2. VESTIGIAL SIDEBAND MODULATION
Introduction
VSB is implemented by frequency discrimination but the filtering process does not completely eliminate
the unwanted band. In fact, some of the desired band is also partially filtered out.
A VSB modulator and its filter’s frequency response are shown below.
• The response of the VSB filter is denoted as HVSB(f).
• A transition band around the frequency fc.
3. Vestigial sideband (VSB) is a type of amplitude modulation (AM ) technique (sometimes called VSB-AM)
that encodes data by varying the amplitude of a single carrier frequency, portions of one of the redundant
sidebands are removed to form a vestigial sideband signal.
In AM, the carrier itself does not fluctuate in amplitude. Instead, the modulating data appears in the form of
signal components at frequencies slightly higher and lower than that of the carrier. These components are
called sidebands.
The lower sideband (LSB) appears at frequencies below the carrier frequency; the upper sideband (USB)
appears at frequencies above the carrier frequency.
The actual information is transmitted in the sidebands, rather than the carrier; both sidebands carry the same
information. Because LSB and USB are essentially mirror images of each other, one can be discarded .
VSB transmission is similar to single-sideband (SSB) transmission, in which one of the sidebands is
completely removed.
4. In VSB transmission, however, the second sideband is not completely removed, but is filtered to remove
all but the desired range of frequencies.
VSB is a compromise between DSB and SSB. To produce SSB signal from DSB signal ideal filters
should be used to split the spectrum in the middle so that the bandwidth of band pass signal is reduced by
one half. In VSB system one sideband and a vestige of other sideband are transmitted together.
The resulting signal has a bandwidth > the bandwidth of the modulating (baseband) signal but < the DSB
signal bandwidth.
The exact frequency response requirements on the sideband filter in SSB-SC system can be relaxed by
allowing a part of the unwanted sideband called vestige to appear in the output of the modulator. Due to
this, the design of the sideband filter is simplified to a great extent. But the bandwidth of the system is
increased slightly.
5. GENERATION OF VSB WAVE
To generate a VSB signal, we have to first generate a DSB-SC signal and then pass it through a
sideband filter as shown in below Figure. This filter will pass the wanted sideband as it is along with a
part of unwanted sideband.
The following are the drawbacks of SSB signal generation,
Generation of an SSB signal is difficult.
Selective filtering is to be done to get the original signal back.
Phase shifter should be exactly tuned to 90°.
To overcome these above mentioned drawbacks, VSB modulation is used.
6. ADVANTAGES OF VSB
• The main advantage of VSB modulation is the reduction in bandwidth. It is almost as efficient as the SSB .
• Due to allowance of transmitting a part of lower sideband, the constraint on the filter have been relaxed . So
practically, easy to design filters can be used .
• It possesses good phase characteristics and makes the transmission of low frequency components possible .
APPLICATION OF VSB
• VSB modulation has become standard for the transmission of television signal . Because the video signal
need a large transmission bandwidth if transmitted using DSB-FC or DSB-SC techniques .
VSB is mainly used in TV broadcasting for their video transmissions. TV signal consists of
Audio signal – transmitted by FM
Video signal – transmitted by VSB
A video signal consists a range of frequency and fmax = 4.5 MHz.
7. FREQUENCY SPECTRUM
Instead of transmitting only one sideband as SSB, VSB modulation transmits a partially suppressed sideband
and a vestige of the other sideband; It needs half of the spectrum. (ie.)
If it transmitted using conventional AM, the required BW is 9 MHz (BW=2fm). But according to the standard,
TV signal is limited to 7 MHz only . So, to reduce the BW, a part of the LSB of picture signal is not fully transmitted.
• Original: low pass signal
• DSB: moved to fc and –fc.
• SSB: only transmit one sideband:
The power & bandwidth can be reduced by 50%.
• USB: Move pos (neg) freq to fc (-fc)
• LSB: Move neg (pos) freq to fc (-fc)
8. COMPARISON OF VARIOUS MODULATION TECHNIQUES
Sl. No. DSB-SC SSB VSB
1
Less transmitted power than full
AM and all the transmitted
power is useful.
Good bandwidth utilization
(message signal bandwidth =
modulated signal bandwidth)
Offers a compromise
between SSB and DSB-SC
2
Requires a coherent carrier at the
receiver; This results in
increased complexity in the
detector(i.e. synchronizer)
Good power efficiency
Preferred in long distance
transmission of voice signals
VSB is standard for
transmission of TV and
similar signals
3
Suited for point to point
communication involving one
transmitter and one receiver
which would justify the use of
increased receiver complexity.
Demodulation is harder as
compares to full AM; Exact
filter design and coherent
demodulation are required
Bandwidth saving can be
significant if modulating
signals are of large
bandwidth as in TV and
wide band data signals.
9. HILBERT TRANSFORM
Magnitude and phase characteristic of Hilbert transform
The Hilbert transformer is a system that produces a phase shift of -90◦ for all positive frequency components and a
phase shift of -90◦ for all negative frequencies. The amplitudes of all frequency components of the input signal are
unaffected by transmission through the system
10. Properties of the Hilbert transform
defines the Hilbert inverse transform.
A signal x(t) and its Hilbert transform (t) have
• The same amplitude spectrum
• The same autocorrelation function
• x(t) and (t) are orthogonal
• The Hilbert transform of (t) is -x(t)
