# DIGITAL MODULATION

Mar 9 • Engineering Sample Papers, General • 6494 Views • 6 Comments on DIGITAL MODULATION

DIGITAL MODULATION AND INFORMATION THEORY

1.WHAT IS COMPANDING?

Ans .COMPANDING: Companding is a non uniform process of digital modulation. In non uniform process signal to noise ratio is very week. If we want to increase the signal to noise ratio then we have to increase the value of delta or step size. For this we use compandig. Companding is the sum of compression and expansion. Before applying the signal  ,the high frequency signal is attenuated and low frequency signal is amplified in the transmitter. The reverse case is happened on the receiver end.

Companding = Compression + Expansion

There is two law of companding:i) A-law companding

ii)µ-law companding

Ans: ADM meaning is Adaptive Delta Modulation. In the case of delta modulation the step size is fixed. so it can not exactly match up with the modulating signal. In the case of adaptive delta modulation size of the delta or step size can be changed according to the modulating signal. Also noise interference is very less. When the signal’s amplitude is very high in that case step size can be changed according to the modulating signal. If the signal’s amplitude is less then step size can be decreased.

3. EXPLAIN ARQ?

Ans: ARQ is automatic  repeat request . In this case when the some bit is applied to the controller or buffer it stores the value. Then it applied to a encoder which encodes it and at the receiver end decoder receive it and send a acknowledgement or ACK /NCK. If the correct bit is received or there is no error then it will give ACK and if error is there then NCK is received and it will then send the received one by using the retransmission path. controller supply the stored bits.

4. WHAT IS SAMPLING THEOREM?

A continuous signal can be sampled in its discrete value and can be recovered back when  fs>= 2fm. Here fs is the sampling frequency and fm is the maximum frequency of the modulating signal.

5. DIFFERENCE BETWEEN PCM & PAM?

Pam is pulse amplitude modulation.

PCM is pulse code modulation.

PCM digital signal but PAM is not the digital signal.

In PAM we divide the amplitude of the signal into pulse.

In PCM we covert the signal into a coded form.

6.WHAT IS ENTROPY?

Entropy is the average of the information rate. In entropy we observe the fluctuation of the information flow.

7. CONVERSION OF ANALOG SIGNAL TO DIGITAL

Firstly the analog signal is applied to the PAM circuit. Here the signal is sampled by using sampling circuit. After this it is applied to PAM circuit and it is converted to the quantized form and applied to  the encoder. Encoder encodes it and convert the sampled form into the code form or binary form and then its applied to the decoder where by using the holding circuit it decodes it.

8. WHY WE USE AMPLIFIER IN CASE OF LONG DISTANCE  COMMUNICATION?

In the case of long distance communication, the signal losses its gain or power. So to amplify or to increase the power of the signal we have to use amplifier, otherwise we loss the signal or the appropriate signal is not received.

9.WHAT IS REDUNDANT BIT?

When we  transfer some bits, we add some redundant bit or extra bit with it. This extra bits are called the redundant bit.

10.WHY WE USE THIS BIT?

We use this bits to make the transmitted bits error free. At the receiver end error free bits are received.