Sunday, August 15, 2010
Learning The Analog World
Before I begin digging deep into the VoIP part of my studies I spent the last 3-4 weeks studying more about the PSTN world. Mainly how analog voice signals is converted into electric and eventually binary signals. This process is called Pulse Amplitude Modulation (PAM) if your converting analog to electrical values and Pulse Code Modulation (PCM) when you're converting the PAM values to binary.
An engineer named Dr. Harry Nyquist noticed that human voice only uses between 20-9000 Hz. He figured out if you take samples of the voice signals at twice it's frequency rate, you can accurately send the signal over a medium and play it back without any noticeable sound distortion.
The problem with that if you sampled the highest frequency range that a human can reach (18,000 digital samples per second), it would require a lot of bandwidth for each voice call. To remedy this he lowered the frequency range to 4000 Hz (8,000 samples per second) since rarely do humans ever speak above this range and if they did it would probably be pretty annoying.
With that said, the 8000 sample rate per second plays nicely with the digital world. The rule states that each sample is 8 bits (1 Byte).
Therefore 8000 samples * 8 bits = 64,000 bps (64 kbps)
Does 64 kbps sound like a familar term you hear? It should as 64 kbps equals the same size as one ds0 channel in a T1. Remember a T1 has twenty four 64 kbps ds0 channels that makes up the entire T1 speed of 1.536 Mbps (24 * 64,000 bps = 1,536,000 bps) with T1 framing signaling the T1 size is actually 1.544 Mbps.