1. Pulse Modulation CHAPTER 4 Part 3
Differential PCM (DPCM)
Delta Modulation (DM)
Noise in Delta Modulation
Digital Modulation
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2. 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.
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3. 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.
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4. 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.
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5. EKT343-Principles of Communication Engineering
DPCM decoder
Predictor input:
Predictor output:
PCM Decoder: to recover yn from the DPCM sequence
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6. 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
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7. EKT343-Principles of Communication Engineering
Cont’d…
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8. Operation of Delta Modulation
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9. 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.
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10. Delta Modulation - Example
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11. Noise in Delta Modulation
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12. 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
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13. 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
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14. 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.
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15. 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

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

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

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

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

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

21. 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

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