Old Techo wrote:With FM (frequency modulation) the audio volume level is what causes the centre radio frequency to deviate. So soft is a small deviation and loud is large deviation. The loud limit will be set so as not to cause the deviation to go too far and break into the next channel. A bit like a large vehicle taking up more than 1 parking space.
Perhaps PD can add some value here or correct my fading recollections?
Adequately explaining the theory of how distortion of the audio signal is caused when the FM deviation is too high is a bit hard unless the readers appreciate that FM transmissions have side bands just as AM signals. However a simple explanation is that if you don't get all the side bands through the bandwidth filter in the receiver then you will get a distorted reception. This is more severe in FM as there are more side bands in a FM signal than the two in an AM signal.
A good explanation of
frequency modulation is in that link. The subject is started with the simple explanation of how FM and AM signals get their name. There is even a moving diagram showing the difference. That is how the signal looks when viewed with an oscilloscope. What the oscilloscope displays is the sum of the individual signals at the output of the transmitter. In the real world the output of a transmitter has many signals making up the transmitted signal. You need a spectrum analyser to display them. If you look at the table under the heading "Bessel functions" you will see how the number of side bands increase with increasing modulation deviation. In a nutshell that is why an audio signal of 3 kHz bandwidth requires a transmitted bandwidth of 16 kHz in a 25 kHz channel transmission where as an AM signal only requires 6 kHz.
Asking me to explain things has elevated the complex nature of discussion and taken the thread off topic. I suggest if members want to discuss things further then we should do it on the PM system.
