Question about FM audio

[Disclord]

Does the deviation of an FM audio signal determine the frequency response? I've have several books on the LaserDisc videodisc format and the specs for LD's analog audio channels list a deviation of +-100 kHz. The final, decoded, audio has a bandwidth of 20 Hz to 20 kHz, but I have seen elsewhere that the actual upper limit is on the order of 80kHz. The RCA CED videodisc format used FM carriers with 50 kHz deviation - and the final frequency response was 30 Hz to 15 kHz - it did make it out to 20 kHz, but about 5 db down.

So, what determines the frequency response in an FM encoded audio signal? Is it the deviation or something else? I have read lots of stuff on FM encoding, but can never find the answer to that specific question.

 

[Fnord]

No.

A deviation of +/-100kHz means only that the peak-to-peak amplitude of the modulating signal (1Vpp) will deviate the frequency of the carrier by +/-100kHz.

That is, when the audio reaches +1 volts, a 99.9 MHz carrier will deviate to 100.0 MHz, when the audio reaches -1 volts, the same carrier will deviate to 99.8 MHz.

Frequency modulation is a method of converting amplitude variations into variations in frequency.

Simple, eh?

 

[BenBurch]

The deviation encodes an amplitude variation. The underlying base "carrier" frequency of the signal determines how often you can vary the encoded amplitude and hence the frequency response.

EDIT: Damn you FNORD, you beat me to it!

 

[MortFurd]

The carrier and any intermediate frequencies along with the demodulator used in recovering the audio determine the maximum bandwidth.

The bandwidth of your audio signal cannot ever be more that half of the carrier frequency. There's long explanation for this. Google "Shannon' Theorem" and "NyQuist cutoff" for more info. For radio receivers, you almost never directly demodulate the carrier. You convert it to a lower frequency first and demodulate that. Analog two-way radios use two stages - carrier down to 21. something MHz, then down to 455kHz. The 455kHz is then demodulated to provide audio.

The demodulator converts the frequency variations in the carrier back to voltage variations (audio signal.) The demodulator has its own (designed) bandwidth. This probably has more to do with the bandwidth than anything else.

There's more to it, but that should get you started in the right direction.