I can understand the following text (Data Communications and Networking: 4th Edition, Berhouz Forouzan, Ch.5, page 179) which says that a property of a single sine wave carrier signal (phase, frequency or amplitude) can be changed to represent the pattern in digital data: 
But I fail to understand why something like human voice (as in a telephonic conversation) can't be similarly mapped onto a single sine wave signal by changing one of the characteristics of the wave (Frequency for example). Doesn't at any instant of time, human voice has a particular frequency and amplitude. Why can't that be represented by modulating a single sine wave? I am asking this because the same book says that in order to transfer human voice etc, we need a composite signal having many constituent sine waves of many frequencies:
Please explain this to me in simple terms why it is so. What's different between transmitting something like human voice on one hand and a digital data pattern on the other?And what are other "stuff" like human talk which necessitates use of a composite signal?
NB: I will appreciate if you can also tell IF human talk can be sampled, converted to a digital pattern, and THEN transmitted over a SINGLE sine wave signal. Thank you.
Best Answer
There are several confusions going on here. I can see what the text you cite is trying to say, but also how it can be easily misinterpreted.
The first section is talking about how to modulate a single sine wave (let's call that the "carrier"), to carry another signal. In the text's example, this other signal is digital, but it doesn't need to be.
AM radio is a great example of modulating a carrier using amplitude to carry a audio signal. FM radio is the same except it modulates the frequency. Phase modulation is also used elsewhere, so that part of the first quote is all true.
The misleading part is giving the impression the result is still just a "single sine wave". It's not. As soon as you change something about a sine wave, you no longer have a single sine wave. This may sound unintuitive, but a AM radio carrier of 1 MHz modulated with a 1 kHz audio signal is actually the combination of three sine waves, at 999 kHz, 1.000 MHz, and 1.001 MHz. Getting into why that is true is beyond the scope of this answer. You'll either have to learn a bunch of Fourier analysis or trust me on this.
The second part correctly points out that a true "single sine wave" can't carry any dynamic information. This is again part of the semantics of "single sine". A true single sine doesn't vary in frequency, amplitude, or phase. If it did, you can show by Fourier analysis that it's not really a single sine anymore, just like the AM carrier modulated with 1 kHz wasn't a single sine anymore.
Basically, a periodically changing sine wave can be mathematically decomposed into a set of separate single sines, each with their own amplitude, frequency, and phase. There is therefore no such thing as a changing single sine. This is why a true single sine doesn't carry any dynamic information.