[opendtv] Re: Toward digital TV
- From: Mark Aitken <maitken@xxxxxxxxxx>
- To: opendtv@xxxxxxxxxxxxx
- Date: Mon, 19 Dec 2005 11:25:01 -0500
"Cheers!" on that one mate!
S J Birkill wrote:
>I'm sorry, but I must step in and inject a chord of reality into the
>speculations in this list about European, and specifically UK, DTT. We may
>be across an ocean from ATSC-land, but we're not on Mars, and the facts are
>on record:
>
>1. Analog TV in the UK is neither exclusively nor even predominantly low
>power or local. The first level of coverage, the UHF main stations, have
>typically a pair of 40kW PSP transmitters per service, combining with three
>or four other services into a usually omnidirectional HRP antenna, giving
>500kW or 1MW ERP per channel. The original (1960s) main stations are
>located close to major population centres (London, Birmingham, Cardiff,
>Manchester, Newcastle etc.) with an average spacing between stations of
>some 80 miles. They serve by far the majority of the country's population.
>The second layer fills the gaps: these transmitters are generally located
>in rural areas rather than close to any one major city (Heathfield,
>Hannington, Sudbury, Sandy Heath, Waltham, Bilsdale etc.) They are also in
>the 500kW class, at intervals of about 50 miles.
>
>Only after that do we get down to the relay stations, with power levels
>anywhere from a handful of watts up to 10kW. Unlike much of the USA, where
>cities are on the plains or in wide basins, the UK's history has placed
>settlements preferentially in valley locations, often shielded by steep
>hillsides from geographically distant main stations. Western and northern
>parts of England, and most of Wales and Scotland in particular, are
>difficult to serve from main stations, and are littered with low power
>transposer sites, each serving its own small pocket of population. Even
>some sizeable UK cities (Sheffield, Bristol) are notoriously hilly and are
>covered, according to local topography, by a combination of distant main
>stations and local relays. Here in Sheffield for instance, antennas can be
>seen pointing not only to the local high power station (Emley Moor, 20
>miles) but also to those in adjacent regions (Belmont, 55 miles; Waltham,
>45 miles), as well as to the local relays.
>
>So it is not at all unusual for fixed (rooftop) antennas to receive their
>TV over a path in excess of 40 miles, and in places as much as 70 miles. In
>the low-rise city suburbs set-top antenna use is widespread, though 'main
>set' installations in private houses generally use rooftop or loft-space
>yagis, not so much because of low field strengths as to overcome multipath
>or co-channel. Of course the UK differs from continental Europe in that
>apartment-dwelling (and with it the communal antenna system) is not so
>widespread here, outside the inner-city areas.
>
>2. Digital TV uses the very same transmitting stations, but at an ERP (per
>mux) nominally 20dB (pending analog switch-off) below the analog peak sync
>value. So the signal paths and service areas are essentially the same.
>Multipath isn't such an issue, but CCI is, and digital coverage is often
>limited by the presence of co-channel analog signals from distant stations.
>Where field strengths are low, impulse interference can be the principal
>impairment to indoor-antenna reception. In addition, the comprehensive
>nature of the original 4-channel UHF analog service plan has meant that
>(again, pending ASO) in some areas one or more digital multiplexes has had
>to be allocated a frequency outside the locally-grouped antenna sub-band,
>so new wideband receiving antennas have been needed.
>
>3. Most of the London area is well served by the 1MW analog, 6.5kW digital
>ERP from Crystal Palace in South London. The (Villiers Street?) ground
>floor location described is a mere 6.1 miles from CP, though shielded from
>the direct signal by the buildings across the Thames, and by the heavy
>girderwork of the Hungerford rail bridge. Not an untypically difficult UK
>city-centre location for indoor antenna use, and field strength should be
>quite adequate even for a receiver of modest performance. So we needn't be
>surprised if a retailer just up the road from there can demonstrate set-top
>DTT reception, and a second-floor apartment-dweller get by quite happily
>with a Silver Sensor.
