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Transcript
4.2 Digital Transmission
Outlines
□
□
□
□
Pulse Modulation (Part 2.1)
Pulse Code Modulation (Part 2.2)
Delta Modulation (Part 2.3)
Line Codes (Part 2.4)
□ Sampling analog information signal
□ Converting samples into discrete pulses
□ Transport the pulses from source to
destination over physical transmission
medium.
Cont’d...
□ Four (4) Methods
1. PAM
2.
3.
4.
PWM
PPM
PCM
Analog Pulse Modulation
Digital Pulse Modulation
Cont’d...
□ Analog Pulse Modulation
□ Carrier signal is pulse waveform and the
modulated signal is where one of the
carrier signal’s characteristic (either
amplitude, width or position) is changed
according to information signal.
Pulse Amplitude Modulation (PAM)
• The amplitude of pulses is varied in accordance with the
information signal.
• Width & position constant.
Pulse Width Modulation (PWM)
□ Sometimes called Pulse Duration Modulation
(PDM).
□ The width of pulses is varied in accordance
to information signal.
□ Amplitude & position constant.
Cont’d...
Pulse Position Modulation (PPM)
• Modulation in which the temporal positions of the pulses are
varied in accordance with some characteristic of the
information signal.
• Amplitude & width constant.
Advantages & Drawbacks of Pulse
Modulation
□ Noise immunity.
□ Relatively low cost digital
circuitry.
□ Able to be time division
multiplexed with other pulse
modulated signal.
□ Storage of digital streams.
□ Error detection & correction
□ Requires greater BW to
transmit & receive as
compared to its analog
counterpart.
□ Special encoding &
decoding methods must be
used to increased
transmission rates & more
difficult to be recovered.
□ Requires precise
synchronization of clocks
between Tx & Rx.
Sampling
□ A process of taking samples of information signal
at a rate of Nyquist’s sampling frequency.
□ Nyquist’s Sampling Theorem :
The original information signal can be reconstructed at the receiver
with minimal distortion if the sampling rate in the pulse modulation
system equal to or greater than twice the maximum information
signal frequency.
fs >= 2fm (max)
Cont’d...
□ Two basic techniques used to perform
the sampling function:
□ Natural sampling
□ Flat-top sampling
Natural Sampling
□ Tops of the sample pulses retain their natural
shape during the sample interval.
□ Frequency spectrum of the sampled output
is different from an ideal sample.
□ Amplitude of frequency components
produced from narrow, finite-width sample
pulses decreases for the higher harmonics
□ Requiring the use of frequency equalizers
Natural Sampling
Flat-top Sampling
□ Common used in PCM systems.
□ Accomplish in a sample-and-hold
circuit
□ To periodically sample the continually
changing analog input voltage & convert
to a series of constant-amplitude PAM
voltage levels.
□ The input voltage is sampled with a
narrow pulse and then held relatively
constant until the next sample is taken.
Cont’d…
□ Sampling process alters the frequency
spectrum & introduces aperture error.
□ The amplitude of the sampled signal
changes during the sample pulse time.
□ Advantages:
□ Introduces less aperture distortion
□ Can operate with a slower ADC
Flat-top Sampling
END OF PART 2.1