30 KHz to place channels that occupy about 20 KHz of signal. (These are generated with +/-5 kHz FM modulation)
NBFM typically used a channel spacing of 12.5 to 15 KHz to place channels that occupy about 11 KHz of signal. (These are generated with +/-2.5 kHz FM modulation)
So if a WBFM receiver is trying to listen to a NBFM signal the sound will be relatively soft, and you may here the adjacent channel (If one is there).
If a NBFM receiver is trying to listen to a WBFM signal the sound will be relatively loud and may be distorted since some of the signal may go beyond the "window" the receiver is designed to receive.
Now we get another level of confusion, because your scanner is designed to be able to tune the channels it was designed to receive so it has a certain channel step size. It may seem to be a continuous tuning receiver, but in fact it can only tune to specific frequencies.
If the world was perfect that step size would only have to match up with the channel spacing for the band you are in, but history has screwed that up.
Over time channels have been spaced at both 25 and 30 kHz spacing on certain bands, and over time channels have been assigned on the lines between the normal channel spacing positions. For this reason scanners are designed with various (but minimized) channel step sizes.
With the move to narrowband, and therefore the creation of additional channel positions (tick-marks) many new channels may not fall onto the step positions of a scanner designed only for WBFM use.
In general if you can get the receiver to a frequency "within" the signal, it will receive the signal. so if your scanner will not go exactly to the channel center it may not be a significant problem. (much like listening to a NBFM signal with a WBFM receiver.
I hope this helps.