Delta Modulation and Demodulation Trainer

INDIAN INSTITUTE OF TECHNOLOGY BHUBANESWAR SCHOOL OF ELECTRICAL SCIENCES

ST2155

Delta Modulation & Demodulation Trainer (Delta, Adaptive Delta & Sigma Delta) ST2155

1.

Safety Instructions

2.

Introduction

3.

Features

4.

Technical Specifications

5.

Theory I. Delta Modulation II. Delta Demodulation III. Adaptive delta modulation IV. Delta Sigma Modulation

6.

Experiments •

•

•

7.

Experiment 1 Study of Delta Modulation Demodulation Experiment 2 Study of Adaptive Delta Modulation and Demodulation Experiment 3 Study of Delta Sigma Modulation and Demodulation

Frequently Asked Questions

COMMUNICATION COMMUNICATI ON SYSTEMS LABORATORY

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Safety Instructions Read the following safety instructions carefully before operating the instrument. To avoid any personal injury or damage to the instrument or any product connected connected to it. Do not operate the instrument if suspect any damage to it. The instrument should be serviced by qualified personnel only.

For your safety: Use proper Mains cord

: Use only the mains cord designed for this instrument. Ensure that the mains cord is suitable for your country.

Ground the Instrument

: This instrument is grounded through the protective earth conductor of the mains cord. To avoid electric shock the grounding conductor must be connected to the earth ground. Before making connections to the input terminals, ensure that the instrument is properly grounded.

Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and marks on the instrument. Use only the proper Fuse

: Use the fuse type and rating specified for this instrument.

Use in proper Atmosphere : Please refer to operating conditions given in the manual. 1.

Do not operate in wet / damp conditions.

2.

Do not operate in an explosive atmosphere.

3.

Keep the product dust free, clean and dry.


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Safety Instructions Read the following safety instructions carefully before operating the instrument. To avoid any personal injury or damage to the instrument or any product connected connected to it. Do not operate the instrument if suspect any damage to it. The instrument should be serviced by qualified personnel only.

For your safety: Use proper Mains cord

: Use only the mains cord designed for this instrument. Ensure that the mains cord is suitable for your country.

Ground the Instrument

: This instrument is grounded through the protective earth conductor of the mains cord. To avoid electric shock the grounding conductor must be connected to the earth ground. Before making connections to the input terminals, ensure that the instrument is properly grounded.

Observe Terminal Ratings : To avoid fire or shock hazards, observe all ratings and marks on the instrument. Use only the proper Fuse

: Use the fuse type and rating specified for this instrument.

Use in proper Atmosphere : Please refer to operating conditions given in the manual. 1.

Do not operate in wet / damp conditions.

2.

Do not operate in an explosive atmosphere.

3.

Keep the product dust free, clean and dry.


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Introduction The ST2155 Delta Modulation & Demodulation Trainer demonstrates the delta, adaptive delta and sigma delta modulation and demodulation schemes. It covers the concepts of delta modulation demodulation, slope overloading, adaptive delta modulation demodulation, sigma delta modulation demodulation and amplitude overloading. Know your ST2155 trainer better :

The trainer requires ±12V and ±5V (200mA) dc signal which can be obtained from Scientech Power Supply Model AD-01 Synchronized & Adjustable Amplitude Sine Wave Generators of 1, 2, 3 & 4 KHz and separate adjustable D.C. level are provided on board. Various test points on the trainer make user understand, the complete process that takes place for the Delta modulation and demodulation process. Six sampling clock frequencies of 50, 100, 200 and 400 KHz are provided on board. These frequencies can be observed at test points 6, 7, 8 and 9 respectively. Transmitter and receiver clocks are switch selectable and can also be observed at test points 10 and 11 respectively. respectively. Integrators in transmitter and receiver blocks are provided with manual and automatic gain control. Manual gain control in both transmitter and receiver is done by switches labeled A & B. Automatic gain adjustment is done by control circuit. A switch is provided for the selection of manual or automatic gain control. control.


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Technical Specifications Crystal Frequency

:

6.400 MHz

Sampling Clock Frequency

:

50, 100, 200 & 400 KHz (Switch selectable)

On Board Generator

:

Synchronized & Adjustable Amplitude Sine Wave Generator of 1, 2, 3 & 4 KHz Separate Adjustable D.C. level

Integrator

:

Four integrator gain settings Normal, X 2, X 4, X 8

Low Pass Filter

:

Fourth order Butterworth (Cut Off Frequency- 4.8 KHz)

Test Points

:

43

Interconnections

:

2 mm socket

Power Supply

:

± 5V, ± 12V DC, 200mA

Dimensions (mm)

:

W 325, H 90, D 255

Weight

:

1.1 Kgs (approx.)

COMMUNICATION SYSTEMS LABORATORY

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Theory of Delta Modulation 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 is encoded into n-bit data streams. In delta modulation, the transmitted data is reduced to a 1-bit data stream. Its main features are: 1. The analog signal is approximated with a series of segments 2. Each segment of the approximated signal is compared to the original analog Wave to determine the increase or decrease in relative amplitude. 3. The decision process for establishing the state of successive bits is determined by this comparison 4. 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 over sampling techniques, that is, the analog signal is sampled at a rate several times higher than the Nyquist’s rate. Delta modulation is a system of digital modulation developed after pulse code modulation. In this system, at each sampling time, say the K th sampling time, the difference between the sample value at sampling time K and the sample value at the previous sampling time (K-1) is encoded into just a single bit. i.e. at each sampling time we ask simple question. Has the signal amplitude increased or decreased since the last sample was taken? If signal amplitude has increased, t hen modulator's output is at logic level 1. If the signal amplitude has decreased, the modulator output is at logic level 0. Thus, the output from the modulator is a series of zeros and ones to indicate rise and fall of the waveform Figure 1 shows the block diagram of delta modulator.


