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1)What are the limitations of delta modulation? Explain Adaptive and linear delta modulation system .Discuss its merits over DPCM.

The limitation of delta modulation is using uniform step size which leads to quantization noise.

The merit of DM over DPCM is it has one bit quantization by which the bit rate is reduced

Adaptive Delta Modulation:

The performance of a delta modulator can be improved significantly by making the step size of the modulator assume a time-varying form. In particular, during a steep segment of the input signal the step size is increased. Conversely, when the input signal is varying slowly, the step size is reduced. In this way, the size is adapted to the level of the input signal. The resulting method is called adaptive delta modulation (ADM).There are several types of ADM, depending on the type of scheme used for adjusting the step size. In this ADM, a discrete set of values is provided for the step size.

2) With a relevant block diagram ,describe the operation of a DPCM

Differential pulse code modulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value.DPCM code words represent differences between samples unlike PCM where code words represented a sample value.

Basic concept of DPCM - 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.Realization of basic concept is based on a technique in which we have to predict current sample value based upon previous samples and we have to encode the difference between actual value of sample and predicted value (the difference between samples can be interpreted as prediction error).Because it's necessary to predict sample value DPCM is form of predictive coding.

DPCM compression depends on the prediction technique, well-conducted prediction techniques lead to good compression rates, in other cases DPCM could mean expansion comparing to regular PCM encoding.

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Quantizer input:

e ’ x − xˆ

e is called prediction error.

Quantizer output:

qn, quantization error

Prediction filter is usually implemented by a linear predictor in which the last r samples are used to predict the values of the next sample.

Predictor input:

x * ’ y + xˆ

Predictor output: xˆ

3)With a neat diagram explain how a DM system works?

Delta modulation (DM or Δ-modulation)is an analog-to-digital and digital-to-analog signal conversion technique used for transmission of voice information where quality is not of primary importance. DM is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples are encoded into n-bit data streams. In delta modulation, the transmitted data is reduced to a 1-bit data stream. Its main features are:

the analog signal is approximated with a series of segments

each segment of the approximated signal is compared to the original analog wave to determine the increase or decrease in relative amplitude

the decision process for establishing the state of successive bits is determined by this comparison

only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent whereas a no-change condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample.

To achieve high signal-to-noise ratio, delta modulation must use oversampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist rate.

Derived forms of delta modulation are continuously variable slope delta modulation, delta-sigma modulation, and differential modulation. Differential pulse-code modulation is the super set of DM.

Encoder and decoder of delta modulation

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The output waveform of delta modulation which leads to either 1 or 0 bit based on step size is given

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The error we get because of delta modulation is slope overload and granular distortion to overcome this we go for adaptive delta modulation

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4) Explain about ISI and EYE PATTERN

In telecommunication, intersymbol interference (ISI) is a form of distortion of a signal in which one symbol interferes with subsequent symbols. This is an unwanted phenomenon as the previous symbols have similar effect as noise, thus making the communication less reliable. ISI is usually caused by multipath propagation or the inherent non-linear frequency response of a channel causing successive symbols to "blur" together. The presence of ISI in the system introduces errors in the decision device at the receiver output.. ISI in a PCM or data transmission system experimentally is to apply the received wave to the vertical deflection plates of an oscilloscope and to apply a sawtooth wave at the transmitted symbol rate R (R = 1/T) to the horizontal deflection plates. The resulting display is called an eye pattern because of its resemblance to the human eye for binary waves. The interior region of the eye pattern is called the eye opening. An eye pattern provides a great deal of information about the performance of the pertinent system.

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One of the causes of intersymbol interference is what is known as multipath propagation in which a wireless signal from a transmitter reaches the receiver via many different paths. The causes of this include reflection (for instance, the signal may bounce off buildings), refraction (such as through the foliage of a tree) and atmospheric effects such as atmospheric ducting and ionospheric reflection.

Eye pattern

The width of the eye opening defines the time interval over which the received wave can be sampled without error from ISI. It is apparent that the preferred time for sampling is the instant of time at which the eye is open widest.

The sensitivity of the system to timing error is determined by the rate of closure of the eye as the sampling time is varied.

The height of the eye opening, at a specified sampling time, defines the margin over noise.

An eye pattern, which overlays many samples of a signal, can give a graphical representation of the signal characteristics. The first image below is the eye pattern for a binary phase-shift keying (PSK) system in which a one is represented by an amplitude of -1 and a zero by an amplitude of +1. The current sampling time is at the center of the image and the previous and next sampling times are at the edges of the image. The various transitions from one sampling time to another (such as one-to-zero, one-to-one and so forth) can clearly be seen on the diagram.

The noise margin - the amount of noise required to cause the receiver to get an error - is given by the distance between the signal and the zero amplitude point at the sampling time; in other words, the further from zero at the sampling time the signal is the better. For the signal to be correctly interpreted, it must be sampled somewhere between the two points where the zero-to-one and one-to-zero transitions cross. Again, the further apart these points are the better, as this means the signal will be less sensitive to errors in the timing of the samples at the receiver.

The effects of ISI are shown in the second image which is an eye pattern of the same system when operating over a multipath channel. The effects of receiving delayed and distorted versions of the signal can be seen in the loss of definition of the signal transitions. It also reduces both the noise margin and the window in which the signal can be sampled, which shows that the performance of the system will be worse (i.e. it will have a greater bit error ratio).

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5)Give the full name of PCM, DM, DPCM and ADPCM. Describe their fundamental differences.

PCM: Pulse Code Modulation

DM: Delta Modulation

DPCM: Differential Pulse Code Modulation

ADPCM: Adaptive Differential Pulse Code Modulation

PCM: The analog speech waveform is sampled and converted directly into a multibit digital code by an A/D converter. The code is stored and subsequently recalled for playback

DM: Only a single bit is stored for each sample. This bit 1 or 0, represents a greater than or less than condition, respectively as compared to the previous sample. An integrator is then used on the output to convert the stored nit stream to an analog signal.

DPCM: Stores a multibit difference value. A bipolar D/A converter is used for playback to convert the successive difference values to an analog waveform.

ADPCM: Stores a difference value that has been mathematically adjusted according to the slope of the input waveform. Bipolar D/A converter is used to convert the stored digital code to analog for playback.

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