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In quantizing telephone signals the range of signals

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discrete sample values. In quantizing telephone signals, the range of signals to be transmitted is divided into 256 intervals (8 bits ).
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Linear Quantization The quantizing intervals are of equal size . Quantization noise : the difference between the input signal and the quantized output signal Signal-to-quantized-noise ratio S/N= 6n+1.76 dB n=8, S/N=49.76 dB In other words, every added binary digit increases the ratio by 6dBs
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Linear Quantization Errors occur on every sample except where the sample size exactly coincides the mid-point of the decision level. If smaller steps are taken the quantization error will be less . - However, increasing the steps will complicate the coding operation and increase bandwidth requirements.
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Non-Linear Quantization With linear quantization, the signal to noise ratio is large for high levels but small for low level signals. Therefore, non-linear quantization is used.
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Non-Linear Quantization The quantizing intervals are not of equal size. Small quantizing intervals are allocated to small signal values (samples) and large quantization intervals to large samples so that: - the signal-to-quantization distortion ratio is nearly independent of the signal level . S/N ratios for weak signals are much better but is slightly less for the stronger signals.
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Non-Linear Quantization Companding: A process in which compression is followed by expansion. Two separate laws are used A-Law adopted by ITU-T for 30 channel PCM(Europe, Pakistan, India etc). µ -law used mainly in USA, Canada and Japan (24 channel PCM).
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ENCODING PCM signal to be transmitted is obtained by encoding the quantizing intervals . Allocation of 8-bit word is done to each individual sample . An 8-digit binary code is used for 128 positive and 128 negative quantizing intervals. First bit used for all PCM words for all positive intervals is ‘1’ and for negative intervals is ‘0’
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MULTIPLEXING The 8-bit PCM words of a number of telephone signals can be transmitted consecutively in repeated cycles. A PCM word of one telephone signal is followed by PCM words of of all other telephone signals arranged in consecutive order. This results in PCM TIME DIVISION MULTIPLEX signal
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MULTIPLEXING Multiplexing function is carried out fully electronically. A switch moves from one input to other. The PCM-TDM signal is then available at the output of the switch. The time interval within which a PCM word is transmitted is known as Time Slot . A bit train containing one PCM word each from all inputs is known as Pulse Frame .
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Receiver Side Demultiplexing PCM-TDM signal i.e. the PCM words of the telephone signals are distributed to the individual lines. Decoding PCM words in the PCM signal i.e. a signal amplitude is allocated to each word, which is equal to the midpoint value of the particular quantizing interval. Result is a PAM signal.
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