28-12-2012, 06:33 PM
Signal Encoding Techniques
1Signal Encoding.ppt (Size: 1.2 MB / Downloads: 119)
Digital Data, Digital Signal
Digital signal
discrete, discontinuous voltage pulses
each pulse is a signal element
binary data encoded into signal elements
Some Terms
unipolar
polar
data rate
duration or length of a bit
modulation rate
mark and space
Interpreting Signals
need to know
timing of bits - when they start and end
signal levels
factors affecting signal interpretation
signal to noise ratio
data rate
bandwidth
encoding scheme
Comparison of Encoding Schemes
signal spectrum
clocking
error detection
signal interference and noise immunity
cost and complexity
Nonreturn to Zero-Level(NRZ-L)
two different voltages for 0 and 1 bits
voltage constant during bit interval
no transition I.e. no return to zero voltage
such as absence of voltage for zero, constant positive voltage for one
more often, negative voltage for one value and positive for the other
Nonreturn to Zero Inverted
nonreturn to zero inverted on ones
constant voltage pulse for duration of bit
data encoded as presence or absence of signal transition at beginning of bit time
transition (low to high or high to low) denotes binary 1
no transition denotes binary 0
example of differential encoding since have
data represented by changes rather than levels
more reliable detection of transition rather than level
easy to lose sense of polarity
Multilevel BinaryPseudoternary
one represented by absence of line signal
zero represented by alternating positive and negative
no advantage or disadvantage over bipolar-AMI
each used in some applications
Multilevel Binary Issues
synchronization with long runs of 0’s or 1’s
can insert additional bits, cf ISDN
scramble data (later)
not as efficient as NRZ
each signal element only represents one bit
receiver distinguishes between three levels: +A, -A, 0
a 3 level system could represent log23 = 1.58 bits
requires approx. 3dB more signal power for same probability of bit error