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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Digital Communications: Message Formatting and PCM Techniques, Slides of Digital Communication Systems
A series of lecture notes from muhammad ali jinnah university's digital communications course (ee3723) covering message formatting, pcm, dpcm, and adpcm. Topics include character coding, pulse code modulation, differential encoding, linear prediction coding, and delta modulation. The notes also discuss the advantages and limitations of pcm and the need for coding speech at low bit rates.
Typology: Slides
2011/2012
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EE4723 : Digital Communications II
Week 3:
Message Formating
A review of PCM, DPCM and ADPCM
Delta Modulation
Formatting
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)
Example 1: In ASCII alphabets, numbers, and symbols are encoded using a 7-bit code
A total of 27 = 128 different characters can be represented using a 7-bit unique ASCII code (see ASCII Table, Fig. 2.3)
Encoding
Advantages of PCM
- Robustness to noise and interference
- Efficient regeneration
- Efficient SNR and bandwidth trade-off
- Uniform format
- Ease add and drop
- Secure
Virtues, Limitations and Modifications of PCM
Consider a finite-duration impulse response (FIR) discrete-time filter which consists of three blocks :
- Set of p ( p : prediction order ) unit-delay elements ( z -1)
- Set of multipliers with coefficients w 1 , w 2 ,… wp
- Set of adders ( )
Linear Prediction Coding (LPC)
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
Need for coding speech at low bit rates , we have two aims in mind:
- Remove redundancies from the speech signal as far as possible.
- Assign the available bits in a perceptually efficient manner.
Adaptive quantization with backward estimation (AQB).
Adaptive Differential Pulse-Code Modulation (ADPCM)
ADPCM
Adaptive prediction with backward estimation (APB).
8-16 kbps with the same quality of PCM