DELTA MODULATION

&DEMODULATIO

N

DM is a modulation technique in which the message signal is encoded into a sequence of

binary symbols.

DM is the simplest form of differential pulse-code modulation (DPCM) where the difference between successive samples is encoded into 1 bit data stream.

Only the change of information is sent, that is, only an increase or decrease of the signal amplitude from the previous sample is sent.

DELTA MODULATION

DM MODULATOR

The difference signal d(t) is given as:

∆(t)= x(t)-xs(t)Where; -x(t) is the message signal-xs(t) is the reference signalwhich is the former sampling value

However the difference signal d(t) will be converted by a limiter. Hence the obtained signal is given as:

∆(t)={+ ∆ if ∆(t)>0}∆(t)={-∆ if ∆(t)<0}

If the difference signal ∆(t) is larger than zero, this means that the reference signal xs (t) is lower than the low frequency signal x(t).

The estimated value is too small and we need to increase the next estimated value by ∆; on other words, we only need to send a variation value, which is ∆ during the transmission.

Nevertheless, if the difference signal d(t) is lower than zero, then we only need to transmit a variation value which is -∆.

ILLUSTRATION OF DM SIGNAL

DM MODULATOR

DISTORTIONS IN DM SIGNALS

In order for the sequence of samples xs to increase as fast as the input, the following condition must be satisfied :

Slope overload distortion In this distortion type , the step size delta ∆ is too

small for the staircase approximation to follow the input signal.

Granular Noise In contrast to slope-overload distortion, granular noise occurs when ∆ is too large relative to the local slope characteristics of the input waveform.

SLOPE OVERLOAD DISTORTION &

GRANULAR NOISE

Signal-to-noise ratio of DM is larger than signal to-

noise ratio of PCM.

In PCM, all the bits that are used to code a sample

are transmitted, whereas in DM only one bit for one

sample is transmitted.

PCM VS. DM