| Quantized signal differs from the actual | | | | modulation. At the receiver, pulse code |
| signal. It leads to introduction of same | | | | modulation is translated back into |
| distortion called quantizing noise. We use | | | | corresponding pulse amplitude modulated |
| word noise here since the error so introduced | | | | pulses which are then demodulated using |
| is random in nature. Random nature is | | | | conventional methods. At the output of an |
| resulted because the difference between the | | | | idealized pulse code modulation receiver we |
| quantized level or digit and actual signal at | | | | get the quantized wave. The binary numbers |
| that instant of time is completely | | | | are sent back to front to indicate each |
| unpredictable that is random. Evident are | | | | sample. Number five is sent as 1010 instead |
| there that the maximum quantization error | | | | of 0101. Similarly number eleven is sent as |
| equals half the sampling unit. All error | | | | 1101 instead of 1011. In the receiver an |
| equals one by thirty two of the total signal | | | | integrated resistance capacitance circuit is |
| range. We should not assume that signal to | | | | used for conversion. The pulses are applied |
| quantizing noise ratio of this system is | | | | to the integrating circuit and are then |
| thirty two is to one because neither the | | | | sampled and discharged immediately after the |
| signal nor the instantaneous quantizing error | | | | arrival of the pulse. The time constant of |
| has always its maximum value. | | | | resistance capacitance circuit is so chosen |
| | | | that the charge due to one pulse decays to |
| By using statistical methods we can calculate | | | | exactly to one half of its value by the time |
| quantizing noise for a given number of | | | | the next pulse arrives. The each last pulse |
| quantizing levels. Quantizing noise reduces | | | | contributes to the total output in the |
| as the number of standard levels increases | | | | preceding one micro second duration the |
| but increase in the number of quantizing | | | | second last pulse will have decayed to half |
| levels necessitates large number of bits to | | | | of its original value. Thus its contribution |
| send them and requires large bandwidth. In | | | | to the output is half volt. Similarly |
| practical system for speech transmission | | | | contribution of each third last pulse will be |
| hundred and twenty eight quantizing levels | | | | one by fourth volts and that of each fourth |
| are sufficient. The signal is first sampled | | | | last will be one by eighth volts and so on. |
| and converted to pulse amplitude modulated | | | | Output pulses are obtainable only if a pulse |
| signal. Then the pulse amplitude modulated | | | | is received in indicated positions. |
| signal is quantized and encoded and | | | | |
| supervisory signals are added. The signal is | | | | Tymon Hytem has worked in the electronics |
| then sent directly through cable or is | | | | feild for the past 15 years. He enjoys |
| modulated and transmitted. The pulse code | | | | helping people decide on electronic gadgets |
| modulation is highly immune to noise hence | | | | from telephones to XM Radio and choosing the |
| amplitude modulation may be used resulting in | | | | perfect XM Satellite Radio system for their |
| pulse code modulation and amplitude | | | | needs. |