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January 2018

DIGITAL TERRESTRIAL TRANSMISSIONS



INTRODUCTION

Terrestrial transmissions of television signals provide consumers an efficient yet low cost means to receive television programming. For the Broadcaster, terrestrial transmissions provide an extremely low cost means to simultaneously reach tens of thousands of viewers.

India currently has more than 1400 terrestrial TV Transmitters, broadcasting analog TV signals in the VHF and UHF frequency bands. This is probably the world's largest network of terrestrial TV transmitters.



WHY DIGITAL ?

A key shortcoming in analog, terrestrial TV transmission, particularly in urban areas, is the presence of multiple images or "ghosts". Ghosts appear on the receiving TV screen when the signal from the TV transmitter arrives at the TV set through multiple paths. As a result, such distortion is also called "Multi Path Distortion."

T h e mechanism of Multi Path Distortion is shown in Figure 1. A signal arrives at the TV set directly from the transmitter. This causes the 1st image on the TV screen. A 2nd signal arrives at the TV receiver after bouncing off building A. This causes the 2nd image on the TV screen. A 3rd image is caused by another signal arriving at the TV receiver after it bounces off building B. As a result there are 3 images on the TV set. Depending on the relative signal strengths of the 3 signals received, the ghost images could be so prominent as to make TV viewing unacceptable.


DIGITAL MODULATION TERRESTRIAL TRANSMISSION - O.F.D.M.

The biggest concern for proper reception of terrestrial broadcast is multi path distortion, or " Ghosts ".

T h i s h a p p e n s when a s i g n a l arrives at the r e c e i v i n g antenna from m u l t i p l e paths or d i r e c t i o n . T h e s e m u l t i p l e signals add up at the antenna, creating multiple images or "Ghosts" on the TV screen. This distortion is most pronounced in densely populated cities particularly those with high rise buildings.

Analog transmissions cannot prevent "Ghosts". The only hope is to realign the antenna to minimise the extent of ghosts. Hence it was a top priority for engineers to device a Digital Modulation scheme that w o u l d eliminate any possibility of ghosts images.

Further t h e Terrestrially transmitted T e l e v i s i o n signal should preferably not interfere with o t h e r terrestrial transmissions such as those for wireless radio etc.

To overcome these problems engineers have created a modulation scheme that appears to be extremely complex.

Orthogonal Frequency Division Multiplexing (OFDM) is a type of Frequency Multiplexing. In Frequency Multiplexing, multiple carriers are used at different frequencies, as shown in Fig.2

Each carrier is separated by an unused band of frequencies called a "Guard Band". Ofcourse, the guard band is a waste of the bandwidth resource.

A Digital Terrestrial transmission (DVB-T) for a single television channel can utilise upto 8000 separate carriers ! Even a Digital Audio broadcast which requires much smaller amount of data to be transmitted compared to a Television channel, employs 1500 separate carriers.

To fit these large number of carriers into the typical 8 MHz bandwidth allocated for terrestrial broadcasts, engineers employed a further Orthogonal variation. Orthogonal here refers to a phase difference of 90 Degrees between two adjacent carriers. This is shown in Fig.3

Using Orthogonal Frequency Division Multiplexing (OFDM) Modulation, 2 Adjacent Carriers will overlap without causing any interference because the two carriers are out of phase by 90 degrees. The overlapping of carriers avoid wastage of frequency bandwidth.

OFDM causes less interference to analog transmissions than an analog signal would, because it doesn't have the same strong carrier and subcarrier elements. Also, because there is a specific spacing between carriers of the same phase (guard interval), the signal is immune to multi path reflections or " Ghosts ".

Further, OFDM Modulation can be used in so called Single Frequency Networks, where a chain of transmitters can all use the same frequency for transmission.


Hence, top priority was allocated to devising a means to avoid Multi Path Distortion or ghost images for Digital Terrestrial Transmissions (DTT).



SOLVING THE MULTIPATH WITH OFDM

In order to address the multi-path problem, engineers devised Orthogonal Frequency Division Multiplexing (OFDM) for Digital Terrestrial transmissions. OFDM combines a large number of carriers and the TV signal is divided over this large number of carriers.

It must be noted that OFDM is not a type of digital modulation. It is only a means of bunching together multiple carriers, at different frequencies. As a simple example, a Channel Combiner in an analog CATV Headend 'multiplexes' or combines multiple channels that have already been modulated at different frequencies.

Worldwide; OFDM variations using 2,000 (2K) carriers and 8,000 (8K) carriers has been practically used. Earlier DTT systems utilised 2K carriers, to reduce the system complexity and costs. As Integrated Circuits (ICs) were developed, it became practical to use 8K systems, and these are now almost universally deployed for DTT transmissions.


MULTI-CARRIER TRANSMISSIONS

In contrast to single-carrier modulation, multicarrier modulation spreads the signal evenly across thousands of carrier frequencies, thus reducing the dependence on a specific carrier. Each carrier is modulated with a small part of the encoded transmission signal thus minimising the effects of frequency selective fading or interference due to difficult transmission conditions.


ORTHOGONAL FDM

To efficiently use the spectrum, and carry a larger number of frequency carriers, in the same bandwidth, 2 adjacent carriers are placed at right angles (Orthogonal) to each other.

This is similar to the Horizontal and Vertical polarizations utilised in satellite transmissions so that twice as many channels can be beamed from the same satellite.


GUARD INTERVAL

One of the advantages of the OFDM is that it can be configured to trade-off bit-rate against robustness by using a guard interval in the transmission of digital symbols. The guard interval allows for the suppression of echoes caused by multipath propagation. Due to the large number of carriers used in OFDM, the digital signal symbols are spread across a large part of the available spectrum, thus enabling relatively long symbol durations compared to single carrier modulation. The long symbol duration means that a portion can be used to guard against strong echoes, usually arriving within the first few microseconds of the start of the symbol.

