In TV broadcast both the sound signal
and the video signal are to be conveyed to the viewer using radio
frequency. These two signals have very distinct features. The audio
signal is a symmetrical signal without continuous current but the frequency
does not exceed 20 kHz. The video signal consists of a logical component, the
sync and the field sync and an analogue part according to the line picture scanning.
This unsymmetrical signal thus has a continuous component. The frequency
bandwidth also extends from 0 to 5 MHz. The two signals modulate the carrier
waves whose frequencies and types of modulations are as per established
standards. These modulated carriers are further amplified and then
diplexed for transmission on the same line and antenna. This technique is
used with High Power TV Transmitters. However for LPTs i.e. transmitters
operating at sync peak power less than 1 kW, both the signals (video and audio)
are modulated separately (In most of the present day TV transmitters the
picture signal is amplitude modulated while the audio signal is frequency
modulated) but amplified jointly using common vision and aural
amplifiers. Both of these systems have merits and demerits. In the
first case (separate amplification) special group delay equalisation circuit is
needed because of errors caused by diplexer while in the second case inter
modulation products are more prominent and special filters for suppressing them
are required. Hence technique of joint amplification is suitable only for
LPTs and not for HPTs.
Though frequency modulation has certain
advantages over amplitude modulation, its use for picture transmission is not
permitted due to large bandwidth requirements, which is not possible due to
very limited channel space available in VHF/UHF bands. Secondly as the power
of the carrier and side band components go on varying with modulation in the
case of FM, the signal with frequency modulation after reflection from nearby
structures at the receiving end will cause variable multiple ghosts, which will
be very disturbing. Hence use of FM for terrestrial transmission of
picture signal is not permitted.
Thus amplitude modulation is invariably
used for picture transmission while frequency modulation is generally used for
sound transmission due to its inherent advantages over amplitude
modulation. As the picture signal is unsymmetrical, two types of
modulation is possible.
i) Positive modulation
Wherein the increase in picture brightness causes increase
in the amplitude of the modulation envelope.
ii) Negative modulation
The increase in picture brightness
causes reduction in carrier amplitude i.e. the carrier amplitude will be
maximum corresponding to sync tip and minimum corresponding to peak white.
In television though positive modulation
was adopted in initial stages, negative modulation is generally adopted
(PAL’B uses negative modulation) now a days, as there are certain advantages
over positive modulation.
Advantages of Negative Modulation
i)Impulse noise peaks appear only
in black region in negative modulation. This black noise is less
objectionable compared to noise in white picture region.
ii) Best linearity can be maintained for picture region and any
non-linearity affects only sync which can be corrected easily.
iii) The efficiency of the transmitter is better as the peak
power is radiated during sync duration only (which is about 12% of total line
duration).
iv) The peak level representing the blanking or sync level may
be maintained constant, thereby providing a reference for AGC in the receivers.
v) In negative modulation, the peak power is radiated during
the sync-tip. As such even in case of fringe area reception, picture
locking is ensured, and derivation of inter carrier is also ensured.
VESTIGIAL SIDE BAND TRANSMISSION
Another feature of present day TV
Transmitters is vestigial side band transmission. If normal amplitude
modulation technique is used for picture transmission, the minimum transmission
channel bandwidth should be around 11 MHz taking into account the space for
sound carrier and a small guard band of around 0.25 MHz. Using such
large transmission BW will limit the number of channels in the spectrum
allotted for TV transmission. To accommodate large number of channels in
the allotted spectrum, reduction in transmission BW was considered
necessary. The transmission BW could be reduced to around 5.75 MHz by
using single side band (SSB) AM technique, because in principle one side band
of the double side band (DSB) AM could be suppressed, since the two side bands
have the same signal content.
It was not considered feasible to suppress one complete
side band due to difficulties in ideal filter design in the case of TV signal
as most of the energy is contained in lower frequencies and these frequencies
contain the most important information of the picture. If these
frequencies are removed, it causes objectionable phase distortion at these
frequencies which will affect picture quality. Thus as a compromise only a part
of lower side band is suppressed while taking full advantage of the fact that:
i) Visual disturbance due to phase errors are severe
and unacceptable where large picture areas are concerned (i.e. at LF) but
ii) Phase errors become difficult to see
on small details (i.e. in HF region) in the picture. Thus low modulating
frequencies must minimize phase distortion where as high frequencies are
tolerant of phase distortions as they are very difficult to see.
The radiated signal thus contains full
upper side band together with carrier and the vestige (remaining part) of the
partially suppressed LSB. The lower side band contains frequencies up to
0.75 MHz with a slope of 0.5 MHz so that the final cut off is at 1.25 MHz.
RECEPTION
OF VESTIGIAL SIDE BAND SIGNALS
Corresponding to the VSB characteristics
used in transmission an amplitude versus frequency response results. When
the radiated signal is demodulated with an idealized detector, the response is
not flat. The resulting signal amplitude during the double sideband
portion of VSB is exactly twice the amplitude during the SSB portion.
