Meaning of frequency and bandwidth of a signal, despite the fact that we do not know the signal First of all, I am completely new to the domain of signal processing.
As far as I know, a signal can be represented with an infinite integral of infinitesimal complex exponentials, which is known as a Fourier Transform. From calculating the Fourier Transform of a signal, we can find the bandwidth of it as well.
We tend to say that voice signals have a bandwidth starting from 50Hz and ending approximately at 10kHz. At the same manner, we say that radiowaves and microwaves lie on a specific bandwidth. How is it possible to know the bandwidth of a signal, if we don't know the signal itself? I mean, when speaking, the output signal produced by my mouth is always different as time proceeds, but we know that the bandwidth of that signal is always at the aforementioned frequency interval. How is that possible to know?
Some signals are generated by processes that are physically capable of generating frequencies only in a specific frequency range. For example, the human vocal tract can only generate signals between roughly 50 and 10,000 Hz. This does not mean that every human can achieve that range, or that there are people who can generate frequencies slightly above or below. When it is said that human voice has a certain bandwidth, it does not mean that every instance of human voice will actually cover that entire range; it only means that you will not often find frequencies outside that range.
Other signals are limited in bandwidth by design. One example is voice in analog telephony; the voice is filtered to a range of approximately 300 to 3400 Hz, and it is impossible to find frequencies outside that range in the system. Another example is analog television: the signal is designed to have a bandwidth of 6 MHz, and except for equipment malfunction, you'll never find a TV signal with frequencies outside that range.