>
>4. But... those legacy 'professional' rooftop antenna installations out in
>suburbia are not all they may appear. Many date from the end of the 1960s
>when UHF PAL colour took over from VHF 405-line black and white. It's not
>at all unusual to see broken or missing elements, drooping booms and
>quarter-inch open-braid single-shielded co-ax. The DTG and the
>Confederation of Aerial Industries are working hard to educate installers
>and the public on the importance of a suitable antenna, correctly installed
>and with good quality feeder. The public is resistant, and often incredibly
>tolerant of the slow decline in their analog picture quality. But they're
>not so happy to accept glitches in their digital reception, especially when
>the upgrade coincides with the purchase of a large plasma screen. So it
>says a lot that the COFDM system is so robust that in the majority of cases
>it delivers customer satisfaction without an antenna upgrade. That this can
>happen is surely at the root of Freeview's success in the Sky-sceptic,
>technophobe majority of middle England.
>
>5. The suggestion that first-generation COFDM receivers had better tuners
>is belied by even the most cursory test of the products themselves, and by
>studies carried out by Digital Television Group, the UK industry's
>independent and confidential testing organisation. The first COFDM STBs
>were built for the failed On-Digital (later ITV Digital) pay service; most
>used a basic analog TV tuner, with outboard IF filtering and amplification
>prior to the ADC. Intermodulation performance at both RF and IF was poor,
>AGC was not optimized for digital, and typical noise figures were in the
>region of 9 to 12 dB. Demods were 2-chip or first-generation single-chip
>jobs, and frankly a little rough. These units had poor echo tolerance and
>struggled, mostly failed, to meet the DTG's target sensitivity of -78dBm
>(QEF BER with a 2k 64QAM 2/3 1/32 signal in a Gaussian channel) and analog
>ACI protection ratio of -35dB. And they just threw their toys out the pram
>when presented with interfering analog signals above about -25dBm -- a bag
>of boosters and another of attenuators was part of every installer's kit.
>
>The first boxes sold for Freeview were modified (or in some cases
>unmodified) On-Digital products -- the card slot wasn't needed. But with
>the first generation of Freeview-specific STBs in 2002 came an
>engineering-led drive for improved performance, as typified by my own
>SetPal design, long championed here by Dermot. With purpose-designed tuners
>these achieved 3.5dB typical noise figures, and combined with the latest
>COFDM demods they pushed QEF sensitivity to -83dBm (64QAM, equal to -87dBm
>with 16QAM) and ACI PRs through the -40dB mark -- in some cases as high as
>-45dB.
>
>The conventional tuner-can manufacturers had to sharpen their act, and
>newer designs from the likes of Philips and Thomson were soon up there with
>the original SetPal, giving even the cheapest boxes (now <$50 retail)
>almost the same performance margins. Despite the cut-throat price
>competition rampant in UK consumer electronics, most current Freeview
>receivers are so good there is very little to choose among them in terms of
>RF performance -- their technology has evolved to be the perfect fit for
>the Freeview plug-and-play DTT model.
>
>6. I'd also like to demolish the old recurring myths about COFDM's alleged
>power budget inferiority (2dB, was it?) relative to 8VSB, for equivalent
>payload and error protection, and the mystical belief that somehow the
>European system degrades faster than the US ouside a 'sweet spot' of range
>from the transmitter. The first is based upon DVB/ETSI specification
>implementation margin minima rather than the real sensitivities achieved by
>almost all demods since 2002, and in an unrealistic Gaussian scenario at
>that. Any real differences are so small they'd be absorbed in cable length
>or antenna gain variations, rather than demanding transmitter power
>increase. The second has nothing to do with modulation; it originates from
>one flawed field study and has been perpetuated within this list to the
>level of dogma, by those who surely know better. But I haven't the time to
>present my case right now.
>
>Season's Greetings to all!
>
>Stephen
>
>
>
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--
><> ><> ><> ><> ><> ><> ><>
Regards,
Mark A. Aitken
Director, Advanced Technology
<>< <>< <>< <>< <>< <>< <><
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