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Delta modulator Figure 1 The Delta Modulator works as follows:

The analog signal which is to be encoded into digital data is applied to the +ve input of the voltage comparator which compares it with the signal applied to its -ve input from the integrator output. The comparator's output is logic '0' or '1' depending on whether the input signal at +ve terminal is lower or greater then the -ve terminals input signal. The comparator's output is then latched into a D-flip-flop which is clocked by the transmitter clock. Thus, the output of D-flip-Flop is a latched 'l' or '0' & synchronized with the transmitter clock edge. This binary data stream is transmitted to receiver and is also fed to the unipolar to bipolar converter. This block converts logic '0' to voltage level of + 4V and logic 'l' to voltage level - 4V. The Bipolar output is applied to the integrator whose output is as follows: a.

Rising linear ramp signal when - 4V is applied to it, (corresponding to binary 1)

b.

Falling linear ramp signal when + 4V is applied to it (corresponding to binary 0).

The integrator output is then connected to the -ve terminal of voltage comparator, thus completing the modulator circuit. Let us understand the working of modulator circuit with the analog input waveform applied as below:


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Technique of Delta Modulation Figure 2

Suppose at some time-instance t = 0, the integrator output voltage is lower than the analog input. This causes the voltage comparator voltage to go high i.e. logic '1'. This data is latched in the D- Flip- Flop at the rising edge of transmitter clock., The latched '1' output of D- flip is translated to - 4V by the unipolar to bipolar converter block. The integrator then ramps up to catch analog signal. At the next clock cycle t = 1, the integrator output becomes more than the analog input, so a '0' is latched into D-Flip-Flop. The integrator now ramps downward as +4V voltage signal from unipolar to bipolar converter appears at its input. Thus, the ramp signal again tries to catch the fallen analog signal. As we can observe, after several clock cycles the integrator output is approximation of the analog input which tries to catch up the analog input at each sample time. The data stream from D-flip-flop is the delta modulators output. The delta demodulator consists of a D-Flip-Flop a unipolar to bipolar converter followed by an integrator and a low pass filter. Figure 3 shows the block diagram of delta demodulator.

Delta De Modulator Figure 3


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Delta Demodulation The delta demodulator works as follows:

The delta demodulator receives the data from D-Flip-Flop of delta modulator. It latches this data at every rising edge of receiver clock, which is delayed by half clock period with respect to transmitter clock. This has been done so that the data from transmitter may settle down before being latched into the receiver Flip-Flop. The unipolar to bipolar converter changes the output from D-Flip-Flop to either - 4V or + 4V for logic '1' and '0' respect ively. In case of modulator when the output from unipolar to bipolar converter is applied to integrator, its output tries to follow the analog signal in ramp fashion and hence is a good approximation of the signal itself. The integrator's output contains sharp edges, which are 'smoothened out' by the low - pass filter, whose cut-off frequency is just above the audio band. Delta modulation offers many advantages as listed below: •

Simple circuitry

•

Cheap

•

Single bit encoding allows us to transmit more information at some sampling rate for given system bandwidth.

Unfortunately, the practical use of delta modulation is limited due to the following several drawbacks. a. Noise

Noise is defined as any unwanted unpredictable random waveform accompanying the information signal. Whenever the signal is received at the receiver, it is always accompanied by noise. b. Distortion

Distortion means that the receiver's output is not the true copy of the analog input signal at the transmitter. Distortion in delta modulation occurs due to following causes: When the analog signal is greater then the integrator output, the integrator ramps up to meet the analog signal. The ramping rate of integrator is constant. Therefore, if the rate of change of analog input is faster than the ramping rate, the modulator is unable to catch up with the information signal. This causes a large disparity between the information signal and its quantized approximation. This phenomenon is known as slope - overloading and causes the loss of rapidly changing the information.


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Information Loss due to Rapid input Changes Figure 4 The problem of slope overloading can be solved by increasing the ramping rate of the integrator. But as the effect of the large step-size added large sharp edges at the integrator's output and hence it adds to noise problem faced at receiver.

Increasing sampling rate cannot be the solution to the slope - overloading problem as it determines how fast the samples are taken and not the ramping rate of the integrator.

Effect of Sampling Rate on Quantization Noise & Step-Size c.

Figure 5 Another problem with delta modulation is that it can not pass DC information. This is not a serious limitation of speech communication but for the systems like video (picture) transmission DC level does not provide information about brightness level of the picture .

The above stated limitations of the delta modulation may be traded for acceptable price in speech application but is totally unsatisfactory for music or video signals.


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Adaptive Delta Modulation Delta modulation system is unable to chase the rapidly changing information of the analog signal, which gives rise to distortion & hence poor quality reception. This is known as slope overloading phenomenon. The problem can be overcome by increasing the integrator gain (i.e. step-size). But using high step-size integrator would lead to a high quantization noise. Quantization Noise:

It is defined as error introduced between the original signal, & the quantized signal due to the fixed step size in which the signal (quantized) is incremented. As the error is random in nature & hence unpredictable, it can be treated as noise. High quantization noise may play on small amplitude signals. The solution to this problem is to increase the integrator gain for fast-changing input & to use normal gain for small amplitude signals. The basic idea is to increase the integrator's gain when slope overload occurs. If still it is unable to catch up with the signal, the integrator gain is doubled again. The integrator on board has four available gains standard, standard X2, standard X4, and standard X8. The integrator thus adopts it self to the gain where its lowest value can just overcome the slope overloading effect. See figure 8.

