Mumbai university > Electronics engineering > Sem 4 > Frequency modulation

Marks : 5

Year : Dec 2015

In PCM system, N number of binary digits are transmitted per quantized sample. Hence the signalling rate and transmission channel bandwidth of the PCM system are very large.

These disadvantages can be overcome by using the delta modulation.

Principle of operation-

Delta modulation transmits only one bit per sample instead of N bits transmitted in PCM. This reduces its signalling rate and bandwidth requirement to a greater extent.

In the basic of a linear DM a staircase approximated version of the sampled input signal is produced.

The original signal and its staircase representation are then compared to produce a difference signal.

This difference signal is quantized in to two levels namely ± δ corresponding to the positive and negative difference respectively.

That means if the corresponding signal x’(t) lies below x(t) at the same sampling instant, then the approximated signal is increased by “δ”.

Whereas if x’(t) is greater than x(t) at the sampling instant, then x’(t) is decreased by “δ”.

D.M. OUTPUT

The DM output is’1’ if the staircase increase signal x’(t) is increased by “δ”

Whereas the DM output is 0 if signal x’(t) is decreased by “δ”

In DM the present value of x(t) is compared with the approximate value x’(t) and the result of the comparison is transmitted.

Advantages

Low signalling rate and low transmission channel bandwidth because in DM only one bit is transmitted per sample.

The delta modulator transmitter and receiver are less complicated to implement as compared to the PCM.

Disadvantages

The two distortions slope overhead error and granular noise are present.

Practically the signalling rate with no slope overhead error will be much higher than that of PCM.