The guard interval in OFDM modulation is the portion of the symbol duration that is kept empty for echo suppression.

The DVB-T standard specifies that guard intervals can be set to between 1/4 and 1/32 of the symbol duration. The longer guard intervals are needed for environments in which strong signal echoes are encountered, such as cities with multiple, tall buildings.

The effective symbol duration is that part of the symbol that is left after the guard interval is taken into account. Figure 4. shows the relationship between the OFDM symbol duration, the effective symbol duration and the guard interval.

As indicated earlier, there are 2 carrier modes in the DVB-T OFDM specification: 2K carriers and 8K carriers.

In 2K mode the symbol duration is 224 microseconds, and in the 8K mode it is 896 microseconds.

Thus the maximum permissible guard interval 1/4 in 2K mode protects against echoes within a 56 microsecond window and in 8K mode the same guard interval protects against echoes within a 224 microsecond window.

Clearly, a 8K carrier system, provides a much higher degree of Ghost Suppression than a 2K system.


MOBILE TRANSMISSION

With the problem of Ghost Images addressed, engineers turned their attention to enabling DTT for even mobile applications. These could be reception of digital terrestrial TV transmissions in trains and buses. Another application could be news gathering from a moving vehicle such as a helicopter.

When either the transmitter or the receiver is in motion, Doppler shift occurs. This causes the transmitted frequency to shift, due to the relative movement between the transmitter and the receiver. Doppler shift is responsible for the pitch of a train's whistle to changes as it approaches and recedes from a person on the ground.

OFDM signals can be continuously adapted so that the Doppler effect is estimated and each channel correctly compensated.


QAM & QPSK MODULATION

As pointed out earlier, OFDM is not a type of digital modulation. It is only a multiplexing scheme that it a method used to combine multiple frequency digital carriers.

The digital signals need to be first modulated before they are carried as 8,000 separate carriers using OFDM.

The digital modulation can either be done as QAM (Quadrature Amplitude Modulation) or as QPSK (Quadrature Phase Shift Keying).

QAM is the standard modulation used in digital cable TV systems. QAM provides a large amount of data to be carried over a relatively small bandwidth. However, for good reception, QAM signals must be received at high signal strengths. (Typically above 60 dBU.)

On the other hand, QPSK is used in satellite transmissions. It requires more bandwidth but provides excellent reception for even very week signals.

Hence, Digital Terrestrial Transmissions (DTT) can utilise either QAM or QPSK digital modulation, in addition to OFDM.


DTT IN INDIA

It is a little known fact that Doordarshan commenced Digital Terrestrial Transmission in India more than 3 years ago. In fact Doordarshan had even demonstrated ghost free reception of Digital TV in a moving car, during the SCAT 2005 Tradeshow in October 2005. The car drove through Cuffe Parade - an area which has multiple tall buildings separated by narrow roads. Such an environment is typically a nightmare situation for receiving ghost-free terrestrial TV. However; perfect reception was demonstrated in the moving car.


5 DTT CHANNELS

As with all other DD Transmissions, the DTT transmissions are Free To Air. While no subscription fee is payable, consumers will have to purchase at their own cost, the necessary DTT receivers.

Currently a bouquet of 5 DTT channels is transmitted from each of the 16 centres. These are: DD National, DD News, DD Bharati, DD Sports, DD Regional/DD Kisan are being relayed.


DTT RECEPTION

Even though DTT transmissions have been available in Delhi, Mumbai and a few other metro cities for several years, they have gone almost completely un-noticed. The reasons for that are two-fold:

1. DTT requires the purchase and use of special additional DTT STBs for each TV, by the consumer.

2. Most of the relevant Doordarshan channels are 'Must Carry' by law, on all Cable TV and DTH platforms, which extensively cover the cities. As a result, there is no reason for the consumer to invest extra in a DTT STB and antenna, to receive channels already visible via cable & DTH.


MOBILE DTT

This time around, DD has adopted DVB-T2, which enables excellent reception in moving vehicles like busses & trains during commute in the cities.

A few TV manufacturers offer built in DTT tuners in their select models. These TVs can be used to directly receive DTT transmissions.

The price of DTT receivers has fallen to reasonable levels, even though their availability is scarce.

Doordarshan has listed on line shopping portals from where DTT STBs, antennae can be purchased.


DTT ON MOBILE PHONES

The primary consumer friendly feature of Doordarshan's DTT transmissions is that they can be received on a large number of compatible mobile phones, when a special receiver is added to the Mobile phone. (See Figure 6).

A DTT USB dongle is to be added to a mobile phone to receive DTT transmissions.


USB OTG MOBILES

Almost all mobile phones have a USB port. However, a key requirement for this application is that the USB port on the mobile phone must be capable of hosting another USB device. This is termed as USB On The Go: USB OTG or simply OTG. A quick search on the net will tell you if your mobile phone is UTG enabled.

A fairly comprehensive list of UTG mobile phones sold in India is available at:

http://www.91mobiles.com/list-of-phones/ android-phones-with-otg

Doordarshan has even identified an online vendor who offers DTT dongles for mobile phones at: http://tinyurl.com/dttdongle

The DTT app 'TV-On-Go' can be downloaded from the Google Play store.


NO INTERNET REQUIRED

Since the DTT dongle receives DTT transmissions off the air, it does not require / consume internet connectivity / data.

Unlike other TV streaming apps on mobile phone which consume substantial data for live TV streaming, there is no running cost to receive the 5 DD channels, when the DTT dongle is used. n