3-26
Pulse-Code Modulation
Pulse-code modulation can easily be decoded, provided the pulse-code groups have been transmitted
in reverse order; that is, if the pulse with the lowest value is transmitted first, the pulse with the highest
value is transmitted last. A circuit that will provide a constant value of current without regard to its load is
known as a current source. A current source is used to apply the pcm pulses to an RC circuit, such as that
shown in figure 3-23, view (A). The current source must be capable of supplying a linear charge to C1
that will increase each time a pulse is applied if C1 is not allowed to discharge between pulses. In other
words, if C1 charges to 16 volts during the period of one pulse, then each additional pulse increases the
charge by 16 volts. Thus, the cumulative value increases by 16 volts for each received pulse. This does
not provide a usable output unless a resistor is chosen that allows C1 to discharge to one-half its value
between pulses. If only one pulse is received at T1, C1 charges to 16 volts and then begins to discharge.
At T2 the charge has decayed to 8 volts and continues to decay unless another pulse is received. At T3 it
has a 4-volt charge and at T4 it only has a 2-volt charge. At the sampling time, a 1-volt charge remains;
this charge corresponds to the binary-weighted pulse train of 0001. Now we will apply a pcm signal
which corresponds to the binary-coded equivalent of 7 volts (0111) in figure 3-23, view (A). View (B) is
the pulse code that is received. Remember that the pulses are transmitted in reverse order. View (C) is the
response curve of the circuit. At T1 the pulse corresponding to the least significant digit is applied and C1
charges to 16 volts. C1 discharges between pulses until it reaches 8 volts at T2. At T2 another pulse
charges it to 24 volts. At T3, C1 has discharged through R1 to a value of 12 volts. The pulse at T3
increases the charge on C1 by 16 volts to a total charge of 28 volts. At T4, C1 has discharged to one-half
its value and is at 14 volts. No pulse is present at T4 so C1 will not receive an additional charge. C1
continues to discharge until T5 when it has reached 7 volts and is sampled to provide a pam pulse which
can be peak detected. This sampled output corresponds to the original sampling of the analog voltage in
the modulation.
