Understanding IP Communications: Cisco VoIP September 17
So people generally understand Voice over IP as making phone calls over the internet instead of through phone lines. Speaking in the same general manner, this is correct.
This post is going to explain Voice and VoIP further using the Cisco CallManager product line for reference (CallManager 4.2 specifically).
Voice Telephony Fundamentals
It is known that the human ear can hear a range of frequencies from 20 Hz to 20,000 Hz, on the other hand the human voice generates frequencies from 300 - 3300 Hz. So what does this have to do with telephony? With standards in place and the long and short of how things came to be in the development of telecommunications/signal processing, the audible frequencies applicable to telephones are from 0 - 4,000 Hz, a relative frequency range of the human voice.
A critical component to the fundamental understanding of Voice is the Nyquist Theorem. The theorem, simply stated and paraphrased says, “for you to exactly reconstruct an audible signal (as in the case of telecommunications) you must sample it at two times the frequency being sampled, in this case two times 4,000 hz which is the max audio frequency for telephony. 2 x 4,000 = 8,000 Hz. So how is the sound reconstructed? You must take miniature recordings of x length(for example 8 bit long recordings), y number of times per second. This process is called sampling.