On completion of this module, you will be able to:

• Explain the Delta Modulation and its feature
• Know the Advantages and limitations of Delta Modulation
• Understand Adaptive Delta Modulation
• Explain Differential Pulse Code Modulation
• Companding
• Digital Pulse Modulation Techniques

Delta Modulation

• Delta Modulation (DM) is a type of DPCM in which the difference signal is encoded into just a single bit per sample.
• In DM, an incoming message signal is oversampled (i.e. at a rate much higher than the Nyquist rate) to purposely increase the correlation between adjacent samples of the signal.
• In delta transmission, only one bit is transmitted at a time. By this technique an analog signal can be encoded into bits.
• Both the signals, analog signal x (t) and quantized signal x’ (t) are applied to a comparator.
• If, x’ (t) > x (t) then the comparator output goes high and if x’ (t) < x (t) then the comparator output goes low.
• Thus the comparator output is either 0 or 1. Which is hold by sample and hold circuit for the entire clock cycle period.

Fig. :Process of Delta Modulation

Transmitter of DM

Fig. :Transmitter of DM

Fig. :Input output characteristic of DM

• The basic principle of delta modulation may be formalized in the following set of discrete time relations.

• Here, S denotes half of the step-size &del; as indicated in figure. So the two output levels are +-S.
• Now, assuming zero initial condition of the accumulator, it is easy to see that

Receiver of DM

Fig. : Receiver of DM

• The input to the demodulator is a binary sequence and the demodulator normally starts with no prior information about the incoming sequence. u(kT_s) closely represents the input signal with small quantization error q(kT_s) i.e.

• That is the error signal. It is the difference of two consecutive samples at the input except the quantization error (when quantization error is small).

Features of DM

• No effective prediction unit: The prediction unit of a DPCM coder is eliminated and replaced by a single-unit delay element.
• A 1-bit quantizer with two levels is used. The quantizer output simply indicates, whether the present

• Performance of the DM scheme is dependent on the sampling rate. Most of the above comments are acceptable only when two consecutive input samples are very close to each other.

Advantages of DM

• DM transmits only one bit for one sample. Thus the signaling rate and transmission channel Bandwidth is quite small for DM.
• The transmitter and receiver implementation is very much simple for delta modulation. There is no A/D converter involved in DM.

Disadvantages of DM

• The performance of the DM encoder is limited by two types of distortion
  1. Slope-overload Distortion
  2. Granular Noise

Slope-overload Distortion

• This type of distortion is due to the use of a step size delta that is too small to follow portions of the waveform that have a steep slope.
• The result that the approximation signal falls behind the message signal.
• It can be reduced by increasing the step size.

Granular Noise

• This results from using a step size that is too large in parts of the waveform having a small slope.
• Granular noise can be reduced by decreasing the step size.

Condition for avoiding slope overload

• If an input signals changes more than half of the step size (i.e. by s ) within a sampling interval, there will be slope-overload distortion.
• So, the desired limiting condition of the input signal x(t) for avoiding slope-overloading is,

Signal Power

Adaptive Delta Modulation

• The performance of delta modulator can be improved significantly by making step size time –varying.
• So, the DM with variable step size is known as Adaptive Delta Modulation (ADM).
• The step size is increased when the waveform has steep slope and is decreased when the waveform has a relatively small slope.

• In this way, the step size is adapted to the level of the input signal.
• Adaptive Delta Modulation is also called Continuously Variable Slope Delta Modulation (CVSD).
• Adaptive Delta Modulation designed to overcome slope overload and granular noise issues.

Transmitter of ADM

• The logic for step size control is shown in the diagram.

• The variation (increases or decreases) of step size depends on one bit quantizer output.
• If one bit quantizer output is high, then step size may be double for next sample.
• If one bit quantizer output is low, then step size may be reduced by one step.

Receiver of ADM

• In the receiver of ADM, the first part generates the step size from each incoming bit.

• Same process followed as that in transmitter.
• The previous input and present input decides the step size.
• The low - pass filter then smoothen out the staircase waveform to reconstruct the smooth signal.

Advantages of ADM

• Better signal to noise ratio.
  1. Because of the reduction in slope overloading distortion and idle noise, SNR becomes better than the delta modulation.
  2. The slope overloading and granular noise can be reduced by varying the step size in ADM.
• The dynamic range of the adaptive delta modulation is wider than the simple delta modulation due to the variation of the step size.
• Utilization of bandwidth is better than delta modulation.

Companding

• Non-uniform quantization is achieved by, first passing the input signal through a compressor.
• The output of the compressor is then passed through a uniform quantizer.
• At the receiver, the signal is restored to its original form by using an expander.
• This complete process of compressing and expanding the signal before and after uniform quantization is called Companding.
• Companding = Compression + Expansion.


Fig. : Companding Curve for PCM

Introduction to DPCM

• In PCM, each sample of the waveform is encoded independently of all the other samples
• Differences between samples are expected to be smaller than the actual sampled amplitudes
• Fewer bits are required to represent the differences
• Coding a difference is based on the fact that most source signals show significant correlation between successive samples. So encoding uses redundancy in sample values which implies lower bit rate

Differential PCM

• In ( Differential PCM)DPCM instead of sending samples, the difference between the successive samples at time t and t-1 is transmitted
• Since the difference is small therefore only few bits are required for transmission. Hence BW required is small
• The DPCM works on the principle of prediction. The value of the present sample is predicted from the past samples


Fig. : DPCM Transmitter and Receiver