Principal of Adaptive Delta Modulation Figure 8


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Control Word

Integrator Gain

00

Standard

01

Standard X2

10

Standard X4

11

Standard X8

Table 1 The input to the control circuit is t he latched data from the D Flip-Flop. The counter is reset whenever 'high' appears at the output of the control circuit. Both the counter & the control circuit are clocked by the same TX clock. The input to the integrator from the counter is a two-bit control word, which controls the gain of the integrator. When the output of counter is '00' the gain is lowest (standard) where as it is highest (standard X8) for counters output '11'.

Adaptive Delta Modulator Figure 9

The adaptive delta modulator works as Follows: The control circuit compares the preset data bit from D flip-flop with the previous two data bits. Its output to the counter is high when the three bits are identical, the control circuits output goes low, thus letting the counter advance with every clock cycle. This advancement continues till the output from the control circuit does not go 'high'. Each time the counter is incremented from ‘00’ integrator gain is doubled till the counter


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reached '11' where it remains in that state until it is reset by the counter. Similarly, the adaptive delta demodulator is a like delta demodulator except for two blocks namely, the control circuit & the counter. They function in the same way as in modulator part, except for the fact that they are clocked by the receiver clock. Consider the adaptive delta modulator in operation. In normal case, when slope overloading is not occurring, the integrator output always hunt above & below the analog input even after it has caught up with it. The output from the D-Flip-Flop is a constant ly changing from ' l' to '0' at each TX clock edge. Even when the analog input is changing at a slightly higher rate, the integrator ramp output is able to catch it in two clock cycles. Thus, the output of the D-Flip-Flop is never a three or more consecutive '0' or ‘1’s. The changing input to the control circuit ensures that its output to the counter is high & hence the counter is reset at every clock cycle. Thus the control word from counter is always '00' forcing the integrator gain at its lowest value, thereby reducing quantization noise. Here the adaptive delta modulator is behaving just as a delta modulator. Suppose, now a fast changing analog signal appears at the input of the modulator such that the slope overloading occur. The integrator output no longer follows the analog signal but it spends its time trying to catch up the analog signal (either it ramps down or up continuously). As a result of continuous ramping in one direction, the D-FlipFlops output is either '0' or '1' for three or more consecutive time. As soon as the third continuous 1/0 is sensed by the control circuit its output goes low. The counter now advances to 01 doubling the integrator gain. This increases the ramping rate of the integrator & it is able to catch the analog signal faster. In the next clock cycle if the same situation continues the counter advances to ‘10' thus forcing the integrator gain to quadruple its standard value. This situation continues till the counter advances to '11' where it remains locked until the control logic does not detect a change in the bit level at its input As soon as the control circuit detects a change in the bit level, its output goes high, thus resting the counter & thus normalizing the integrator gain.


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Delta Adaptive Demodulator Figure 10 The Demodulator work as follows:

The adaptive delta demodulator control circuit receives the same bit stream as the transmitted one except for the fact that it is received after a half clock cycle delay. The functioning of the receiver's control circuit & counter is same as that of the transmitter's block. Therefore, the demodulator output which itself is a good approximation of the analog input signal accepts for the inhere spikes. The output from integrator is passed to a low pass filter to 'smooth out 'the waveform. Thus, adaptive delta modulation system is thus able to reduce slope-over load error at an expense of small increase in quantization error. It turns out that in matter of speech transmission the reduced slope error provides a net advantage in spite of slight increase in quantization error & that the adaptive delta. Modulator can operate at the bit rate of 32 KB/S with performance comparable to that obtained using PCM at 64 KB/S.


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Delta Sigma Modulation The delta modulation & adaptive delta modulation suffers from two serious limitations, namely, they are not able to pass DC level information, which is a must in video systems. The signal - to - noise ratio decreases as the signal frequency increases. Both these serious limitations can be overco me by delta sigma modulation. The delta modulator on-board can pass DC level information if the integrators are zeroed before hand (by level adjust presets) & the gains of integrator at two ends (TX & RX) are exactly equal. This is possible on ST2105 board but not in real life operation where TX & RX are separated by hundreds of kms & they are expected to work over a long period without additional setting up. The inherent short coming of the delta modulator to pass DC level information is due to the fact that it passes the information about the change in the voltage level & not the actual voltage level itself e.g. encodes the change from -2V to -1V same as it would encode a change from +2V to +3V. In delta sigma modulation the integrator is added in front of the delta sigma modulator. This simple arrangement makes a big change in the circuit behavior. It now responds to actual voltage levels rather than the change in the voltage level. The effect of the addition of integrator before the delta modulator can be done away by adding a differentiator at the end of the receiver. The System looks as shown in figure 12.

Delta Sigma Modulator Figure 12 The system shown in the figure 12 & 13 can be simplified further. Since the voltage comparator is highly non-linear device cannot transfer the two integrations at its input to integration at its output.

The voltage comparator functionally can be thought of as a unity gain differential amplifier followed by a zero-crossing detector. If the voltage comparator is replaced by this circuit, the no. of integrators can be reduced to one (at the output of unit-gain differential amplifier).Similarly, since the low pass filter is a linear circuit element the effect of integrator at the input of LPF is nullified by the differentiator present at its output. Therefore, these two blocks are redundant in the receiver. The resulting circuit looks as in figure 14 & 15.


