Signals

A signal is simply information travelling from one place to anther. It could be in the form of sound waves travelling through the air or as electrical waves travelling in a wire.

An oscilloscope is basically just a voltmeter. The electrical signal is an alternating voltage and the oscilloscope shows us how this varies with time.

The simplest signal (analogue) is a pure tone with a single frequency present. You would need some kind of electronic equipment to produce a perfectly pure tone as the sounds made by musical instruments contain a variety of harmonics. These are higher frequencies which are present in different amounts. They will affect the tone of the sound. One of the reasons why we can easily tell apart the same note played by different instruments, e.g. a saxophone and a cello.

A frequency spectrum tells us      a) what frequencies are present in a signal and        b) their relative amplitudes.

Frequency spectrum for a pure tone and with a few harmonics added.

Early synthesisers would create tones by simply adding higher harmonics to a base frequency. This Mini-Moog is one of the first.	The spectrum for a voice contains lots of different frequencies. This was recorded over 15 seconds. How would a man and woman be different?
Below is the frequency spectrum of a note played on a kalimba, an African thumb piano. I'm sure you recognised it.	How about a graphic equaliser on a T shirt?