The Secret about Pulse Code Modulation.

In pulse code modulation the total amplitude range of the signal is divided into a number of standard levels at equal intervals. These levels are transmitted in the binary code. Hence the actual number of theses standard levels is a power of two such as sixteen, thirty two, sixty four or one hundred and twenty eight. By the so called quantizing process the level actually sent at any sapling instant is the one nearest to the standard or quantum level. Thus if the signal is 10.2 volts at any time it is sent as the digit ten since ten volts is the standard amplitude nearest to 10.2 volts. This digit ten is sent at that time instant as a series of pulses corresponding to number ten. But there are sixteen levels hence four binary places are required. Thus the analogue number ten becomes binary number 1010 and may be sent as POPO, where P is equal to pulse and zero is equal to no pulse. Actually it is often sent as binary number back to front that is as 0101 or OPOP to make demodulation easier.

The signal is continuously sampled and quantized. Each sample magnitude is being converted to the nearest standard amplitude. The quantized number is coded using binary code, converted into corresponding back to front binary number and then sent. If adequate quantizing levels are used, the result closely resembles the corresponding analog transmissions.

Generally a supervising bit is added to each binary code group representing a quantized sample. Each group of pulses denoting a sample, referred to a word, is thus expressed by number of bits plus one bit where two raised to power number of bits is the chosen number of standard levels.

The quantizing signal somewhat differs from the actual signal. This amounts to introduction of same distortion, called quantizing noise. The word noise is used here since the error so introduced is random in nature. This random nature results because the difference between the quantized level or digit and actual signal at that instant of time is completely unpredictable that is random.

It is obvious that the maximum quantizing error equals half the sampling unit. Thus the maximum error equals one by thirty two of the total signal range. But it is wrong to assume that the signal to quantizing noise ratio of this system is thirty two is to one because neither the signal nor the instantaneous quantizing error has always its maximum value. The above consideration and several others influence the quantizing noise. Thus quantizing noise for a given number of quantizing levels can be calculated only using statistical methods. Quantizing noise reduces as the number of standard levels increases.

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Tymon Hytem has worked in the electronics feild for the past 15 years. He enjoys helping people decide on electronic gadgets from telephones to XM Radio and choosing the perfect XM Satellite Radio system for their needs.