Pulse Modulation CHAPTER 4

Part 3

□Differential PCM (DPCM)□Delta Modulation (DM)□Noise in Delta Modulation□Digital Modulation

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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.

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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.

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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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DPCM decoder

• Predictor input:

• Predictor output:

• PCM Decoder: to recover yn from the DPCM sequence

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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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Cont’d…

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Operation of Delta Modulation

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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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Delta Modulation - Example

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Noise in Delta Modulation

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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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Cont’d…

• To avoid slope overload, σ = Amax ωm / fs

Where σ=step sizeAmax = Maximum signal

amplitudeωm = = 2 πfm . For voice

signal, fm = 800 Hz.

fs =sampling rate

• average power of granular noise,N0=σ2 B / 3 fs

Whereσ=step sizeB = signal bandwidthfs =sampling rate

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Example 1The 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. Finda)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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Digital Modulation• 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