Terrestrial Television
Terrestrial Television is a mode of television broadcast
Uses Radio waves through transmitting and receiving antenna
Terrestrial Television operates on TV channels in VHF and UHF Band.
VHF : 30 - 300 MHz
Band I (40-68 MHz) TV Channel ≠ 4
Channel Spacing - 7 MHz
Band II (88-108 MHz) FM Sound Broadcasting
Channel Spacing - 100 KHz
Band III (174-230 MHz) CH ≠ 5 – CH ≠ 12
Channel Spacing – 7 MHz
UHF : 300 – 3000 MHz
Band IV (470 – 606 MHz) CH ≠ 21 – CH ≠ 37
Channel Spacing - 8 MHz
Band V (606 – 798 MHz) CH ≠ 38 – CH ≠ 61
Channel Spacing - 8 MHz
TV Transmitters in DDK Chennai
Modulation of Video and Audio Signal
In TV broadcast both the sound signal and video signal are to be conveyed to the viewer using radio frequency.
The two signals modulate the carrier waves whose frequencies and type of modulations are as per established standards.
Image frequency (IF)
In most of the present day TV transmitters the picture signal is amplitude modulated while the audio signal is frequency modulated on different carriers separated by 5.5 MHz. PICTURE I F = 38.9 MHz SOUND I F = 33.4 MHz
TV Channel (VHF)
Video modulation
Carrier is transmitted with full power
One complete sideband is also transmitted
Only part of the second sideband is transmitted
Why Vestigial side band(VSB)
DSB takes too much BW (BT = 2W) for the TV channel
SSB is too expensive to implement
VSB is compromise between DSB and SSB
W BW 2W
Easy to implement receiver
Types of Modulation
Positive modulation
The increase in picture brightness causes increase in the amplitude of the modulation envelope
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
CCVS
Modulated carriers
Why negative modulation
Pictures contain more information towards white than black and hence the average power is lower resulting in energy saving. (Bright picture points correspond to a low carrier amplitude and sync pulse to maximum carrier amplitude).
Impulse noise peaks appear only in black region in negative modulation. This black noise is less objectionable compared to noise in white picture region.
Best linearity can be maintained for picture region and any non-linearity affects only sync which can be corrected easily.
In CCIR (Consultant Committee for International Radio) system sound carrier is 5.5 MHz above the vision carrier and is frequency modulated. The maximum frequency deviation is 50 KHz.
Also the ratio of vision and sound power is 10:1 (20:1 is also employed in some countries)
If we assume maximum audio signal is 15 KHz the band width is 130 KHz.
According to Carson’s Rule the bandwidth is 2 x (Maximum frequency deviation + highest modulating frequency).
However, calculated value(using Bessel’s function) of Bandwidth is 150 KHz i.e. 75 KHz on either side of sound carrier.
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.
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).
TV Transmitters
High Power TV Transmitters
both the signals (video and audio) 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.
Low Power TV Transmitters
both the signals (video and audio) are modulated separately but amplified jointly using common vision and aural amplifier.
Schematic Block Diagram of a TV Transmitter
Functional description
Video Input :1V pp; Audio Input :+10dBm
Video input is band limited to 5Mhz, amplified and inverted
compensated for group delay in DVC
nonlinearity precorrection in IF corrector
Amplitude modulated at 38.9 MHz IF in Modulator
Ring modulator/double diode balanced modulator
SAW filter shapes the modulated IF to VSB format
VCXO generates IF carrier
Audio signal is pre-emphasised , amplified and frequency modulated
VCO modulator with centre frequency 33.4 MHz
Group Delay
Is a measure of the transit time of a signal through a device , versus frequency. It is a useful measure of phase distortion.
Equalizers
Correct the waveform distortion of the signal by suitably modifying the amplitude and /or group delay characteristics.
Pre-emphasis
Boosting of higher frequency components
From Analog to Digital
More than one program/service in one channel through efficient compression (source coding)
Powerful error correction mechanisms ensure proper decodeability
Quality of reception can be determined more easily.
Distribution costs cheaper per service
Quality vs Distance
Schematic Block Diagram of Digital Transmitter
Source and channel coding
Source coder removes non-essential and redundant parts of the picture signal and performs the bit-rate reduction process.
Digital compression techniques enable the original data to be compressed so that it can readily be transmitted over the existing transmission path.
Digital compression techniques allow many different programmes to be simultaneously transmitted over a single channel.
Channel encoder optimises the data for the particular modulation system and transmission channel.
DVB-T Modulator (FEC)
Special carriers in DVB-T
Continual pilots
- fixed position in spectrum
- fixed position in constellation diagram
- used for automatic frequency control
Scattered pilots
- var. position in spectrum
- fixed position in constellation diagram
- used for channel estimation and correction
TPS carrier
- fixed position in spectrum
- BPSK modulation
- fast information channel from Tx to Rx
Digital Terrestrial Transmitter
UHF Antenna – Super turnstile array
To obtain an omnidirectional radiation in the horizontal plane
Two crossed dipoles are used in a turnstile arrangement
They are fed in quadrature from the same source by means of an extra quarter wavelength line
Terrestrial Television is a mode of television broadcast
Uses Radio waves through transmitting and receiving antenna
Terrestrial Television operates on TV channels in VHF and UHF Band.
