DTV Across The Pond

 Submitted by Dermot Nolan

With the current debate raging in the US about 8VSB vs. COFDM here is a piece on DTV in the UK. 

DTV began in the UK on November 15 1998, twenty-nine years to the day after PAL color television. It uses DVB-T COFDM transmission (mode 64QAM, FEC 2/3, guard interval 1/32, 2000 carrier version.) There are currently over thirty stations in the DTV grid providing 37 SDTV/EDTV channels in 6 x 8 MHz UHF channels broadcast from each station. UK population coverage has now passed 75% and is expected to reach 90% by Christmas 1999. Each channel has a payload data rate of 24. 1 Mbit/sec carrying an ensemble of DTV services. Stations transmit between 20 - 27 dB below their PAL counterparts to achieve equivalent coverage typically 50-60 miles radius, although in some instances DX -COFDMers are receiving DTV 90 miles away! Minimum clear sky C/N to successfully decode is 18.5 dB in very easy locations and in the worst possible urban/hilly multipath areas is around 25dB. Although these numbers may seem interesting it is to be remembered that a Grade 5 PAL picture requires a C/N of 44dB. Therefore the existing installed antenna base is perfectly adequate to receive DTV. In most locations simple 10 element antennae suffice and in Grade B contours eighteen elements, which are preinstalled for analogue use, usually suffice.

The COFDM DTV system is designed to work with today's antenna technologies:

1.  Over 80% of existing rooftop antennae work first time, and in predicted coverage areas this figure rises to 95% after any modifications or upgrades needed.

2.  In City and Grade A contours portable indoor antenna reception of DTV is widespread using either simple rabbits ears or amplified indoor antennae ($30). Unlike 8VSB, COFDM works very effectively with amplified signals as the interferers are added constructively and the system can handle 0dB echoes common in urban areas. Indoor DTV reception has become increasingly popular in condos for those who would otherwise have to wait for channelised MATV systems to be upgraded to DTV by building owners.

3.  The system is immune to multipath, which is why you will frequently see off-air demonstrations of DTV in downtown retail shopping malls, rather than server sourced demos. The BBC conducted a detailed DTV reception survey concluding that, to date, no areas have been detected where DTV reception is prevented by multipath impairment. (The chosen COFDM mode handles echoes as long as 28uS, but itís believed that the worst echoes in the UK are under 20uS). 

A wide variety of STB and IDTV receivers are available from major international CE vendors. If you subscribe to ONdigital, the pay DTV service provider, STB's are free. Minimum monthly subs are $10. Otherwise the STB costs $600 unsubsidized. IDTVs , 16x9, Dolby surround etc retail for $1500 + and are available with/without CA systems.

With the free STB business model current DTV penetration stands at 300,000 DTV users (equivalent to 1.2 million homes in the US market) and is expected to reach 500,000 before Christmas. The 'plug and play' nature of COFDM DTV, its immunity to multipath, reuse of legacy antennae systems, the channel line-up, the introduction of widescreen TV, and its robust reception on indoor antennae have been contributory factors to growing consumer acceptance nine months after launch.



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