This characteristic is shown in Fig.
Fig- Response for VSB reception
In order to equalize the amplitude, the
receiver response is designed to have an attenuation characteristics over the
double side band region appropriate to compensate for the two to one
relationship.
This attenuation characteristic, the
so-called nyquist slope, is assumed to be in the form of a linear slope over
the + 750 kHz (DSB region) with the visual carrier located at the mid point (-6
dB point) relative to SSB portion of the band. Such a characteristic exactly
compensates the amplitude response non-symmetry due to VSB.
Typical receiver
characteristics
Modern practice for purposes of circuit and IF filter
design simplification, also provides an attenuation of the upper end of the
channel such that colour sub carrier is also attenuated by 6 dB as shown in
Fig.
Typical
receiver IF characteristics
TV receivers have Nyquist
characteristics for reception which introduces group delay errors in the low
frequency region. Notch filters are used in receivers as aural traps in
the vision IF and Video amplifier stages. These filters introduce GD
errors in the high frequency region of the video band. These GD errors
are pre-corrected in the TV transmitters (using RX pre corrector) so that
economical receiver filter design is possible. The group delays of the RX
and TX with pre-correction are shown in Fig.
Group delay curves
Depth of Modulation
Care must be taken to avoid
over-modulation at peak-Luminance signal values to avoid picture distortions
and interruptions in vision carrier. The peak white levels when over modulated
tend to reduce the vision carrier power or even cause momentary interruptions
of vision carrier. These periodic interruptions due to accidental over
modulation result in interruptions of the sound carrier in inter carrier
receiver systems which produces undesired sound buzz in the receiver output.
Therefore, to prevent this effect,
the maximum depth of modulation of the visual carrier by peak white signal
values is specified as being 87.5%. This 12.5% residual carrier (white
level) is required because of the inter-carrier sound method used in TV
receiver .
Carrier
signal and modulation envelop
The depth of modulation is set by using
a ramp signal or step signal as given in the manual. It should be 87.5%
for 100% modulation (i.e. m = 1).
Inter Carrier
The TV receivers incorporate inter
carrier principle. According to our system, the inter-carrier i.e. the
difference between the vision transmitter frequency and sound transmitter
frequency is 5.5 MHz. Hence it is to be ensured that even when the
modulating video signal is at white peak, 12.5% of residual carrier is left so
that sound can be extracted even at the peak white level, where the carrier
power is minimum.
Power Output
The peak power radiated during the
sync. tip or sometimes the carrier power corresponding to black level is
designated as the vision transmitter power. This power is measured by
using a thruline power meter after isolating the aural carrier. The power
read on thruline meter is multiplied by a factor of 1.68 to get the peak power
(vision) radiated. As transmitter output is connected to an antenna,
having a finite gain, the effective radiated power (ERP) is obtained by
multiplying the peak power by the antenna gain (w.r.t a half wave
dipole). Hence a 100 W LPT using transmitting antenna having a gain of 3
dB w.r.t a half wave dipole will have an ERP of 200 W or 53 dBm or 23 dBW.
In TV broadcasting, the sound
signal is transmitted by frequency modulating the RF sound carrier in
accordance with the standards. The sound carrier is 5.5 MHz above the
associated vision carrier. The maximum frequency deviation is + 50 kHz
which is defined as 100 per cent modulation in PAL-B system. In the case
of NTSC, the maximum deviation permissible is + 25 kHz.
Standards
The characteristics of the TV signal in
sections 1 and 2 refer to CCIR B/G standards. Various other standards
are given in Table .
Table
1
Vision/sound carrier spacing
channel width |
Frequency Range
|
Vision sound carrier spacing
|
5.5 MHz
|
Channel width
|
7 MHz (B) VHF
8 MHz (G) UHF
|
Sound Modulation
|
FM
|
FM deviation (maximum)
|
+ 50 kHz
|
Standards
|
American
|
European PAL
|
No. of lines per frame
|
525
|
625
|
No. of frames per second
|
30
|
25
|
Field Frequency Hz
|
60
|
50
|
Line Frequency Hz
|
15750
|
15625
|
Channel MHz
|
6
|
7
|
Video BW, MHz
|
4.2
|
5
|
Colour subcarrier, MHz
|
3.58
|
4.43
|
Sound System
|
FM
|
FM
|
Maximum sound deviation kHz
|
25
|
50
|
Intercarrier frequency MHz
|
4.5
|
5.5
|
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