Analog Wave

What is it that the needle in Edison's phonograph is scratching onto the tin cylinder? It is an analog wave representing the vibrations created by your voice. For example, here is a graph showing the analog wave created by saying the word "hello":

Click here to hear the word.

This waveform was recorded electronically rather than on tinfoil, but the principle is the same. What this graph is showing is, essentially, the position of the microphone's diaphragm (Y axis) over time (X axis). The vibrations are very quick -- the diaphragm is vibrating on the order of 1,000 oscillations per second. This is the sort of wave scratched onto the tinfoil in Edison's device. Notice that the waveform for the word "hello" is fairly complex. A pure tone is simply a sine wave vibrating at a certain frequency, like this 500-hertz wave (500 hertz = 500 oscillations per second):

Click here to hear the tone.

You can see that the storage and playback of an analog wave can be very simple -- scratching onto tin is certainly a direct and straightforward approach. The problem with the simple approach is that the fidelity is not very good. For example, when you use Edison's phonograph, there is a lot of scratchy noise stored with the intended signal, and the signal is distorted in several different ways. Also, if you play a phonograph repeatedly, eventually it will wear out -- when the needle passes over the groove it changes it slightly (and eventually erases it).