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SINGLE SIDEBAND {SSB} MODULATION
DSB modulation results in doubling the
bandwidth of a given message signal. Thus both
DSBSC and DSBLC are wasteful of channel
bandwidth.
It can be seen from the following figure that
transmission of both side bands is unnecessary
since the information in both sidebands is
redundant.
SSB MODULATION
Two Methods
• Frequency discrimination - filtered DSB
• Phase discrimination
Each sideband
pair {whether
upper or lower}
contains the
complete
information or
the original
message.
Frequency Discrimination
The most straight forward way to generate SSB is
to first generate DSB and the suppress the side
bands by filtering.
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Frequency Discrimination
Frequency Discrimination Difficulties
In practice the construction of the sideband filter
is difficult.
To produce a perfect SSB spectrum requires an
ideal filter characteristic at fc. Because this is
impossible, we instead put a restriction on the
message spectrum.
IF M(f) contains no low frequency components
then there will be no significant components in
the vicinity of fc after modulation.
Frequency Discrimination Difficulties
This condition permits use of a less than perfect
sideband filter.
Frequency Discrimination Difficulties
The need to have a message signal without low
frequency content limits this method to voice
transmissions. Since voice signals have little or
negligible frequencies below 300Hz.
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Phase Discrimination SSB Generation
It is possible to generate SSB by proper phasing
of signals. The method essentially uses two
product modulators to create two DSB-SC signals
that are phase shifted by 90°. The two DSB
signals are then combined to cancel one of the
side bands.
Phase Discrimination SSB Generation
Phase Discrimination SSB Generation
Continuing..
Phase Discrimination SSB Difficulties
A major problem in the design of phase-shift SSB
systems is the construction of the 90° phase shift
network for m(t), because all frequency
components must be shifted by exactly 90°. Such
systems can work efficiently for only a very
restricted message bandwidth.
Likewise;
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Demodulation of SSB Signals
To recover the baseband signal m(t) from the SSB
signal we have to shift the spectrum by +/- fc.
This requires synchronous detection similar to
that of DSBSC.
The process is;
Vestigal Sideband Modulation {VSB}
VSB is a compromise between SSB and DSB
modulation. In VSB one sideband is passed
completely and just a trace or ‘vestige’ of the
other sideband is retained.
Demodulation of SSB Signals
The spectrums;
VSB Generation
Start with a DSBSC signal and then pass through
a sideband shaping filter.
Advantages:
• Improved BW over DSB modulation
• Can be recovered using and envelope detector.
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VSB Generation
VSB Demodulation
LPF
VSB Demodulation
Complimentary Filters
The VSB filters Hv(f) are required to be
‘complimentary’ in the vicinity of fc. These type
filters have many applications in electronics and
their designs are well understood. Therefore,
VSB filter design (unlike SSB) does NOT pose
special difficulties.
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B/W Television Spectrum
In order to transmit a two dimensional (picture)
information via a one dimensional coordinate
system ( time), it is necessary to employ a
scanning technique.
The TV camera is designed to focus on an image
on a two dimensional photo- sensitive array.
B/W Television Spectrum
A rough video band width can be found as;
B/W Television Spectrum
The signal resulting from scanning the array is a
voltage vs time waveform that varies with the
brightness of the elements. This is a bw video
signal.
In bw TV; the raster scan consists of
- 525 vertical lines scanned 30 times/sec.
- 4:3 aspect ratio means there are 525 x 700
picture elements [pixels]
- So 525 x 700 x 30 = 11,025,000 picture
elements [one frame] must be transmitted
every second.
B/W Television Spectrum
In practice a bandwidth of 4 MHz is typical.
If DSB was used 2 x 4 MHz = 8 MHz would be
required. However, by employing VSB, channel
allocations of only 6 MHz are required.
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Comparison of AM Systems
1. Large carrier systems have the advantage of
simple detection at the receiver. The
disadvantage is that they are less power
efficient at the transmitter when compared
to a suppressed carrier system.
2. Suppressed carrier systems are more power
efficient than large carrier systems. The
disadvantage is the complex receivers
required to perform synchronous
demodulation of DSBSC signal.
Comparison of AM Systems
3. SSB requires minimum bandwidth and
minimum transmitted power possible. Thus is
long distance communication where signal
repeaters are necessary SSB is a good choice
since reducing the number of repeaters is
paramount to reduce the system cost. The added
transmitter complexity is thus justified.
4. VSB requires a bandwidth less than that of
DSB and the bandwidth savings is justified when
modulating signals with large bandwidths.
COMMERICIAL BROADCAST AM
Broadcast AM stations are assigned at 10kHz
spacing from 540-1600 kHz.
Each transmitter sends a DSBLC signal at it’s
assigned carrier frequency.
An AM receiver can pick up any of these signals
by tuning to the proper carrier frequency. The
selected station is then demodulated using an
envelope detector.
The problem is that production of stable tuning
amplifiers and filters that cover the entire AM
band are hard to build.
COMMERICIAL BROADCAST AM
This gave rise to the heterodyne receiver used
today.
In the heterodyne receiver, the incoming
modulated signal is translated to a new center
frequency that is independent of the station
selected.
This intermediated frequency (IF) is fixed. The
signal is then amplified at the IF before
demodulation.
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