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Delta Sigma Modulator Figure 13

Delta Sigma Modulator Figure 14


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Delta Sigma Modulator Figure 15 To see how the circuit functions, suppose that initially the TX data is at logic '1' & the input remains constant at +2V.

Since logic 1 is present at the input of unipolar - to- bipolar converter, its output is 4V, which appear at -ve terminal of unity gain differential amplifier. Its output soon switches to +6V. This +6V appears at the input of integrator which ramps down at a fast rate. As soon as it crosses zero level, the zero-crossing detectors output goes zero which is latched in the coming clock cycle. Thus the D-Flip-Flops output goes low & a zero is transmitted. The zero's appearance at input of unipolar to bipolar converter makes it switch its output to +4V. With +4V input to the unity gain differential amplifier, its output goes to -2V level. Since -2V signal is present at input of integrator; it ramps up slowly, as the rate of ramping depends on the input applied to it. The ramp will not be able to cross-up zero at this slow rate, so again a zero is latched at the next clock cycle. Thus, the inference that can be drawn is that smaller the signal at integrator input, slower it will ramp up or down thus latching the same data bits for more clock periods. Thus the mark to space ratio conveys the information about the mean level of the signal & the signal & the modulator's output depends on the actual signal amplitude itself & not only on its change. The demodulator's work here is to extract the mean level information from the markto space ratio transmitted to it. The received data is latched into D-Flip-Flop by the receiver clock, which has been before being latched. The level changer provides the +ve & -ve voltages needed to reconstruct the input analog signal. It also inverts the signal & compensate for the inversion at transmitter automatically. The work of the low pass filter is to remove the spikes introduced by switching action of unipolar to bipolar converter.


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Advantage offered by Delta Sigma Modulation:

1.

The first & the foremost advantage of delta sigma modulation is its ability to transmit DC Level information.

2.

Receiver Simplicity, since there is no integrator in the demodulator circuit. With no integrator in the receiver, we do not have the problem of its quantization noise or ramping rate limitation, which leads to slope overload at high frequency signals.

3.

With absence of integrator in receiver, the errors due to noise disturbances during transmission are no cumulative.

4.

The signal - to - Noise ratio is independent of frequency of analog signal unlike in delta modulation where it decreases as the frequency of information signal increases.


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Experiment 1 Objective: Study of Delta Modulation Demodulation Equipments Required:

1.

ST2155trainer with power supply cord

2.

Oscilloscope with connecting probe

3.

Connecting cords

Connection Diagram:

Figure 1.1


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Procedure: Initial setup of ST2155:

Clock frequency selector switches

: A = 0 and B = 0 position.

Integrator (1) blocks switches position: Gain control switch position

: Left-hand side.

Switches position

: A=0 and B=0 position.


: Right-hand side

Switches position


1.

Connect the mains supply cord to the Trainer.

2.

Make connection on the board as shown in the figure 1.1.

3.

Switch 'ON' the trainer power supply and oscilloscope.

4.

In order to ensure for correct operation of the system, we first take the input to 0V. So connect the '+' input of the delta modulator's voltage comparator to 0V and monitor on an oscilloscope the output of integrator 1 (TP13) and the output of the transmitter's unipolar to bipolar converter (TP 29) If the transmitter's unipolar to bipolar converter output has equal positive and negative output levels, integrator’s output will be a triangular wave centered around '0' Volts, as shown in figure 1.2 (Case A). However, if the unipolar to bipolar converter’s negative level is greater than the positive level, the integrator's output will appear as shown in figure 1.2 (Case B). Should the unipolar to bipolar converter’s positive output level be the greater of the two levels, the integrator's output will resemble that shown in figure 1.2 (Case C).

5.

The relative amplitudes of the unipolar to bipolar converter’s positive and negative output levels can be varied by adjusting the level adjust preset in the unipolar to bipolar converter circuit 1 block. When it is turned anticlockwise, the negative level increases relative to the positive level, when turned clockwise, the positive level increases relative to the negative level. Prove that you can obtain all the three waveforms shown in figure 1.2 by turning the potentiometer from one extreme to another. Try explaining the reason behind it.


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Case A: Bipolar output: Positive level = Negative level

Case B: Bipolar output: Positive level < Negative level

Case C: Bipolar output: Positive level > Negative level Figure 1.2

6.

Adjust the transmitter's level changer preset until the output of integrator 1 (TP 13) is a triangular wave centered around 0 Volts, as shown in figure 1.2 (Case A). The peak-to-peak amplitude of the triangle wave at the integrator's output should be 1.8V (approx), this amplitude is known as the integrator step size. The output from the transmitter's bistable circuit (TP 19) will now be a stream of alternate '1' and '0', ‘s' this is also the output of the delta modulator it self. The delta modulator is now said to be 'balanced' for correct operation.

7.

Examine the signal at the output of integrator 2 (TP 41) at the receiver. This should be a triangular wave, with step size equal to that of integrator 1, and ideally centre around 0 Volts. If there is any DC bias at the output of integrator 2, remove it by adjusting the receiver's level adjust preset (in the unipolar to bipolar converter circuit 2 block). This preset adjusts the relative amplitudes, of the positive and negative output levels from the receiver's unipolar to bipolar converter circuit. Only when these levels are balanced will there be no offset at the output of integrator2.

8.


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Outputs at TP 13 and TP 41 respectively Volts/Div: 1V Time/Div: 20 usec

Clock Frequency: 50 KHz Input Signal : 0V

The receiver's low pass filter (whose cut off frequency is 4.2 KHz.) then filters out the higher - frequency triangular wave, to leave a DC level at the output of filter (TP 43). If the receiver's level adjust preset has been adjusted correctly, this DC level will be '0' volt. The delta demodulator also is now balanced for correct operation. 9.

