0195136047.pdf

15.3 DIGITAL COMMUNICATION SYSTEMS 717

Analog

message f(t)

Band-limited message

Presampling

lowpass

filter

Sampler

A

Compressor

(optional)

Uniform

quantizer

Waveform

formatter

Binary PCM

to channel

Binary

encoder

Timing

control

Quantized

samples

Figure 15.3.6Block diagram of a PCM system transmitter.

PCM plus

broad-band

channel noise

Original PCM plus errors Quantized samples plus noise

Expandor

(optional)

PCM

reconstruction

Binary

decoder

f(t) plus noise

Timing

Message

reconstruction

(low pass filter)

Figure 15.3.7Block diagram of a PCM system receiver.

pulses of amplitudeAare 0 andA, whereas those associated with polar pulses (of amplitudes±A)
areAand−A. It is, of course, better for the receiver if the ratio of the pulse-caused voltage to the
noise rms voltage is the largest possible at the time of measurement. Figure 15.3.8 shows PCM
reconstruction circuits for unipolar, polar, and Manchester waveforms. The following notation is
used:

• VT: Preset threshold, which is zero for polar and Manchester PCM. In the unipolar system,
  it is equal to half the signal component of the integrator’s output level(A^2 Tb/ 2 )at the
  sampling time when the input has a binary 1. (After the sample is taken, the integrator is
  discharged to0Vinpreparation for integration over the next bit interval.)


• D: The difference between the integrator’s output andVTat the end of each bit interval of
  durationTb.IfD≥0, binary 1 is declared; ifD<0,a0isdeclared.


• Square-wave clock: Generates a voltageAfor 0<t<Tb/2 and−AforTb/ 2 <t<Tb,
  with its fundamental frequency 1/TbHz. The product of the clock and the incoming