communication engineering lab report, 5th sem, electrical engineering, IIT Delhi, eep306

1. Umang Gupta (2010EE50564)
Indra Bhushan(2010EE50548)
Vivek Mangal(2010EE50566)
Jitendra Kumar Meena (2009EE50483)
19-9-12
Experiment
Delta Modulation
Aim: To demonstrate modulation and de-modulation of Delta Modulation Scheme.
Introduction:
Delta modulation is a technique to convert analog-to-digital and digital-to-analog signal.
This is used for voice transmission. In this modulation, signal is sent in differential form, the
data is encrypted/transmitted in 1 bit.
The analog signal is approximated with series of segments and each segment is compared to
original analog to determine the change in relative amplitude. Hence only change in
information is sent and if no change occurs it remains on the same state. Delta-modulation
has to be oversampled to achieve high signal to noise ratio. If not oversampled there are
problem of step over-load and granularity.
Observations and remarks:
Theoritical observations:
Matlab code for Delta modulation of a 50Hz wave with 2000Hz sampling frequency
% %signal sampling
fs=1/2000;
tn=0:fs:1/25;
m=.5*sin(2*pi*50*tn);
% %Delta modulation-demodulation
StepSize=1/5;
encode=dm_encoder(m, StepSize);
decode=dm_decoder(StepSize,fs, encode);
m_output=lpf(100, .1, decode);
functions used:

2. encoder function:
function cn=dm_encoder(x, StepSize)
xlen = length(x);
accum(1) = 0;
for i=1:xlen
if(x(i)>=accum(i))
e_n(i)=1;
accum(i+1) = accum(i) + e_n(i) * StepSize;
else
e_n(i)=-1;
accum(i+1) = accum(i) + e_n(i) * StepSize;
end
end
cn = e_n < 0;
Decoder function:
function [Sn]=dm_decoder(StepSize,fs,cn)
xlen = length(cn);
Ts=1/fs;
n=0:Ts:Ts*(xlen-1);
xlen = length(cn);
accum(1) = 0;
for i=1:xlen
if(cn(i)==0)
accum(i+1) = accum(i)+StepSize;
else
accum(i+1) = accum(i)-StepSize;
end
end
Sn=accum(2:xlen+1);
Low pass filter:
function Sa=lpf(tap, cf, Sn)
b=fir1(tap,cf);
Sa = conv2(Sn,b,'same');
Graphs from Matlab:

3. Input signal
Decoded signal projected on input signal

4. Signal for transmission
Demodulated output signal

5. Practical observations:
Waveforms for sampling freq 100Khz, 50Khz and 25 Khz respectively

6. on measuring the sampling frequnecy it comes out to be 100,50 and 25 for above three
waveforms.
Waveform showing staircase output

7. Wavefrom showing the transmitted signal (signal 4)
Note that there is some delay and also it is inverted.
Below are the two plots showing the demodualation. Note that signal 4 is demodualated
signal.

8. Here the signal is erraneous because of smaller stepsize.
Concluding remarks:
Though modulation and demodulation is easy for delta modulation technique, but however
it is not used much because it includes noise in the channel, which is undesirable.