Disconnect the voltage comparators '+' input from 0V, and reconnect it to the ~1 KHz output from the function generator block; the modulator's analog input signal is now a 1 KHz sine wave. Monitor this analog signal at the voltage comparator's '+' input (TP 15) together with the output of, integrator 1 (TP 13).Trigger the scope on the same analog signal which is applied to the voltage comparator's '+' input (TP 15). Note how the output of the transmitter integrator follows the analog input, as was illustrated in figure 1. Note :- It may be necessary to readjust slightly the transmitter's level adjust preset (in the unipolar to bipolar converter circuit 1 block) in order to obtain a stable, repeatable trace of the integrator's output signal.

10.

Display the data of the transmitter's bistable (at TP 19), together with the analog input at TP 15 (again trigger on this signal), and note that the 1 KHz sine wave has effectively been encoded into a stream of data bits at the bistable's output, ready for transmission to the receiver.


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11.

For a full understanding of how the delta modulator works, examine the output of the voltage comparator (TP 16), the bistable's clock input (TP 19), and the unipolar to bipolar output (TP 29)

12.

Display the output of integrator 1 (TP 13) and that of integrator 2 (TP 41) on the scope. Note that the two signals are very similar in appearance, showing that the demodulator is working as expected. Outputs at TP 13 and TP 41 respectively Volts/Div: 5V Time/Div: 500usec

Clock Frequency: 50 KHz Input Signal : 1 KHz, 10Vpp

13.

Display the output of integrator 2 (TP 41) together with the output of the receiver's low pass filter block (TP43). Note that although the integrator’s output has been smoothed out somewhat by the low pass filter, some unwanted 'ripple' still remains at filter's output This 'ripple' is due to the 'quantization noise' at the integrator's output, which is caused by the relatively large integrator step size. This step size can be reduced by increasing the rate at which the system is clocked (i.e. the sampling frequency).This reduces the sampling period, and hence the time available between samples for the integrators to ramp up and down.

14.

The current system clock frequency is 50 KHz. This is set by the A, B switches in the clock frequency selector block, which are currently in the A= 0, B= 0 positions. While monitoring the same signals, increase the system clock frequency to 100 KHz, by putting the switches in the A = 0, B = 1 positions. Note :- If the integrator's output (TP41) no longer gives a stable trace after changing the clock frequency, make a slight adjustment to the transmitter's level adjust preset (in the unipolar to bipolar converter circuit 1 block), until the trace is once again stable.


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Notice that, at the integrator's (TP41), the frequency of the triangular error signal doubles, and the peak-to-peak amplitude of that error signal (i.e. the step size) is halved. Examine the ripple at the low-pass filter's output (TP43). Note that this is now less than it was before. 15.

By changing the system clock frequency to first 200 KHz (clock frequency selector switches in A=l, B=0 positions), and then to 400 KHz (switches in A=l, B=1 positions), note the improvement in the low - pass filter's output signal (TP43). Once again, it may be necessary to adjust slightly the transmitter's level adjust preset, in order to obtain a stable oscilloscope trace.

16.

Using a system clock frequency of 400 KHz compare t he low pass filter's output (TP43) with the original analog input (TP15). There should now be no noticeable difference between t hem, other than a slight delay. Output waveforms at TP13 and TP43 respectively Volts/Div: 5V Time/Div: 500usec

Clock Frequency: 400 KHz Input Signal frequency: 1 KHz, 10Vpp

17.

While continuing to monitor the transmitter's analog input (TP15) and the receiver's low-pass filter output (TP43), disconnect the comparator's + input from the 1 KHz sine wave output, and reconnect it to 2 KHz, 3 KHz and 4 KHz outputs in turn and using system clock frequency of 50 KHz . Note that, as the frequency of the analog signal increases, the low pass filter's output becomes more distorted and reduced in amplitude.

18.

In order to understand what has caused this distortion, leave the comparator's + input connected to the 4 KHz sine wave output of the function generator, and examine the output of integrator 2 (TP41). Note that the integrator's output no longer approximates the analog input signal, but is tr iangular.


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Compare this with the output of integrator 1 (TP13), and note that the two signals are exactly the same; the problem obviously starts in the delta modulator circuit Output waveforms at TP13 and TP43 respectively Volts/Div: 5V Time/Div: 200usec

Clock Frequency: 400 KHz Input Signal frequency: 4 KHz, 10Vpp Slope Overloading

19.

Compare the 4 KHz analog input signal (TP15) with the output of integrator 1 (TP13) it should now become clear what has happened. The analog signal is now changing so quickly that the integrator's output cannot ramp fast enough to 'catch up' with it, and the result is known as 'slope overloading.'

20.

Although the system clock frequency i.e. the sampling frequency determines how often the integrator's output direction can change (up or down), it does not affect how quickly the integrator's output can ramp up and down. Consequently, changing the system clock frequency will not help the slope overload problem. Prove this by changing the clock frequency selector switches, and noting that the problem is still present. Return the switches to the A= 1, B=1 (400 KHz clock frequency) position before continuing.

21.

If slope overloading is to be avoided in a practical delta modulation system, the transmitter integrator must be able to ramp up or down at a rate which is at least as great as the maximum rate of change at the transmitter's analog input. If the incoming analog signal is a sine wave, its maximum rate of change occurs at the zero crossing point, and is proportional to both the frequency and the amplitude of the sine wave.


