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Digital Communication_ppt ppt ppt
Typology: Summaries
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22
Analog to digital converter employs two
techniques:
๏ 2. Quantization: The process of dividing
the maximum value of the analog signal into a fixed no. of levels in order to convert the PAM into a Binary Code.
The levels obtained are called โquanization levelsโ.
37
Quantization Noise
Illustration of the quantization process (^37)
into PCM is called coding, the inverse operation is called decoding
Time
V o l t a g e
111 110 101 100 011 010 001 000
L e v e l s B i n a r y C o d e Time^ s
7 6 5 4 3 2 1 0 V o l t a g e
Sampling, Quantization and Coding
Time^2727
๏ This scheme sends only the difference between pulses, if the pulse at time tn+1 is higher in amplitude value than the pulse at time tn, then a single bit, say a โ1โ, is used to indicate the positive value.
๏ If the pulse is lower in value, resulting in a negative
value, a โ0โ is used.
๏ This scheme works well for small changes in signal values between samples.
๏ If changes in amplitude are large, this will result in large errors.
The process of delta modulation
DM system. ( a ) Transmitter. ( b ) Receiver. (^43)
๏ Distortions in DM system
that of approximated digital signal over long duration,than this difference is called Slope overload distortion.
original signal is called as Granular noise. It is similar to quantisation noise.
Granular noise occurs when step sizeโฒ is large relative
to local slope m(t).
There is a further modification in this system,in which
step size is not fixed.
That scheme is known as Adaptive Delta Modulation.
Condition to avoid slope overload distortion
๐ด๐ โค
๐ 2 ๐๐๐๐๐
๐ โฅ
2 ๐๐๐๐ด๐ ๐๐
๐ด๐ is the message signal amplitude, ๐๐ is the message signal frequency, ๐๐ is the sampling frequency and d is the step size.
To get a better performance we can use
๐ = ๐๐๐๐๐ด๐ ๐๐ N is the step size control factor
