2/23/2012 Muhammad Ali Jinnah University, Islamabad Digital Communications EE3723 1
EE4723 : Digital Communications II
Week 3:
Message Formating
A review of PCM, DPCM and ADPCM
Delta Modulation
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Message Formating-Digital Communication-Lecture Slides, Slides of Digital Communication Systems
This lecture was delivered by Asma Ali at Quaid-i-Azam University for Digital Communication course. It includes: Message, Formating, PCM, DPCM, ADPCM, Delta, Modulation, Formatting, Transmit, Receive, Character, Coding
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2011/2012
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EE4723 : Digital Communications II
Week 3:
Message Formating
A review of PCM, DPCM and ADPCM
Delta Modulation
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Formatting
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A textual information is a sequence of alphanumeric characters
Alphanumeric and symbolic information are encoded into
digital bits using one of several standard formats, e.g, ASCII,
EBCDIC
Character Coding (Textual Information) docsity.com
Example 1:
In ASCII alphabets, numbers, and symbols are encoded using a 7-bit code
A total of 2 7 = 128 different characters can be represented using a 7-bit unique ASCII code (see ASCII Table, Fig. 2.3) docsity.com
Encoding
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Advantages of PCM
1. Robustness to noise and interference
2. Efficient regeneration
3. Efficient SNR and bandwidth trade-off
4. Uniform format
5. Ease add and drop
6. Secure
Virtues, Limitations and Modifications of PCM
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Consider a finite-duration impulse response (FIR) discrete-time
filter which consists of three blocks :
1. Set of p ( p : prediction order ) unit-delay elements ( z
2. Set of multipliers with coefficients w
1
, w
2
,… w
p
3. Set of adders ( )
Linear Prediction Coding (LPC)
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Differential Pulse-Code Modulation (DPCM)
Usually PCM has the sampling rate higher than the Nyquist rate. The encode signal contains redundant information. DPCM can efficiently remove this redundancy. 32 Kbps for PCM Quality docsity.com
Need for coding speech at low bit rates , we have two aims in mind:
1. Remove redundancies from the speech signal as far as
possible.
2. Assign the available bits in a perceptually efficient manner.
Adaptive quantization with backward estimation (AQB).
Adaptive Differential Pulse-Code Modulation (ADPCM) docsity.com
ADPCM
Adaptive prediction with backward estimation (APB).
8-16 kbps with the same quality of PCM
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