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Hence, the likelihood of slope overloading can be reduced by either reducing the maximum input frequency, or by reducing the maximum input amplitude to the delta modulator. We have already seen how slope overloading can be avoided by reducing the frequency of the analog input signal since there was no problem with the ~1 KHz analog input. Now check that the problem can also be avoided if the amplitude of the input signal is reduced. Do this by slowly turning the ~4 KHz preset anticlockwise. Note that there comes a time when the integrator's output can once again follow the analog input signal. 22.

Another possible way of overcoming slope overloading is to increase t he gain of the integrators so that they can ramp up and down faster, and so can follow even those analog input waveforms those change very quickly. To illustrate this, first return the ~4 KHz preset to its clockwise (maximum amplitude) position, so that slope overloading can once again be seen on the scope. In each of the two integrator blocks, there are two switches labeled A and B. The 2bit binary code produced by these switches selects one of four integrator gains, the lowest gain selected when the switches are in the A=0, B=0 positions. For each increasing step, in the switch code, A=0, B=0 to A=1, B=1, the integrator gain is doubled. Output waveforms at TP15 and TP43 respectively Volts/Div: 5V Time/Div: 200usec


Manual gain setting of Tx. & Rx. Integrators: A=0, B=1

Change the codes produced by the switches (in both integrator 1 and integrator 2 blocks) from A=0, B=0 to A = 1, B=1, to double the gain of the two integrators; note that slope overloading still occurs.


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Then change both sets of switches to the A=1, B=0 position, and finally to the A=1, B= 1, position, to show that slope overloading can be eliminated if the integrator gain is large enough. Once again, it may be necessary to make a slight adjustment to the transmitter's level adjust preset, in order to obtain a stable trace on oscilloscope. Note that, although it is the gain o f integrator 1 alone which determines whether or not slope overloading will occur, integrator 2 must have the same gain if the amplitude of the demodulator's analog output is to be equal in amplitude to the modulator's analog input. Output waveforms at TP15 and TP43 respectively Volts/Div: 5V Time/Div: 200usec


Manual gain setting of Tx. & Rx. Integrators: A=1, B=1

23.

We have observed slope over loading can be overcome by changing anyone of the three following options: a.

Reducing the maximum input frequency to the delta modulator.

b.

Reducing the maximum input amplitude, or

c.

Increasing the integrator gain.

In a practical delta modulation communication system, the signal at the modulator's analog input would normally be in the audio band, so that the maximum input frequency could not be reduced below about 3.4 KHz without losing information. This rules out solution (a) above. The problems with reducing the amplitude of input signal i.e. solution (b) is that smaller input signals then are lost in the quantization noise. They become smaller in amplitude than the integrator's step size.


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Finally, if the integrator gain is increased i.e. solution (c), much of the same problem results as for solution (b), since the larger step size increases the quantization noise and once again 'drowns out' the smaller signals. In experiment 2, we will investigate another solution to the problem of slope over loading which allows us to use high integrator gains for fast-changing analog input signals, and low integrator gains for those smaller signals which would otherwise be 'Drowned out'. Observation Table: Clock Frequency Applied for TX & Rx:

Applied Input S. No.

I/P frequency

1

0KHz

2

1KHz

3

2KHz

4

3KHz

5

4 KHz

Output I/P Voltage

O/P Voltage

Conclusion: Questions:

1.

What is delta modulation?

2.

What is the slope overloading?

3.

How over slope overloading can be reduced?

4.

What is the function of unipolar to bipolar circuit?

5.

What is the function of integrator?


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Experiment 2 Objective: Study of Adaptive Delta Modulation and Demodulation Equipments Required:

1.

ST2155 trainer with power supply cord

2.

Oscilloscope with connecting probe

3.

Connecting cords

Connection Diagram:

Figure 2.1


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Procedure: Initial setup of ST 2155:




: Left-hand side.

Switches position



: Right-hand side

Switches position


Function Generator’s potentiometers position: 1 KHz to 4 KHz Pots position

: fully clockwise position

1.

Connect the main supply cord to the trainer.

2.

Connect the board as per figure 2.1.

3.

Switch ON the supply of trainer and oscilloscope.

4.

As the gain control switch is towards A & B switches, the gain setting is still manual, connect the voltage comparator's +ve input to 0V & check whether the modulator & demodulator are balanced for correct operation as in delta modulation experimentation. Change the clock frequency selector switches to the A=1, B=1, positions (400 KHz Clock Frequency) before continuing.

5.

Disconnect the voltage comparators '+' input from 0V and reconnect it to the 4 KHz output from the function generator block.

6.

Monitor the 4 KHz analog input at TP14 and the output of integrator 1 at TP13. Note that slope overloading is still occurring, as indicated by the fact that the integrator's output is not an approximation of the analog input signal.

7.

At the transmitter, move the slider of the gain control switch in the integrator 1 block to the right-hand position (towards the sockets labeled A, B). At the receiver, move the slider of the gain control switch in the integrator 2 block to the left-hand position (again towards the sockets labeled A, B). The gain of each integrator is now controlled by the outputs of the counter connected to it. Functionally, the transmitter and receiver are now configured as shown in the figure 9 & 10 i.e. as adaptive delta modulator and demodulator respectively.

8.

Once again examine the 4 KHz analog input at TP14 and the output of integrator 1 at TP13, noting that the" slope overloading problem has been eliminated, and that the integrator's output once again follows the analog input signal. Again, it may be necessary to adjust slightly the transmitter's level adjust preset, in order to obtain a stable trace of the integrator's output signal.


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9.

