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EXPERIMENT CM5: DELTA MODULATION Related course: KEET2202 (Digital Communication and Telephony) OBJECTIVES: To study generation of Delta Modulation signals, detection of Delta Modulation signals, idling noise and overload noise, the following factors that effect of the quality of the DM system (step size and sampling frequency) and adaptive Delta Modulation (ADM) system

EQUIPMENT:

1. Delta Modulation Module – B4330 2. Dual-trace Oscilloscope with probes 3. Electron stabilized power supply (+/- 15 volt) 4. Function/waveform generator-HP33120A 5. Multimeter 6. Connecting wires

PRECAUTIONS: Follow the demonstrator instructions throughout the experiment REFERENCE(S): Refer to the main references of KEET2202 (Digital Communication and Telephony) TEST: TEST 1: Preliminary TEST 2: Generation of Delta Modulation signals TEST 3: Demodulation of DM signals

INTRODUCTION: Delta Modulation (DM) DM is a digitization technique of approximating an analog signal m(t) with a staircase signal f(t).

Two main types of Noise in Delta Modulation system: a) Idling or Granular Noise

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Assuming m(t) is a constant voltage or a slow varying signal. Then f(t) ,is a square-wave approximation. The bigger the step size, the larger is the idling noise. Hence, idling noise may be reduced by keeping the step size small. However, if the step size is too srnall, overload noise may take place. Question: How can idling noise be reduced in the DM Receiver? b) Overload Noise

Overload noise occurs if the rate of change of f(t) is less than the rate of change of m(t).

�/� = ��� < �(�)/�� Overload noise may be avoided by making step size bigger and/or the sampling rate faster. c) Adaptive Delta Modulation (ADM) ADM is a process of adjusting the step size to suit the nature of m(t) in order to reduce both the idling noise and the overload noise. In this Module, the 8 previous bits of the output are presented as input to the DAC and its output helps control the step size.

TEST 1: Preliminary

1. Power up the Delta Modulation module.

2. Switch Toggle Switches SW I to DC Level.

3. Connect Oscilloscope Y1 to-point A1.

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a) Sketch Timing Generator waveform

b) Measure maximum sampling frequency and minimum sampling frequency.

c) Set Sampling Frequency to 64 kHz. Find the measured and calculated sampling pulse width τ in µs.

4. Connect point A2 to earth. [That is m(t) = zero ]

a) The Step Size Controller i. Connect the volt meter to point A7. DC Level = 4 volts. ii. Connect oscilloscope Y1 to point A6. Sketch the waveform.

iii. Measure the frequency, pulse width and pulse height for DC Level = 4, 5, 6, 7, and 8 Volt.

DC Frequency Pulse width Pulse amplitude 4 5 6 7 8

b) Step Size

i. Connect Y2 to point A3 [namely f(t)] ii. Connect oscilloscope Y2 to point A3. Sketch the waveform.

iii. Measure the step size and plot k vs DC.

DC Step size k 4 5 6 7 8

iv. Find how the Step Size k related to the area under the pulse in (ii)

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5. Set DC Level= 5 volts

a) Connect Y1 to point A4 and sketch the waveform

b) Connect Y1 to point A5 and sketch the waveform

c) Connect Y1 to point A8(ADM) and sketch the waveform

TEST 2: Generation of Delta Modulation Signals A) Triangular Waveform

1. Connect a triangular waveform to point A2. Frequency=800 Hz, Amplitude = 2 Volt. 2. Connect Y1 to point A2 and connect Y2 to A3. You may need to invert the Y2. Explain

why you need to invert the Y2. 3. Sketch the waveforms m(t) and f(t). Overload noise occurred? lf not, increase frequency

and/or amplitude so that it occurs. 4. Record the frequency and amplitude. Plot the waveforms of m(t) and f(t). Explain why it

should occur for the given frequency and amplitude. What is the gradient of m(t)? What is the gradient of f(t)? Investigate the effectiveness of the ADM to overcome overload. Switch SW1 towards D/A Connector. Comparing with DM, is this ADM better? If No, explain why?

5. Connect Y1 to point A8. Sketch the waveform.

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B) Sinewave

1. Connect a sine waveform to point A2. Frequency = 800 Hz, Amplitude = 2 Volt. 2. Connect Y1 to point A2 and connect Y2 to A3. 3. Comparing the waveforms m(t) and f(t). Did overload noise occur? If not, increase

frequency and/or amplitude so that it occurs. Record the frequency and amplitude. Plot the waveforms of m(t) and f(t). Observe which portion of the sine waveform overload first occurred. Explain why it should occur for the given frequency and amplitude. What is the gradient of m(t)? What is the gradient of f(t)?

4. Investigate the effectiveness of the ADM to overcome overload. Switch SW1 towards D/

A Connector. Comparing with DM, is this ADM better? If No, explain why. Connect Y1 to point A8. Sketch the waveform.

TEST 3: Demodulation of DM signals A) Settings

1. Connect Tx output to Rx Input. 2. Switch SW1 and SW2 to DC Level (ordinary DM). 3. LP Filter to maximum.

B) Connect a Sine wave to the Tx Input.

1. Frequency 800 Hz and Amplitude 2 volts 2. Connect Y1 to point A2 and Y2 to output of the LP filter. Sketch the waveforms.

3. Increase frequency until overload condition occurs. What is the frequency? Sketch the

waveforms.

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4. Switch SWI and SW2 to ADM. Sketch the waveforms.

5. Do you observe any improvement? 6. Connect Y1 to point A8. Plot the waveform.

7. Is the method of ADM used in this Module good? Then, suggest a better way.

END OF EXPERIMENT