VHF : 30 - 300 MHz
Band I (40-68 MHz) TV Channel ≠ 4
Channel Spacing - 7 MHz
Band II (88-108 MHz) FM Sound Broadcasting
Channel Spacing - 100 KHz
Band III (174-230 MHz) CH ≠ 5 – CH ≠ 12
Channel Spacing – 7 MHz
UHF : 300 – 3000 MHz
Band IV (470 – 606 MHz) CH ≠ 21 – CH ≠ 37
Channel Spacing - 8 MHz
Band V (606 – 798 MHz) CH ≠ 38 – CH ≠ 61
Channel Spacing - 8 MHz
TV Transmitters in DDK Chennai
Modulation of Video and Audio Signal
In TV broadcast both the sound signal and video signal are to be conveyed to the viewer using radio frequency.
The two signals modulate the carrier waves whose frequencies and type of modulations are as per established standards.
Image frequency (IF)
In most of the present day TV transmitters the picture signal is amplitude modulated while the audio signal is frequency modulated on different carriers separated by 5.5 MHz. PICTURE I F = 38.9 MHz SOUND I F = 33.4 MHz
TV Channel (VHF)
Video modulation
Carrier is transmitted with full power
One complete sideband is also transmitted
Only part of the second sideband is transmitted
Why Vestigial side band(VSB)
DSB takes too much BW (BT = 2W) for the TV channel
SSB is too expensive to implement
VSB is compromise between DSB and SSB
W BW 2W
Easy to implement receiver
Types of Modulation
Positive modulation
The increase in picture brightness causes increase in the amplitude of the modulation envelope
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
CCVS
Modulated carriers
Why negative modulation
Pictures contain more information towards white than black and hence the average power is lower resulting in energy saving. (Bright picture points correspond to a low carrier amplitude and sync pulse to maximum carrier amplitude).
Impulse noise peaks appear only in black region in negative modulation. This black noise is less objectionable compared to noise in white picture region.
Best linearity can be maintained for picture region and any non-linearity affects only sync which can be corrected easily.
In CCIR (Consultant Committee for International Radio) system sound carrier is 5.5 MHz above the vision carrier and is frequency modulated. The maximum frequency deviation is 50 KHz.
Also the ratio of vision and sound power is 10:1 (20:1 is also employed in some countries)
If we assume maximum audio signal is 15 KHz the band width is 130 KHz.
According to Carson’s Rule the bandwidth is 2 x (Maximum frequency deviation + highest modulating frequency).
However, calculated value(using Bessel’s function) of Bandwidth is 150 KHz i.e. 75 KHz on either side of sound carrier.
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.
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).
TV Transmitters
High Power TV Transmitters
both the signals (video and audio) 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.
Low Power TV Transmitters
both the signals (video and audio) are modulated separately but amplified jointly using common vision and aural amplifier.
Schematic Block Diagram of a TV Transmitter
Functional description
Video Input :1V pp; Audio Input :+10dBm
Video input is band limited to 5Mhz, amplified and inverted
compensated for group delay in DVC
nonlinearity precorrection in IF corrector
Amplitude modulated at 38.9 MHz IF in Modulator
Ring modulator/double diode balanced modulator
SAW filter shapes the modulated IF to VSB format
VCXO generates IF carrier
Audio signal is pre-emphasised , amplified and frequency modulated
VCO modulator with centre frequency 33.4 MHz
Group Delay
Is a measure of the transit time of a signal through a device , versus frequency. It is a useful measure of phase distortion.
Equalizers
Correct the waveform distortion of the signal by suitably modifying the amplitude and /or group delay characteristics.
Pre-emphasis
Boosting of higher frequency components
From Analog to Digital
More than one program/service in one channel through efficient compression (source coding)
Powerful error correction mechanisms ensure proper decodeability
Quality of reception can be determined more easily.
Distribution costs cheaper per service
Quality vs Distance
Schematic Block Diagram of Digital Transmitter
Source and channel coding
Source coder removes non-essential and redundant parts of the picture signal and performs the bit-rate reduction process.
Digital compression techniques enable the original data to be compressed so that it can readily be transmitted over the existing transmission path.
Digital compression techniques allow many different programmes to be simultaneously transmitted over a single channel.
Channel encoder optimises the data for the particular modulation system and transmission channel.
DVB-T Modulator (FEC)
Special carriers in DVB-T
Continual pilots
- fixed position in spectrum
- fixed position in constellation diagram
- used for automatic frequency control
Scattered pilots
- var. position in spectrum
- fixed position in constellation diagram
- used for channel estimation and correction
TPS carrier
- fixed position in spectrum
- BPSK modulation
- fast information channel from Tx to Rx
Digital Terrestrial Transmitter
UHF Antenna – Super turnstile array
To obtain an omnidirectional radiation in the horizontal plane
Two crossed dipoles are used in a turnstile arrangement
They are fed in quadrature from the same source by means of an extra quarter wavelength line
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