Why does FM require more bandwidth?
IIRC, FM b'cast was made for stereo and required more bandwidth. then
there's a formula i can't remember the makes it so.
BW = 2.Fdev + 2.Fm for FM
BW = 2.Fm for AM - half that for SSB
Fdev is the peak deviation and Fm is the modulating freq. modulation
index, m, i think, is
m = Fdev/Fm - this determines whether it's NBFM or WBFM, but the FCC is
the final authority on what is considered allow in an NB band or
service. it's a f'd up read of the regs. lawyers make more sense
sometimes.
Is there some unstated assumption
included in that?
yeah, i just stated it
Is that to get the same signal/noise ratio?
FM give a better S/N for a given input, all else equal. in fact, the
wider the bandwidth the better the S/N. plus with FM, you don't get
amplitude fading problems. check some specs for different radios. you'll
see sesitivity figures for a given SINAD of 20dB of 1uV say for FM and
for AM i've always seen S/N of 14dB i think for the same input if not
higher input.
Or the
same effective noise as measured by human ears and typical/practical
receiver circuits?
i don't understand the question, but this is electronics, not
philosophy. of course, some distortions aren't noticeable to the ear and
you can screw up an audio amp if you trash one spec trying to achieve
another. like damping versus THD.
Suppose I'm interested in audio and I'm willing to throw many
transistors at the problem. Does modern DSP type processing help
any?
some people don't/didn't like the "mechanical" sound of CDs. it didn't
bother me. if you can throw transistors at a problem, you might come up
with something great. Fender still makes tube amps and i haven't seen a
transistorized fender amp schematic yet.
What sort of modulation would you use if you wanted the
best signal/noise out of a given bandwidth? What sort of question
should I be asking?
one that i can answer
i was doing ok, there. i like spread spectrum,
but that's like comparing apples to oranges. S/N is an analog
measurement and Eb/No is what SS is all about on the RF end. and SS is
wide BW.
i would say your best bet for best S/N in a given bandwith is to
transmit nothing
if you send one narrow band rf pulse with a (sin
x)/x (? the one that occupies the least bandwith) envelope per millenium
in a manner that gets it through one way or another - error correction
or whatever - then the S/N will be determined by the analog/audio
section because it's digital info. but 1 bit per 2 milleniums will have
less data bandwith. we're talking no bit errors here. what i mean is
that the data rate or analog modulation is propotional to the bandwith.
how about CW. morse code. the S/N is infinite if you can copy perfectly.
smoke signals. there's a good one. semaphore on a clear day. but you'd
have to use filters to narrow the visible light spectum to what's
neccessary, i guess.
brs and up yours for that last question ;-) make me think like that.
mike
