18-09-2012, 05:37 PM
Digital Transmission
1Digital Transmission.ppt (Size: 1.32 MB / Downloads: 202)
ANALOG-TO-DIGITAL CONVERSION
A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation.
PCM
PCM consists of three steps to digitize an analog signal:
Sampling
Quantization
Binary encoding
Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.
Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.
Sampling
Analog signal is sampled every TS secs.
Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
There are 3 sampling methods:
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flattop - sample and hold, like natural but with single amplitude value
The process is referred to as pulse amplitude modulation PAM and the outcome is a signal with analog (non integer) values
Quantization Levels
The midpoint of each zone is assigned a value from 0 to L-1 (resulting in L values)
Each sample falling in a zone is then approximated to the value of the midpoint.
Quantization Error and SNQR
Signals with lower amplitude values will suffer more from quantization error as the error range: /2, is fixed for all signal levels.
Non linear quantization is used to alleviate this problem. Goal is to keep SNQR fixed for all sample values.
Two approaches:
The quantization levels follow a logarithmic curve. Smaller ’s at lower amplitudes and larger’s at higher amplitudes.
Companding: The sample values are compressed at the sender into logarithmic zones, and then expanded at the receiver. The zones are fixed in height.
1Digital Transmission.ppt (Size: 1.32 MB / Downloads: 202)
ANALOG-TO-DIGITAL CONVERSION
A digital signal is superior to an analog signal because it is more robust to noise and can easily be recovered, corrected and amplified. For this reason, the tendency today is to change an analog signal to digital data. In this section we describe two techniques, pulse code modulation and delta modulation.
PCM
PCM consists of three steps to digitize an analog signal:
Sampling
Quantization
Binary encoding
Before we sample, we have to filter the signal to limit the maximum frequency of the signal as it affects the sampling rate.
Filtering should ensure that we do not distort the signal, ie remove high frequency components that affect the signal shape.
Sampling
Analog signal is sampled every TS secs.
Ts is referred to as the sampling interval.
fs = 1/Ts is called the sampling rate or sampling frequency.
There are 3 sampling methods:
Ideal - an impulse at each sampling instant
Natural - a pulse of short width with varying amplitude
Flattop - sample and hold, like natural but with single amplitude value
The process is referred to as pulse amplitude modulation PAM and the outcome is a signal with analog (non integer) values
Quantization Levels
The midpoint of each zone is assigned a value from 0 to L-1 (resulting in L values)
Each sample falling in a zone is then approximated to the value of the midpoint.
Quantization Error and SNQR
Signals with lower amplitude values will suffer more from quantization error as the error range: /2, is fixed for all signal levels.
Non linear quantization is used to alleviate this problem. Goal is to keep SNQR fixed for all sample values.
Two approaches:
The quantization levels follow a logarithmic curve. Smaller ’s at lower amplitudes and larger’s at higher amplitudes.
Companding: The sample values are compressed at the sender into logarithmic zones, and then expanded at the receiver. The zones are fixed in height.