[Ari Ben Canaan]

So the bandwidth of FM radio is around 200 KHz.

This implies that FM is able to broadcast a carrie signal of maximum frequency = 100KHz.

However, I am aware they dont broadcast at such a high frequency. Instead they usually broadcast a carrier signal of maximum frequency = 15KHz.

Is this because the hearing range of humans is 20Hz-20KHz ? Hence, a higher frequency would not be heard so it makes no sense in broadcasting and cluttering up the spectrum ?

[Stonebridge]

Yes. If you are broadcasting audio frequency information then there is no need for frequencies above about 20,000Hz.
In fact for speech alone the bandwith is usually very narrow. For example, two-way radios can have a bandwith of around 3kHz.
Music needs a higher bandwidth.
Be careful with your terminology.The carrier wave for FM is between 88 and 108MHz.
It's the modulation frequency of that wave that you are referring to with your question.

[MotsMan]

Stonebridge has it right.

Say you want to transmit some voice signal. This is at a low frequency at around 3000Hz. If you were to try and pick up this signal with no modulation you'd need a huge antenna for your radio.

Thats why in FM you use frequency modulation, by modulating a high frequency carrier wave with your information signal. The frequency of the info signal changes the frequency of the carrier and this new modulated signal is transmitted. This is at a much higher frequency so you dont need as large an antenna.

Other reasons for modulation include you can modulate a carrier so multiple signals can share the same medium.

[Ari Ben Canaan]

(Original post by Stonebridge)
Yes. If you are broadcasting audio frequency information then there is no need for frequencies above about 20,000Hz.
In fact for speech alone the bandwith is usually very narrow. For example, two-way radios can have a bandwith of around 3kHz.
Music needs a higher bandwidth.
Be careful with your terminology.The carrier wave for FM is between 88 and 108MHz.
It's the modulation frequency of that wave that you are referring to with your question.

I realise 2f_{info signal} = bandwidth of AM waves.

So the bandwidth of AM is around 9KHz. In this case would it be right to say that an informational signal of 4.5KHz is the highest that can be modulated and then transmitted ?

[Stonebridge]

Yes. The bandwidth is twice the frequency of the maximum information signal.
This was covered here in one of your previous questions.
http://www.thestudentroom.co.uk/show...ight=bandwidth

[Ari Ben Canaan]

My textbook is very confusing. Its almost as if they're implying that it is an information signal of 15 KHz that can be transmitted.

See 4th line in the red box.

EDIT : Ignore above. I just realised my mistake.... I was just a bit confused.