Dpcm

All about the differential pulse code modulation and adaptive pulse code modulation which is one the classification of pulse code modulation(digital communications).

1. DPCM D ifferential P ulse c ode M odulation By K.Ramya Sree 18BF1A0485 ECE – B Sri Venkateswara College Of Engineering,Tirupati .

2. Why we need DPCM?

3. Short Review of PCM Short review of PCM For the samples that are highly correlated,when encoded by PCM,will leave Redundant information behind. Cause for Redundancy ●High Correlation b/w conjugation bits. ●When sampled at a rate slightly greater that Nyquist rate.(fs>2W) Where W is maximum frequency of the analog i /p signal. Redundancy in the quantizer. Redundant bits means Extra bits.

4. What is high correlation?

5. High correlation with reference to DPCM. During the process of quantization  the signal doesnot change rapidly from one samples to the next sample. With the result that the difference b/w adjacent samples has variance that is smaller than the variance of the signal itself. I.e ; variance (difference b/w adjacent sample) < variance (digitalized signal) So at this case the samples obtained are called as highly correlated samples.

6. Redundant Information bits in PCM Signal x(t) sampled by flat top sampling.

7. The above figure shows a continuing time signal x(t) denoted by a dotted line. This signal is sampled by flat-top sampling at intervals Ts, 2Ts, 3Ts… nTs . The sampling frequency is selected to be higher than the Nyquist rate. These samples are encoded by using 3-bit (7 levels) PCM. The samples are quantized to the nearest digital level as shown by small circles in the above figure. The encoded binary value of each sample is written on the top of the samples. Just observe the above figure at samples taken at 4Ts, 5Ts, and 6Ts are encoded to the same value of (110). This information can be carried only by one sample value. But three samples are carrying the same information means redundant. Now let consider the samples at 9Ts and 10Ts, the difference between these samples only due to the last bit and first two bits are redundant since they do not change.

8. How to process the Redundant Information?

9. Solution DPCM(differential pulse code modulation) To process this Redundant Information and to have better output,it is a wise decision to take the predicted sampled values ,assumed from its previous output and summarize them with the original quantised values.And such process is called as DPCM. DPCM is a derivative of standard PCM.

10. D ifferential  p ulse  c ode  m odulation (DPCM) is a procedure of converting an analog into a digital signal in which an analog signal is sampled and then the difference between the actual sample value and its predicted value (predicted value is based on previous sample or samples) is quantized and then encoded forming a digital value. DPCM Reduces the bit rate. DPCM code words represent differences between samples unlike PCM where code words represented a sample value.

11. DPCM analysis The signals at each point are named as − x( nTs ) is the sampled input xˆ( nTs ) is the predicted sample e( nTs ) is the difference of sampled input and predicted output, often called as prediction error v( nTs ) is the quantized output u( nTs ) is the predictor input which is actually the summer output of the predictor output and the quantizer output

12. Quantizer Output is represented as − v( nTs )=Q[e( nTs )] =e( nTs )+q( nTs ) v( nTs )= e( nTs )+q( nTs )  eqn 1 Where  q ( nT s )  is the quantization error Predictor input u( nTs )=xˆ( nTs )+v( nTs )  eqn 2 u( nTs )=xˆ( nTs )+ e( nTs )+q( nTs ) (from eqn 1) predictor output quantizer output Result :Predictor input is the sum of quantizer output and predictor output , The same predictor circuit is used in the decoder to reconstruct the original input.

13. DPCM Reciever The block diagram of DPCM Receiver consists of  a decoder  a predictor  a summer circuit  The input given to the decoder is processed and that output is summed up with the output of the predictor, to obtain a better output.  In the absence of noise, the encoded receiver input will be the same as the encoded transmitter output.

14. Prediction gain Variance of message signal assumed to be zero mean. Variance of prediction error Variance of quantization error

15. ADPM Adaptive differential pulse-code modulation  ( ADPCM ) is a variant of  differential pulse-code modulation  (DPCM) that varies the size of the quantization step,(variation of step size depends upon amplitude levels) to allow further reduction of the required data bandwidth for a given  signal-to-noise ratio . Also reduces the bit rate. ADPCM implemented using forward Logic for adaptive predictio n Or backward estimation. In forward estimation,unquantized sample of i /p signal is used to obtain the step size. in backward estimation,quantizer o/p samples are used to obtain step size.

16. Advantages and Applicationsof ADPCM Advantages 1)bit rate less 2) Bcz of backward estimation,problems of delay and requriements of buffer are reduced . Applications Voice coding at 32kpbs Secure transmission over radio channels.

17. Any Queries ??

18. Thankyou