kfbkfb said:
According to this information:
http://www.ycars.org/EFRA/Module B/fmapps.htm
When only one sideband and the carrier are transmitted,
the NBFM signal occupies the same bandwidth as an AM signal.
This overcomes one of the drawbacks of wideband FM,
the large bandwidth required.
Maybe the existing chear channel stations could keep using
AM (maybe with C-QUAM Stereo and of course no IBOC)
and the local AM stations could change to NBFM.
Kirk Bayne
That web page is SERIOUSLY wrong...
1) a FM signal can have a MI or DR of >1 and still be NARROW band....in fact, common FM used in two way (commercial, PS, ham, etc) is 5 kHz dev with 3 kHz audio...thus the MI (Modulation Index) is 1.333 and it IS NBFM......WIDEBAND twoway FM is considered 15 kHz deviation and that would have 15/3 or a MI=5....BC FM is 75/15 or 5 as well......
but they are WRONG on NBFM...
2) also they state bandwidth of NBFM is 2*fm or the same of AM...THAT IS WRONG!! AM is 2* mF (modulating Freq).....FM is 2X (DEVIATION + MOD FREQ)....which is much more than AM for the same audio freq....EVERY EE knows that formula....and most hams and broadcast folks know it too...
FM IS used below 30MHz...hams use it on the 10mtr band (29-29.7) and below 29Mhz as well BUT they are limited in deviaton and MI below 29....thus hurting the signal to noise...
COMMERCIAL use of FM has been done on 25 and 26 MHz...some broadcasters still use the 26MHz RPU channel....which is 5 kHz deviation FM...
THE BIG problem on freqs below 30Mhz (and this is seen on the 30-50Mhz range as well) is phase distortion as the skywave signal changes with propogation....on the MW band, FM would sound good locally but DXing, the FM skywave signal would be highly distorted most of the time and unlistenable....Again, their page is wrong.
To quote them:
"NARROW BAND FM (NBFM)
NBFM is widely used in business and public service communications. The DR for NBFM is restricted to values between 0.5 and 1.0. By holding the DR to such small values, only the carrier and the first sideband are of significant amplitude. When only one sideband and the carrier are transmitted, the NBFM signal occupies the same bandwidth as an AM signal. This overcomes one of the drawbacks of wideband FM, the large bandwidth required."
WHO wrote this crap??? FM IS FM not matter what the MI (DR?? noone calls it by that term...Modulation Index is the engineering term for it)....B/W for conventional FM ALWAYS follows Carson's formula...
Even on NBFM, the primary sidebands is NOT where the signal stops....What they meant in the above is taking the carrier and the primary sidebands on either side....I have read and seen articles on SSB-FC FM (singlesideband/fullcarrier FM)....or even SSB-RC (Reduced Carrier) and it would make the b/w less...BUT you now have to choose which sideband side to keep thus offseting the carrier OR you will have something similar to NSTC VSB TV. Aint gonna happen in our lifetime on MW....
There is NO more clear channel AMs....Class A stations are only protected to about 750miles out iirc...they are no longer clear (as say WLS was back in the 60s and 70s with no other 890 anywhere at night)
IBOC just needs to go away and the RECEIVERS need to be fixed...thats where the problem has always been on the AM band...AMAX standards, Noise Blankers, DSP (especially now) filtering and more could be used to improve the audio....I would like to see AM stereo back....but it wont happen...(and Im an old 70s AM Top40 jock....I LOVE WLS' Rewind and the opening song Landecker started with this year said it all...You could hear your music on the AM Radio!
