Scatmag.com

May 2016

TRAI DECLARES NEW MSO-LCO DAS INTERCONNECT AGREEMENTS


The government has forced all Cable TV networks throughout India to shut off their analog cable TV transmissions by end 2016. However, there is no similar dictate or even a deadline for Doordarshan to shut off its analog terrestrial transmissions. This is contrary to practices worldwide where governments lead by example and first shut off their analog terrestrial signals, before dictating similar measures to private sector broadcasting.

Doordarshan's analog terrestrial transmissions still are the widest received TV transmissions in India, reaching almost twice the number of Cable TV + DTH (C&S) homes.


DD TERRESTRIAL GOING DIGITAL

There are many DTT broadcasting standards world over, such as Advanced Television Systems Committee: ATSC(USA), Digital Terrestrial Multimedia Broadcasting: DTMB(China), Digital Video Broadcasting- Terrestrial: DVB-T(Europe) and Integrated Services Digital Broadcasting-Terrestrial: ISDB-T (Japan).

India has adopted the European Union's DVB standard for Cable TV (DVB-C), Satellite (DVB-S) and terrestrial (DVB-T) digital transmissions.

Doordarshan has now started taking its first steps to commence digital transmissions terrestrially.

Doordarshan's target was to complete its major (40 transmitter) DTT roll out by end 2017. The public broadcaster is racing to meet this deadline. It must be noted that there is no mention of any deadline by the government for Doordarshan to discontinue its analog terrestrial transmissions.


DTT BENEFITS

Digital Terrestrial Transmissions (DTT) can replace all analog terrestrial TV transmissions with the following added benefits:

1. TINY BANDWIDTH

Atleast 5 DTT channels can be transmitted in the bandwidth occupied by a single analog terrestrial TV channel.

As a result, the excess bandwidth used for analog terrestrial transmissions can be auctioned at a huge profit, for mobile telephony and other applications.

The single analog channel bandwidth used in DTT will allow DD offer atleast 5 DD channels terrestrially from each of its DTT centres.

2. NO GHOST

DTT complete eliminates image 'Ghosts' which is the biggest irritant in analog terrestrial TV reception. perfect picture clarity is available as long as the signal strength is adequate.



3. MOBILE RECEPTION

A direct result of this is the option to receive TV transmissions in moving vehicles like trains, busses and cars. It can even be received on suitably equipped mobile phones, PCs and tablets.

4. HIGH DEFINITION TV

Only digital TV transmissions can provide High Definition TV (HDTV) & even UHDTV transmissions, that provide far better picture clarity.




DVB-T2

DVB-T2 is an advance version of DVB-T.

It can carry more TV channels in the same bandwidth, and also provide for good reception for portable & mobile applications, such as good reception of TV signals on mobile receivers in cars and trains.

Technically, DVB-T2 is very similar to DVB-T but uses few carriers & offers even 256 QAM, to more productively utilise the bandwidth.


ALL INDIA ROLL OUT

Several years ago, DD commenced DTT in New Delhi & Mumbai.

Doordarshan plans to roll out Digital Terrestrial Transmissions (DTT) to 40 cities throughout the country.

On 25 February 2016, DD commenced DTT in Delhi, Mumbai, Kolkata, Chennai, Guwahati, Patna, Ranchi, Cuttack, Lucknow, Jalandhar, Raipur, Indore, Aurangabad, Bhopal, Bangalore and Ahmadabad.

DTT roll-out has also been promised soon in 3 more cities: Srinagar, Hyderabad and Thiruvananthapuram.



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.




COMMON DD DTT TECHNICAL PARAMETERS
DVB-T2; QPSK Rotated, Code Rate 1/2, FFT 8K, GI 1/8



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 twofold:

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 1). 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 applicationis 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.


DD DTT Reception On Mobile Phones Was Disappointing



ACTUAL TRIALS

To test and experience DTT reception in Mumbai, on a mobile phone, SCaT purchased a 'TVONGO' DTT dongle (Rs. 3,500) from Flipkart.

Figure 2 Shows a photograph of the DTT dongle fitted on an aging Samsung Note-3 mobile phone.

In Mumbai, we received 11 Doordarshan TV channels + 4 Radio channels, on 2 separate data streams referred to by Doordarshan as DD Mumbai S (474 MHz) & DTT Mumbai H.

Most of the channels were duplicated between the 2 data streams. The channels received are listed in Table :




FIELD TESTS IN MUMBAI

The reception outdoors in South Mumbai (the TV transmitting tower is in Central Mumbai) was OK, though at many locations, only one of the S or H data streams were received (but not both).

Indoor reception was either not received at all, or when received, the picture on all c h a n n e l s pixelised and froze approx. once every 5 seconds. It was essential to extend the DTT Dongle's antenna to get any reception at all, in most areas.

T h e reception was certainly very disappointing, and certainly does not justify the purchase of a dongle by consumers.

DTT would be of interest to commuters to watch news or sports while commuting in Mumbai's local trains. Unfortunately, the reception in local trains was either absent or froze too frequently to be of any practical use.

Frankly, we were very disappointed with the results and DTT reception on phones is certainly unacceptable.

Doordarshan Mumbai has had DTT transmissions for almost 5 years, and it is unfortunate for the public that these are of no practical use for mobile applications.

Stationary reception (e.g. in the home or office) of DTT is also of no practical value in Mumbai, since all the DTT channels are compulsorily carried on all cable TV networks and DTH platforms, and there is no reason for the consumer to purchase an additional DTT STB for receiving DTT transmissions at home.


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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 a n t e n n a , 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 would eliminate any possibility of g h o s t s 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 t e r r e s t r i a l 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.4

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.5

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.