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Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier - (so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

The original sinusoid still exists; we have added information to it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

All the other answers here are accurate - I am simply trying to answer from a different perspective.

Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier - (so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

The original sinusoid still exists; we have added information to it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier - (so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

The original sinusoid still exists; we have added information to it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

All the other answers here are accurate - I am simply trying to answer from a different perspective.

2 added 74 characters in body
source | link

Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier  -  (so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

The original sinusoid still exists; we have added information to it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier-(so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier  -  (so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

The original sinusoid still exists; we have added information to it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.

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Consider a single sine wave: as already noted it is a single line in the frequency domain.

Now we will add some information. That could be voice, digital data - anything. This information will usually (always in practice) have some bandwidth, but the instantaneous signal will be at some frequency f(x) at some amplitude A(x).

In amplitude modulation (because it is the simplest), we will have, at any instant, a composite signal of f(carrier) +/- f(information). I am not going to derive that here.

As this information signal varies with time, we will get f(carrier) +/- f(information) where the information is a band of signals, when viewed over time.

So if we start with a simple sinusoid (the carrier) and modulate with some complex information signal H(s), we end up with f(carrier) modified by H(s) in the frequency domain.

The simple sinusoid carries no information and this might be key to understanding the issue. The modulated signal contains a known signal - the carrier-(so we can find it in the frequency domain) that is carrying an information signal.

So: the simple sine does not carry any information except where to find it in the frequency domain. We 'piggyback' the information onto it.

Note: The use of the term information is deliberate, and as further reading for the OP, the definition of information is indistinguishable from that of noise.