- Digital Terrestrial TV in the UK - February, 2002
Graham Vine - Editor, EUROPE
TV is going digital for a number of very good reasons. Compared with traditional, analog methods, the signals are more robust, occupy less bandwidth, are easier to encrypt, and can be adapted more easily for carriage of additional services such as data and interactivity. Much technology has had to be developed to make these potential benefits become a reality, the chief amongst these being MPEG compression.
The UK is well advanced in the roll-out of digital TV services, all of them based on the DVB (Digital Video Broadcasting) set of standards. Three delivery mechanisms are in use: Cable, available in many cities and towns but not in many out-lying areas, Satellite, with good coverage of the UK, and Terrestrial, currently in phase 2 of its roll-out and set to increase coverage from 70% to the high 90%. Experiences and information about terrestrial distribution of digital broadcast TV in the UK are the subjects of this article.
"Snow" on analog television when the signal is too weak
Current analog terrestrial TV in the UK is all in the UHF (Ultra High Frequency) band, from 470 MHz to 860 MHz . By a system of main stations and repeaters, virtually complete coverage of the country has been in place for many years. The band-planning done over 40 years ago was highly successful, allowing for a national network of 4 stations with regionalized programming for some news, current affairs and, in some cases, advertising. Note that all these 4 stations carry a mix of programming styles suitable for viewing right through a whole evening, not streamed according to content (e.g., music, sport, nature). Picture quality, based on the 625-line standard (it's 525 in the US), is generally very good. During periods of extreme atmospheric conditions, however, breakthrough from continental or other out-of-area signals can interfere. A characteristic of analog signals is that they degrade 'gracefully' as signal strength varies, the only visible artifact being an increase in the snow-like 'noise' on the picture (see photo of snow at right, something you don't get with digital TV). Sound is carried as FM (Frequency Modulation) for a mono soundtrack and digital using NICAM (Near Instantaneous Companded Audio Multiplex) for stereo.
A few years ago, before digital technology was ready, the opportunity was
seen to offer more choice by creating a fifth channel. But where to fit it
in in the available band-plan? A starting point was the area around
channel 36/37. This has traditionally been used for the UHF feed from VCRs
(Video Cassette Recorders) and video games into TV sets. As these are
local, within a home, they are easily 'moved' from channel 36 to a
frequency not used in the local area. In TV regions where channel 36 or 37
was unavailable, perhaps because of harmonic or local oscillator
breakthrough, then each area was investigated area by area, selecting an
unused channel that would not cause interference. Another difficulty was
found in areas close to the European mainland. Channel 5 transmitter
antennas in these area had to be made directional, leaving some viewers
without service. Throughout the country, Channel 5 transmitters tend to run on
significantly lower power than the original 4 channels. This then is the
analog status quo, with incomplete coverage for Channel 5 but very good
quality available from the other 4.
For digital television transmission to be receivable using a normal antenna, they must lie within the same UHF band as the analog signals, and they must co-exist for a number of years, until the analog system can be closed down. But, as illustrated earlier, the UHF band is full (to overflowing!) in the UK. Fortunately there are different trade-offs that can be made when dealing with digital signals. Considering interference from the digital transmission onto the existing analog ones, the digital signal appears as random noise to the analog TV and so, in an area of full-strength analog transmissions it is lost in the normal, lively, active image that makes up the TV picture. To aid in this, digital transmissions can be much lower power than their analog equivalents - say one-hundredth of the power. This reduces the risk of interference even more.
Each area's set of 4 main programs occupies a bandwidth of 88 MHz. In order to use the same antenna for digital and analog signals, the digital ones are also, in general, situated within this 88 MHz slot. This is because TV antennas are 'grouped' according to the region being viewed. However, some regions have had to use an out-of-group frequency for the digital signal. In these cases an extra-wide bandwidth antenna is needed.
Each digital transmission carries many TV programs in a so-called multiplex. So a digital band-plan could be designed which has 8 frequencies in use and 10 programs on each giving 80 programs. Reality is a little more complex, however, since there is a trade-off between bandwidth and picture-quality: more picture detail and more movement in the picture calls for higher bandwidth. The authorities running the system need to divide up the available bandwidth according to the needs of the programs being carried. As a result, some programs may appear to have a lower picture-quality than others.
Terrestrial Digital TV antenna on the loft area of the roof
With this information as background, and with the bulk of the transmission system in place for a couple of years, the time had clearly arrived for this writer to give Digital Terrestrial Television (DTT or DTTV) a try-out. The domestic installation here at the Vine Mansion has remained stable for quite a few years, consisting of television receivers in the kitchen, sitting-room and two of the bedrooms. These are all fed from a distribution amplifier whose antenna input goes via the VCR, allowing taped videos to be seen in all rooms, if required, which it isn't.
The only antenna has been in the roof-space (loft area), providing (at a range of about 20 miles from the transmitter) a solid analog signal on the 4 main programs but a rather weak one for Channel 5. When the digital converter box was fitted, there was instant success with some of the channels, but not all. The Nokia converter ('Set-Top Box', STB) has a nice menu system that includes an on-screen signal-strength meter. Some of the frequencies required were in the red band - too weak. Officially, the area I live in is not yet digital-ready, but the results were promising enough to persevere. So an outdoor antenna was fitted to the chimney-stack (photo shown at left), fed via the distribution amplifier for good measure, and then tested again. Complete success! All the active frequencies in my area were in the green band - well up to the signal-strength required.
