Exactly How Loud is Loud? Quantizing Noise

Quantized signal differs from the actual signal. It leads to introduction of same distortion called quantizing noise. We use word noise here since the error so introduced is random in nature. Random nature is resulted because the difference between the quantized level or digit and actual signal at that instant of time is completely unpredictable that is random. Evident are there that the maximum quantization error equals half the sampling unit. All error equals one by thirty two of the total signal range. We should not assume that 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. By using statistical methods we can calculate quantizing noise for a given number of quantizing levels. Quantizing noise reduces as the number of standard levels increases but increase in the number of quantizing levels necessitates large number of bits to send them and requires large bandwidth. In practical system for speech transmission hundred and twenty eight quantizing levels are sufficient. The signal is first sampled and converted to pulse amplitude modulated signal. Then the pulse amplitude modulated signal is quantized and encoded and supervisory signals are added. The signal is then sent directly through cable or is modulated and transmitted. The pulse code modulation is highly immune to noise hence amplitude modulation may be used resulting in pulse code modulation and amplitude modulation. At the receiver, pulse code modulation is translated back into corresponding pulse amplitude modulated pulses which are then demodulated using conventional methods. At the output of an idealized pulse code modulation receiver we get the quantized wave. The binary numbers are sent back to front to indicate each sample. Number five is sent as 1010 instead of 0101. Similarly number eleven is sent as 1101 instead of 1011. In the receiver an integrated resistance capacitance circuit is used for conversion. The pulses are applied to the integrating circuit and are then sampled and discharged immediately after the arrival of the pulse. The time constant of resistance capacitance circuit is so chosen that the charge due to one pulse decays to exactly to one half of its value by the time the next pulse arrives. The each last pulse contributes to the total output in the preceding one micro second duration the second last pulse will have decayed to half of its original value. Thus its contribution to the output is half volt. Similarly contribution of each third last pulse will be one by fourth volts and that of each fourth last will be one by eighth volts and so on. Output pulses are obtainable only if a pulse is received in indicated positions.

About the Author

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.