Lostgallifreyan wrote:
Jerry Stuckle wrote in news:lriogq$kba$1@dont-
email.me:

In an FM transmitter, the amount of deviation is proportional to the
amplitude of the modulating signal. That is, the louder the audio, the
greater the deviation.

Transmitter bandwidth cannot be greater than receiver bandwidth without
loss of signal and distortion. Transmitter deviation can be less than
receiver bandwidth; this only results in lower audio. This is why
commercial FM receivers typically have a bandwidth of 170-180 kHz, when
deviation is only 75 kHz. It makes things less critical.


Ok, so a kind of 'headroom' exists then, but is there some agreeable degree
of reduction on transmitter bandwidth to gain a significant reach in RF
propagation and reduction in intrusion across the band, before the whole
exercise fails due to poor SNR on output? My guess is that there may be, but
the decision could be very different depending on whether speech or music
(let alone stereo) is wanted.

Deviation is something different than bandwidth. To receive an FM signal,
as a rule of thumb you require a receiver with a bandwith of about
twice the deviation plus twice the maximal audio frequency. So to receive
a signal with 15kHz audio and 75kHz deviation, you require a bandwitdh
of about 180kHz.

This is not "to make things less critical". It is a requirement because
of the characteristics of FM modulation, where sideband frequencies are
determined by a Bessel function.

The ratio between deviation and maximal audio frequency, the modulation
index, determines the signal/noise ratio after demodulation for a strong
signal. So for music transmissions on the FM broadcast band a larger
modulation index (about 5) is used than for communication (usually 1 or
a bit less). That means at typical signal strengths encountered in
broadcast use, the signal/noise ratio is better than for small modulation
index.
(for weak-signal work, where you are looking for an intelligible signal
and not for lowest noise, a smaller modulation index performs better
because you can use a narrower receiver bandwidth)