Pulse Amplitude Modulation See Also: Modulation The process of sampling an Analog signal. It is based on Nyquist’s Theorem which states that if signal samples ar taken at twice the highest possible Frequency , the Sound reproduced from the sample will closely approximate the original signal. Since the highest frequency transmitted on the Telephone network is 4,000 Hertz (Hz) , the electrical (see Electricity) signal is sampled 8,000 times a Second. Each sample is a discrete packet of analog information. The essential information, the amplitude, can be measured and converted into a numerical equivalent. By knowing the sample number and the amplitude of the Wave, the numerical information transmitted across the network can closely approximate the original wave. The figure at the right illustrates how sound is transformed into numerical information for transmission using a graph. The horizontal axis represents a time period (one second) during which 8,000 samples are taken. The vertical axis is the amplitude. The solid line represents the original sound wave. The 8,000 samples taken each second are represented by the dots on the original sound wave. In the figure, 20 samples are taken. In time period one, the first sample has an amplitude of 11, in Time period two, the second sample has an amplitude of 13, etc. To ensure that each sample’s numerical information is transmitted across the network along with its amplitude, and the transmitter and receiver are synchronized, framing bits are inserted into the data stream so that the receiver can identify the different time slots for each sample. The amplitude information from the sound wave can now be converted into binary digits (zeros and ones) for transmission across the network. The sample amplitude information from the sound wave is summarized n the table below. Time Slot Amp. Amp. In Binary Time Slot Amp. Amp. In Binary 1 12 01100 11 17 10001 2 13 01101 12 15 01111 3 16 10000 13 11 01011 4 17 10001 14 11 01011 5 16 10000 15 13 01101 6 15 01111 16 12 01100 7 14 01110 17 11 01011 8 18 10010 18 11 01011 9 19 10011 19 13 01101 10 18 10010 20 16 10000 When the information reaches its final destination, the original sound wave is reconstructed. The original sound wave can now be represented by a series of dots on a graph. Connecting the dots with a straight line closely approximates the original sound wave. The smaller the scale and the more dots measured, the closer the reconstructed wave resembles the original wave (see the figure at the right).