Reason to use ADM :To overcome the quantization errors due to

slope overload and granular noise, the step size (delta) is made adaptive to variations in the input signal x(t)

Particularly in the steep segment of the signal x(t),the step size is increased. also if the input is varying slowly, the step size is reduced. Then this method is known as adaptive Delta modulation(ADM)

When signal changes are very slow:

Idea ? Remedy :When signal changes are very fast:

Idea ? Remedy:

ADM transmitter:

One bit quantizer

∑∑

∑∑

Delay Ts

Logic for step size control

)(nTsx )(nTse

)(ˆ nTsx

Tsnu )1(

)(nTsu

Output

accumulator

Transmitter:

The logic for the step size control is added.Depending on the one bit quantizer output

the step size increases or decreases.

Pulse generator

Difference amplifier

Modulator

integratorVariable gain amplifier

Square law device

R

m(t)DM O/P

Gain control i/p

Continuously Variable Slope Delta Modulation (OR) ADM

)(~ tm

)(t

Integrator provides low gain when control voltage is zero and larger gain with increasingly +ve control voltage.

Gain control circuit

ADM receiver:

∑∑XX

Logic for step size control

Delay Ts

Low pass filter

Accumulator

input

output

Advantages: 1)S/N better than LDM 2)dynamic range wider than LDM 3)utilization of bandwidth is better thanDM

For the signal which is highly correlated PCM fails w.r.t. transmission bandwidth.

Ex: Tx’n of picture (or) video information appreciable portions of the signal describe background information containing very little tonal variations.

If we use PCM ,for this codeword values of average background level is repeated same.

One way to improve is digitally encoded differences b/n samples.

Ex: 256 levels (8 bits) picture Txn bandwidth << 4 bit differential encoding txn bandwidth

Reason to use DPCM:To reduce the redundancy for highly correlated

samples so as to increase over all bit rate.

(010)

(100) (1

01)

(110)

(110)

(110)

(101)

(101)

(101)

(100)

Ts

PCM + differential quantizing scheme =DPCM

DM is one bit version(2 levels) of DPCM

1)Sampling+qunatization +encoding.

1)Oversampled +2level quantization +1 bit encoding

1)Oversampled signal+qunatizer+encoder.

PCM DM DPCM

DPCM transmitter block diagram:

One bit quantizer

∑∑

∑∑

Prediction filter

)(nTsx )(nTse

)(ˆ nTsx

)(nTsxq

DPCM signal

predictor

)(nTseqencoder

DPCM works on the principle of prediction.The value of the present sample is predicted from

the past samples. The prediction may not be exact but it is very close to the actual sample value.

= - this is the difference between unquantised input sample and prediction of it

Input to the prediction filter is quantiser output signal

and previous prediction so = + This makes the prediction more and more close to the actual sampled

signal.

)(nTse )(nTsx )(ˆ nTsx

)(nTseq )(ˆ nTsx

)(nTsxq )(nTseq )(ˆ nTsx

We can observe that the quantized error signal

is very small and can be encoded by using small number of bits.

Thus the number of bits /sample are reduced in DPCM.

= +

= +

Sub eq (1) in Eq (2) = + +

)(nTseq

)(nTseq )(nTse )(nTsq qzr66.5

Eq(1)

)(nTsxq )(nTseq)(ˆ nTsx Eq(2)

)(nTsxq )(ˆ nTsx )(nTse )(nTsq Eq(3)

)(nTsxq = )(nTsx + )(nTsq Eq(4)

From eq(4) it does not depend upon the prediction filter characteristics.

DPCM ReceiverDPCM Receiver

Decoder

Prediction filter

∑

DPCM input output+

+

Reconstructs the quantized error signal

Output S/N ratio for DPCM:For PCM :

For DM :

For DPCM :Where Gp = &

v

x

p

n

s 22max

2.3

3228

3

smM Tffn

s

pSNRGn

sp )(

2

2

E

X

2

2

Q

EpSNR

Differences btn PCM, DM,ADM, DPCM1) no of bits :2)Level and step size3)Qzn noise error and distortion4)Transmission bandwidth5)Feedback6)Complexity of implementation