Pulse Modulation CHAPTER 4 Part 3 Differential PCM (DPCM) Delta Modulation (DM) Noise in Delta Modulation Digital Modulation EKT343-Principles of Communication Engineering

Differential Pulse Code Modulation (DPCM) Differential Pulse Code Modulation (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 code words represent differences between samples unlike PCM where code words represented a sample value. predict current sample value based upon previous samples (or sample) and encode the difference between actual value of sample and predicted value (the difference between samples can be interpreted as prediction error). Because it's necessary to predict sample value DPCM is form of predictive coding. DPCM compression depends on the prediction technique, well-conducted prediction techniques lead to good compression rates, in other cases DPCM could mean expansion comparing to regular PCM encoding. EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Cont’d It often happens that in the analogical signals, which are coded in PCM (voices, images, etc), that many next samples show the same quantization level; as a consequence there is the transmission of many equal PCM codes and this is redundant for the reception signal reconstruction. The DPCM coding exploits this redundancy between adjacent samples. DPCM requires fewer bits than the standard PCM. Basic concept of DPCM - coding a difference, is based on the fact that most source signals show significant correlation between successive samples so encoding uses redundancy in sample values which implies lower bit rate. EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering DPCM encoder Quantizer input ; en - prediction error Quantizer output ; qn- quantization error Prediction filter is usually implemented by a linear predictor in which the last r samples are used to predict the values of the next sample. EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering DPCM decoder Predictor input: Predictor output: PCM Decoder: to recover yn from the DPCM sequence EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Delta Modulation (DM) A single-bit PCM code to achieve digital transmission of analog. Use only 1 bit either logic ‘1’ or ‘0’. Logic ‘0’ is transmitted if current sample is smaller than the previous sample Logic ‘1’ is transmitted if current sample is larger than the previous sample EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Cont’d… EKT343-Principles of Communication Engineering

Operation of Delta Modulation EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Cont’d... Analog input is approximated by a staircase function Move up or down one level () at each sample interval (by one quantization level at each sampling time)  output of DM is a single bit. Binary behavior Function moves up or down at each sample interval In DM the quantization levels are represented by two symbols: 0 for - and 1 for +. In fact the coding process is performed on eq. The main advantage of DM is its simplicity. EKT343-Principles of Communication Engineering

Delta Modulation - Example EKT343-Principles of Communication Engineering

Noise in Delta Modulation EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Slope overload distortion occurs when  is too small, the staircase approximation mq(t) can't follow closely the actual curve of the message signal m(t ). large  is needed for rapid variations of m(t) to reduce the slope-overload distortion granular noise occurs when  is too large relative to the local slope characteristics of m(t). granular noise is similar to quantization noise in PCM small  is needed for slowly varying m(t) to reduce the granular noise EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Cont’d… To avoid slope overload, σ = Amax ωm / fs Where σ=step size Amax = Maximum signal amplitude ωm = = 2 πfm . For voice signal, fm = 800 Hz. fs =sampling rate average power of granular noise, N0=σ2 B / 3 fs Where σ=step size B = signal bandwidth fs =sampling rate EKT343-Principles of Communication Engineering

EKT343-Principles of Communication Engineering Example 1 The specification of delta modulation (DM) system used to transmit a voice signal is as below: Sampling rate: 64 kHz. Peak amplitude: 1 V. Voice signal bandwidth: 3.5 kHz. Find a)The minimum permissible value of the step size delta in order to avoid slope overload distortion. b)The average power of granular noise. EKT343-Principles of Communication Engineering

Digital Modulation Positive or negative pulse used for zero or one Using Digital Signals to Transmit Digital Data Bits must be changed to digital signal for transmission Unipolar encoding Positive or negative pulse used for zero or one Polar encoding Uses two voltage levels (+ and - ) for zero or one Bipolar encoding +, -, and zero voltage levels are used

Non-Return to Zero-Level (NRZ-L) 0 – High level 1 – Low level

Non-Return to Zero Inverted (NRZ-I) 0 – no transition 1 – transition at beginning of the interval

Multilevel Binary(Bipolar-AMI) 0 – No line signal 1 – positive or negative level, alternating for successive ones 0 1 0 0 1 1 0 0 0 1 1

Pseudoternary 0 – positive or negative level, alternating for successive zeros. 1 – No line signal. 0 1 0 0 1 1 0 0 0 1 1

Manchester There is a transition at the middle of each bit period. 1  low-to-high transition 0  high-to-low transition

Differential Manchester mid-bit transition is ONLY for clocking. 1  absence of transition at the beginning of the bit interval 0  presence of transition at the beginning of the bit interval

Example 2 Sketch the data wave form for a bit stream 110100110 using NRZL Bipolar AMI Pseudoternary Manchester Differential Manchester