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ADVANTAGE OF DIGITAL COMMUNICATION

It can withstand channel noise and distortion much better than analog.
It is more rugged than analog communication.
The digital messages can be transmitted over longer distances with greater reliability.
Digital hardware implementation is flexible and permits the use of microprocessors, digital switching, and large scale integrated circuits.
It is easier and more efficient to multiplex several digital signals.
The cost of digital hardware continues to half every two or three years, while performance or capacity doubles over the same time period.

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PULSE CODE MODULATION

It allows the transmission of any form of digital data regardless of what it represents.
It is used in telephone to transmit speech signals binary data for use in digital computers.
It can be used to transmit video data over very long distances.

GENERATION OF PCM

The analog signal m(t) is passed through anti-aliasing filter which avoids aliasing(refers to the overlapping of parts of spectra and that overlapping can no longer be separated by filtering ).
The o/p of anti-aliasing filter is sampled using sample and hold circuit.
The flat-top samples are then quantized. The quantized samples are applied to an encoder.
The encoder responds to each sample by the generation of unique and identifiable binary pulse pattern. The combination of quantizer and encoder is called A/D converter.
A/D converter accepts an analog signal and replaces it with a succession of code symbols, each symbol consisting of a train pulses in which each pulse may be interpreted as the representation of a digit in an arithmetic system.
Thus, the signal transmitted over the communication channel in a PCM system is referred as digitally encoded signal.

PCM RECEIVER

It consists of low-pass filter, pulse regenerator, DAC, expander, equalizer filter and reconstruction filter.
The i/p low pass filter is used to limit the noise bandwidth and to complete the waveform shaping.
The regenerator block reshapes the pulses and removes the thermal noise. The digital word is converted into flat top samples using digital to analog converter (DAC) which provides the expansion necessary to compensate for the compression applied at the transmitter.
Last stage is reconstruction low pass filter, which recovers the analog signal by passing only the low frequency part of the spectrum.

DIFFERENTIAL PULSE CODE MODULATION

When sample of signal are encoded by standard PCM system, the resulting encoded signal contains redundant information.
If this redundancy is reduced, then overall bit rate will decrease and number of bits required to transmit signal will be reduced. As a result we can accommodate more signals on the same channel.
The redundancy can be avoided by encoding difference between the samples. This technique is k/a DPCM.
It works on the principle of prediction. The value of present sample is predicted from the past samples.
The prediction may not be exact but it is very close to the actual sample value.
Figure shows the transmitter of DPCM system.

The sampled signal is denoted by m(nT_s) and the predicted signal is denoted by mˆ(nT_s).
The comparator finds out the difference between the actual sample value m(nT_s) and predicted sample value mˆ(nT_s). This is called error and it is denoted by e(nT_s).
e(nT_s)= m(nT_s) - mˆ(nT_s)
The predicted value is produced by using a prediction filter.
The quantizer output signal e_q(nT_s) and previous prediction are added and given as i/p to the prediction filter. This signal is k/s m_q(nT_s).
This makes the production more and more close to the actual sampled signal.
The number of bits per sample are reduced in DPCM.
At receiver, the incoming DPCM binary signal is recovered using regenerator and then decoded to reconstruct the quantized difference signal.
The prediction filter output and quantized difference signal are summed up to give the quantized version of the original signal.

DELTA MODULATION

It is a special case of DPCM in which only the polarity of the difference signal is encoded as binary output.
If the difference signal is +ve, it is encoded as binary 1, whereas if the difference signal is –ve, it is encoded as binary 0. A binary 1 is transmitted as +ve voltage pulse and binary 0 is transmitted as –ve voltage pulse.
In the figure, the comparator compares the analog signal m(t) and feedback signal m’(t). The o/p of comparator is a fixed amplitude pulses whose polarity depends on the difference signal at the comparator input.

The polarity is +ve if the analog signal m(t) is greater than and negative if less than the feedback signal m’(t). The o/p of the comparator is fed to the one i/p of the multiplier.
The other i/p of the multiplier is a periodic train of unipolar impulses p_i(t) at the sampling frequency f_s.
The o/p of multiplier is a sequence of impulses p_o(t) whose polarity depends on the difference signal. The feedback signal m’(t) is the integral of the multiplier o/p p_o(t).
At receiver, a regenerator is used to recover the p_o(t) impulse waveform. This waveform is integrated to produce the staircase approximation similar to that at the transmitter. The analog o/p m”(t) is recovered from the integrator o/p through the use of a reconstruction filter.

Advantages of Delta Modulation
It transmits only one bit for one sample. Thus, the number of bits required for transmission of one signal are less.
The transmitter and receiver signal implementation is very much simpler for delta modulation.

ADAPTIVE DELTA MODULATION

To overcome the quantization errors due to slope overload and granular noise, step size is made adaptive to variations in the i/p signal m(t).
When the i/p is varying slowly, step size is reduced. This method is called Adaptive Delta Modulation.
It can take continuous changes in step size or discrete changes in step size.
The step size decrease or increase according to certain rule depending on one bit quantizer o/p.
If one bit quantizer o/p is high (1), then step size may be doubled for next sample.
If one bit quantizer is low, the step size may be reduced by one step.
In the receiver of adaptive delta modulator, the first part generates the step size from each incoming bit.

Exactly the same process is followed as that in transmitter. The previous i/p and present i/p decides the step size.
It is then given to an accumulator which builds up staircase waveform. The low pass filter then smoothens out the staircase waveform to reconstruct the smooth signal.

ADVANTAGES of Adaptive Delta Modulation

Signal to noise ratio is better than ordinary delta modulation because of the reduction in slope overload distortion and granular noise.
Because of the variable step size, the dynamic range of ADM is wide.
Utilization of B.W is better than delta modulation.

Adaptive Delta Modulation Receiver