Compare the output of integrator 1 (TP13) with that of integrator 2 (TP41); noting that, both are identical in appearance as expected. Output waveforms at TP13 and TP43 respectively Volts/Div: 5V Time/Div: 200usec


Automatic gain control of Tx. & Rx. Integrators

10.

Examine the output of the low pass filter (TP42) and the output of integrator 2 (TP41). The filter has removed the high-frequency components from the integrator's output signal, to leave good, clean 4 KHz s ine wave.

11.

Compare the original 4 KHz analog input signal (at TP15) with the output signal from the receiver's low pass filter at TP43). Note that the demodulator's output signal is identical to the modulator's input signal, but is delayed somewhat.

12.

Disconnect the voltage comparator’s '+' input from the 4 KHz function generator output, and reconnected it in turn to the 3 KHz, 2 KHz and 1 KHz outputs, noting in each case that the demodulator’s output signal is identical to the modulator's input signal, but delayed in time.


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Output waveforms at TP13 and TP43 respectively Volts/Div: 5V Time/Div: 200usec


Automatic gain control of Tx. & Rx. Integrators

13.

The adaptive delta modulator/demodulator system has therefore eliminated slope-overloading problems. To examine in details how it does this, reconnect the voltage comparator's '+' input to the function generator's 4 KHz output, then reduce the system clock (i.e. sampling) frequency to 50 KHz, by putting the clock frequency selector switches in the A=0, B=0 positions. Although a 50 KHz sampling frequency is too low to ensure that an undistorted output is obtained from the demodulator's low pass filter, it does increase the step size to a level, which makes it easier to understand how the system is operating.

14.

Monitor the 4 KHz analog input signal at TP14 and at the output of integrator 1 (TP13). It should now become a little clearer as to how the adaptive delta modulator is operating. It will be noted that the slope of the integrator's output signal is no longer constant, but increases in a series of discrete steps, in order to 'catch up' with the fast-changing analog input signal. If the integrator output does not 'catch up' with the analog input within two clock periods of its direction changing, the slope of the integrator's output signal. (i.e. the integrator gain) is doubled. If it has still not caught up with the analog input signal by the end of the third clock period, the integrator gain will double once again. If the integrator output still lags behind at the end of the fourth clock period, the integrator's gain is doubled once again, to its maximum value. It then remains at this value until the integrator output 'catches up' with the analog input signal. Once the integrators output 'overtakes' the analog input signal, its direction changes, and its rate of change reverts to the minimum value.


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15.

Examine also the test points in the adaptive control circuit 1 block (TP22-25), to have an understanding of how the adaptive delta modulator is operat ing.

16.

While monitoring the outputs of the modulator's binary counter (TP22 and 23), slowly turn the 4 KHz preset anticlockwise, in order to reduce the amplitude of the 4 KHz analog input signal. Notice that once the analog input signal becomes small enough, both the counter's outputs becomes permanently low, causing the integrator to have minimum gain. This happens because the input signal is now so small that the integrator can always follow it, even with minimum gain. The result is that small-amplitude input signals can be transmitted with minimum integrator gain, thereby keeping quantization noise to a minimum at the demodulator’s output.

Observation Table: Clock frequency applied for Tx And Rx:

Applied Input S.No

I/P frequency

1

0KHz

2

1KHz

3

2KHz

4

3KHz

5

4KHz

I/P Voltage

Output O/P Voltage

Conclusion: Questions:

1.

What is adaptive delta modulation?

2.

What is the drawback of delta modulation?

3.

What is the effect of frequency on adaptive delta modulation?

4.

What is advantage of adaptive delta modulation?

5.

Why integrator is required for adaptive modulation?


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Experiment 3 Objective: Study of Sigma Delta Modulation and De modulation Equipments Required:

1. ST2155 trainer with power supply cord 2. Oscilloscope with connecting probe 3. Connecting cords Connection Diagram:

Figure 3.1


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ST2155

Procedure: Initial setup of ST 2155:




: Left-hand side.

Switches position



: Right-hand side

Switches position


Function Generator’s potentiometers position: 1 KHz to 4 KHz Pots position

: fully clockwise position

1. Connect the main supply cord to the trainer. 2. Connect the board as per figure 3.1. 3. Switch ON the supply of trainer and oscilloscope. 10.

The delta sigma system is currently being clocked at 400 KHz, as indicated by the current positions of the clock frequency selector switches (A=1, B=1). Reduce the clock frequency to 200 KHz (switch positions A= 1, B=0), then 100 KHz (A=0, B=1), and finally to 50 KHz (A=0, B=0), while monitoring the output of the demodulator at TP43. Note : That as the clock frequency is reduced, the analog output signal becomes less 'smooth', indicating that clocking frequency affects the quality of the demodulator's output signal. Leave the system clock frequency at 50 KHz, since it is easiest to analyze the operation of the delta sigma system when the clock frequency is minimum.

11.

In order to understand exactly how the delta-sigma modulator works examine the following signals: analog input (TP27), differential amplifier output (TP26), integrator output (TP13), Voltage comparator output (TP16) and unipolar to bipolar converter output (TP29). Note : In the case of signals at test points 13, 26, 16 and 29, trigger the scope on the analog input signal (TP27), and if necessary adjust the transmitter's level adjust preset (in the unipolar to bipolar converter circuit 1 block) in order to obtain a stable, repeatable waveform.

12.

At the demodulator, examine the bistable's data output (TP32), the unipolar to bipolar converter's output (TP33), and the output of the low pass filter (TP43, to understand how the transmitted data stream is demodulated.

13.

Return the system clock frequency to 400 KHz (clock frequency selector switches in A=1, B=1 positions), so that the demodulator's output once again becomes a good, clean sine wave.


