where Ts = time period of the sampling.
In PAM sample generator is a amplitude modulator. In simple version, it contains a AND gate. The signal to be converted is fed to one input of the AND. Pulses at the sampling frequency are applied to the other input of the AND gate to open it during the required time intervals. Thus, the output of the gate consists of pulses at the sampling rate, equal in amplitude to the signal voltage at each instant. The pulses are then passed through a pulse shaping network which gives the flat tops.
Quantization & encoding. Instead of sending a pulse train capable of continuously varying one of the parameters, the PCM technique produces a series of binary code which represents the approximate amplitude of the signal sample at that instant. This process of approximation is called signal quantization. Thus, the combined operations of sampling and quantizing generate a quantized PAM wave form.
The quantizing process is as follows. Let the signal m(t) is to be quantized. Identify the signal excursion (negative peak to positive peak) which is ranges from VL to VH. Divide the total range into M equal intervals. In the center of each of these steps, we locate quantization levels. If the sampled amplitude is in a particular interval, the nearest value is chosen and the corresponding binary digit is approximated as equivalent to the sampled amplitude. The quality of the approximation may be improved by reducing the size of the steps, thereby increasing the number of allowable levels. Fig. 5.5 shows the process of quantization and encoding.
Code Level Volts
7 3.56 2.55 1.54 0.5-S -0.5 ■ 2 -1.5 1 -2.5 -0 -3.5 -
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