My domestic television was actually designed for monitor use and has UHF and baseband ('video') inputs. I tried both methods, and the difference in perceived quality was negligible. What was noticeable was the complete absence of visual interference - snow. The nearest equivalent in the digital domain is the occasional burst of blockiness - see picture below, right (simulated). Sometimes - maybe twice per week - a splat happens, taking up about a quarter of the picture. This happens when a vehicle passes that has an ignition system that isn't properly suppressed. I have only known this to be the case with motorcycles.
The digital picture quality is marginally less crisp than the analogue equivalent. On a good receiving setup, the picture quality from the 625 line PAL system is truly superb, only marred by the 'cross-color' effect. Since an RGB feed is available from the STB (set top digital TV decoder box), full resolution is maintained going into the TV set. Restricting the picture resolution at the encoding transmitter end of the chain actually saves bandwidth (=lower cost, more programs) so that is the trade-off. In practice, the bandwidth used does not peak up to the same rate as, say, DVDs but different economics are at play, and most viewers should prefer the digital version.
Blockiness, the digital equivalent to "snow" (simulated photo)
Most of the programs are sent in the widescreen format. This works very
well, especially using the projector that I borrowed for test purposes.
The only downside is an increasing trend for a normal 4 x 3 aspect-ratio
picture to be displayed in the center 4 x 3 area of the wide screen
instead of filling out the whole 4 x 3 of the monitor. This will become
less important as more households convert to widescreen televisions.
What's on offer?
Firstly, we should have all five of the main national stations, but with more of the programming being in the widescreen format. The two national BBC programs are financed through a license fee, which means no advertising is carried. The other three national programs are financed by advertising. Since the digital signals originate, in general, from the same transmitter masts as their analog equivalents, they are 'regionalized' in much the same way. However, not all of the switching infrastructure is in place to do this. An example of this causing a problem for me was during an East of England program on BBC-2, which I wanted to tape. I would have expected BBC-2 digital to have carried the East of England regional service, but instead, my local transmitter was carrying the London (national) service. The only way I could obtain the particular program I wanted was to revert back to BBC-2 analog, which did carry the proper East of England service for me. I'm sure when all the switching is in place, the regional opt-outs will be carried on digital just as they are on analog.
Like their analog equivalents, the five national programs are Free To Air (FTA). This is an important distinction from some programs which are on subscription or Pay Per View. I am only interested in the FTA programs, so the system suits me very well. Other FTA programs, at the time of writing, are BBC News 24, BBC Knowledge, BBC Choice, ITV-2, ITN, and a shopping program called Shop!
News 24 comprises rolling news and current affairs throughout the day and night. This is ideal for dipping in to when the main news broadcast is at an awkward time. ITN has a similar format. Knowledge is mainly informative, with an emphasis on arts, nature, science, history, geography - that kind of thing. BBC Choice has a hefty element of childrens' programming but also includes the pick of programs from other BBC channels. ITV-2 is an associated program to ITV-1, one of the five national programs. There are some repeats from ITV-1, but there are also specialized programs like a sign-language version of a popular soap opera and back-up information which leads on from some main ITV-1 programs.
Other FTA material is provided as several channels of text information, suitable for browsing through, and text information per-program, accessible by hitting the INFO button on the remote. The proceedings in Parliament are available in sound only. BBCi (i for interactive) has a very nice composite picture, with most of the screen taken up by textual material, but a quarter of the screen showing a miniature of your choice from the six BBC programs, one of which is the audio-only Pariament. A similar format is used in ITV TEXT+.
For those wishing to receive more programs, there are various subscription offers available. Such packages allow viewing of recent feature films and some of the major sporting occasions. There are dozens of these channels available, some of the program material coming from one of the UK's satellite broadcasters, Sky. Most if not all of these programs are 'themed' rather than general interest.
Digital viewing is catching on in the UK at a steady pace. Of the three delivery mechanisms - satellite, cable and terrestrial - the latter has some advantages. It is for others to point out the benefits from satellite and cable. Terrestrial delivery is more suitable to the multi-receiver situation since a normal UHF feed to the location of each TV and STB is all that is needed, so there is no cable interface or dish-control signal to worry about. As TVs with the STB built-in become more widespread, my simple arrangement of an antenna feeding a distribution amplifier could become the norm. DTT can also be the basis for digital TV reception at mobile locations, such as by caravanners (trailer homes).
The STBs for DTT are normally only available as part of a rental or subscription scheme, but that is set to change in Spring 2002 with the release of a new STB from Pace. This will be a low-cast option (perhaps half the price of a typical TV) and will be for purchase outright. It will allow economical upgrading of all the TVs in the home to digital TV reception. I intend to review this device in the coming months. In the meantime I plan to adapt one of my spare TVs to RGB input.
In conclusion, I am pleased with having gone to the trouble of 'going digital'. My additional programs make an interesting alternative to the normal set of 5, and the performance of the system is very pleasing. And if I ever need to extend my program choice, subscribing to extra services will be easy enough, activated through a smart-card system in the front of the STB.
- Graham Vine -
© Copyright 2002 Secrets of Home
Theater & High Fidelity
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