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14.


While examining the modulator’s analog input TP27 and the receiver's low pass filter output; increase the amplitude of the analog input signal to its maximum value, by turning the function generator’s ~1 KHz pot. fully clockwise. Note: The peaks of the demodulator's output signal become flat if peak-to-peak amplitude exceeds that of the transmitter's level changer output, causing the modulator to 'saturate'. The result is a signal at the modulator's output which contains long periods when a continuous stream of' ‘1’s or '0's is transmitted. These long streams of the same digit are responsible for the flattening of peaks at the demodulator output.


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Clock Frequency: 400 KHz Input Signal frequency: 1 KHz, 10Vpp Amplitude Overloading

This amplitude overloading in delta-sigma modulation is equivalent to delta modulation's 'slope over loading', and it occurs whenever the amplitude of the analog input signal exceeds that of the transmitter's level changer output. Examine the signals at the delta-sigma modulator and demodulator, in order to understand exactly how amplitude over loading occurs. 15.

Return the amplitude of the analog input signal to 5 volts P-P by adjusting the ~1 KHz pot. in the function generator block. While monitoring the output of the receiver's low pass filter output (TP43), reduce the gain of the integrator 1 by putting the integrator's A, B switches in the A=0, B=0 position. Note : The demodulator's output becomes distorted, which implies that a minimum integrator gain is necessary in the modulator circuit, in order to guarantee correct operation of the delta modulator system.

Reduce the amplitude of the 1 KHz analog input signal (by turning the ~1 KHz preset anticlockwise), and notice that the distortion can be minimized by using an analog input signal of lower amplitude. Return A, B switches of integrator block 1 to the A=1, B=1 positions before continuing. 16.

Remove the differential amplifier's '+' input from the function generator's ~1 KHz output, and reconnect it in turn to the ~2 KHz, ~3 KHz and ~4 KHz outputs. In each case, adjust the amplitude adjustment potentiometer for a sine wave of 5 volts P-P (to avoid amplitude overloading), and monitor the signal at the demodulator's output (TP43).


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Note: The output is in each case is a good sine wave, and that there is no distortion as the frequency of the analog input signal is increased.






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17.


Finally, connect the differential amplifier's '+' input to the function generator’s D.C. output. Monitor the input (TP27) and the demodulated output (TP43), while turning the D.C. pot. from its fully anticlockwise position to its fully clockwise position. Note that the demodulator's output changes accordingly, indicating that the delta- sigma system can handle D.C. levels. Output waveforms at TP27 and TP43 respectively Volts/Div: 5V Time/Div: 200usec


Clock Frequency: 400 KHz Input Signal: +5V D.C

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Output waveforms at TP27 and TP43 respectively

Volts/Div: 5V Time/Div: 200usec

Clock Frequency: 400 KHz Input Signal: -5V D.C

Conclusion:

Questions:

1.

What do you understand by delta sigma modulation?

2.

What are the advantages of delta sigma modulation?

3.

Why transmitter and receiver clock frequency should be same?

4.

What is the effect of frequency on the reconstructed signals?

5.

Why low pass filter is required while reconstructing the original signals?


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Frequently Asked Questions 1.

What is delta modulation? Ans: 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 is encoded into n-bit data streams.

2.

Write the main features of DM? Ans: Its main features are:

1. The analog signal is approximated with a series of segments 2. Each segment of the approximated signal is compared to the original analog 3. Wave to determine the increase or decrease in relative amplitude. 4. The decision process for establishing the state of successive bits is determined by this comparison 5. 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 nochange condition causes the modulated signal to remain at the same 0 or 1 state of the previous sample. 3.

What are the advantages of delta modulation? Ans: Delta modulation offers many advantages as listed below: •

Simple circuitry

•

Cheap

•

Single bit encoding allows us to transmit more information at some sampling rate for given system bandwidth.


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4.

What are the drawbacks of delta modulation? Ans: The practical use of delta modulation is limited due to the following several drawbacks. a. Noise

Noise is defined as any unwanted unpredictable random waveform accompanying the information signal. Whenever the signal is received at the receiver, it is always accompanied by noise. b. Distortion

Distortion means that the receiver's output is not the true copy of the analog input signal at the transmitter. c. Another problem with delta modulation is that it can not pass DC information. This is not a serious limitation of speech communication but for the systems like video (picture) transmission DC level does not provide information about brightness level of the picture. 5.

What is slope overloading in delta modulation? Ans: When the analog signal is greater then the integrator output, the integrator ramps up to meet the analog signal. The ramping rate of integrator is constant. Therefore, if the rate of change of analog input is faster than the ramping rate, the modulator is unable to catch up with the information signal. This causes a large disparity between the information signal and its quantized approximation. This phenomenon is known as slope - overloading and causes the loss of rapidly changing the information.

6.

How the slope overloading can be minimized? Ans: Problem of slope overloading can be solved by increasing the ramping rate of the integrator. But as the effect of the large step-size added large sharp edges at the integrator's output and hence it adds to noise problem faced at receiver.

7.

How the problem of slope overloading can be overcome? Ans: By using adaptive delta modulation

8.

What is the drawback of delta modulation and adaptive delta modulation? Ans: The delta modulation & adaptive delta modulation suffers from two serious limitations, namely, they are not able to pass DC level information, which is a must in video systems. The signal - to - noise ratio decreases as the signal frequency increases.

9.

By which technique the problem of delta modulation and adaptive delta modulation can be overcome? Ans: By using delta sigma modulation.


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Delta Modulation and Demodulation Trainer

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