Metal in the Sky 2

- DTT in Oxford (a diary)




The Proof of the Pudding

[End of document 9 November 1998: 6 days until official 'launch']


The Damp Squib

- A Distinctively British Launch

15 November 1998

It is ONdigital's launch day. Unfortunately, very few people will be taking part in the festivities because, although the national transmitter network is up and running (with the exception of Caldbeck, which has already suffered storm damage), the plan to rush decoders to the shops at the eleventh hour has failed. Congratulations to Philips, the only manufacturer who has succeeded in coming up with the goods here today. The large nationals (Comet, Currys and Dixons) have been allocated just one Philips decoder as a demonstrator for each of their Oxford stores. They are taking orders and are hoping that supplies will start to arrive "in a week or two". The smaller retailers (we have spoken inter alios to Allders, Apollo 2000, City Television, Radfords, Tempo, Videotec) are still living in hope. To be fair, much the same thing happened at the launch of Sky Digital on 1 October, with the added embarrassment of no picture being receivable on the demonstration set-ups seen!

ONdigital is running an intensive advertising campaign on London Weekend Television, whereas nothing has been seen on Anglia, Central or Meridian except for a report on the ITN news showing the illuminations at Crystal Palace. Perhaps Londoners are actually able to purchase decoders - the poor provinces must wait their turn.

Although it is difficult to comment fairly on a hastily-mustered demonstration in a busy store, it could be said that picture quality seems similar to that of DVD.

[End of document, updated to 15 November 1998: 'launch day']


At Very Long Last...


Heroes of their time: ONdigital, Philips Consumer Electronics and Apollo 2000 Superstores

Thank you, Apollo 2000, for obtaining some stock! It is 24 November, nine days into ONdigital's service. We have finally obtained a Philips DTX 6370 DTT receiver, a bland dark-grey box with the fashionable smiling plastic escutcheon. The plug-and-play simplicity of installation was impressive: the unit soon located all six of the current local multiplexes plus some of those from Hannington, and it assembled a menu of 38 channels. Quite a few of these are evidently still under construction or showing promotional material.

Techie stuff

Connexion to the TV set is normally via SCART (there is also a UHF modulator if required): it is important to set the mode to 'RGB' rather than 'Composite' in order to avoid the dreaded PAL dot crawl effect at the boundaries of saturated colours. Picture and sound quality on all channels is excellent - what a pity there is so much video noise in much of the source material - although occasional motion artifacts and rare blocking, scrambling and momentary black-outs have occurred even on the local signals. Impulsive interference is a factor. We are trying a few experiments with attenuators in case the signal strength is too high (the unit's signal strength meter shows maximum on all six Beckley multiplexes with a loft-mounted log periodic array: transmitter distance 2·8 miles).

It can be said that digital reception is lost at values of attenuation where analogue is still acceptable, which implies a smaller service area. The signal strength meter also shows maximum on the Hannington multiplexes that come through our channelised antenna system, although the Crystal Palace and Sandy Heath signals do not register at all (they are below the 40dBµV receiver threshold even with a serious fringe installation). Nevertheless, the Hannington channels are far more susceptible to freezing and drop-outs and it is clear that perfect digital reception at this distance (32 miles) is significantly more demanding than for high-quality analogue. On the other hand, the immunity to ghosting is impressive.

The concerns about colour quantisation mentioned previously seem insignificant, fortunately. Although dot crawl is not visible, close inspection of contrasty detail in, say, computer-generated weather maps shows that sloping lines (e.g. isobars and the profile of the announcer's head) are attended by curious small artifacts resembling shimmering zikkurats ('mosquitoes'). However, these are generally less obtrusive than the artifacts in equivalent analogue pictures.

Caveat emptor

ONdigital have an efficient and obliging telephone help desk. Prospective purchasers of ONdigital services need to realise that dealers have been issued with a very detailed prediction of service coverage based on postcode, finely subdivided according to the second part of the code in many locations. Because these estimates are quite conservative, and dealers have been instructed to show at best reluctance to issue decoders to customers in areas not predicted to receive good reception, it might be necessary to do some homework (and even to claim migratory tendencies) beforehand. We shall shortly be testing reception at a site in the foothills of the Grampians, which although very much a 'no-sale' area according to the postcode predictor should yield interesting results on the basis of the present evidence.

What's ON?

The BBC-1 regional news programmes are replaced by a 'national' version, as on Sky Digital: we shall need to continue using the analogue system to receive opt-outs. Fortunately, given suitable reception conditions, it is still possible to access multiple ITV-1 regional opt-outs. The additional range of new channels is very welcome. The SDN channels are present but still under construction, including Channel 5; Teletext is not yet available.

Direct comparisons between analogue and digital pictures tend to favour the latter, although the difference is slight with good signals and each system has its strengths and weaknesses: for instance, it is obvious which is the digital signal when a sudden dissolve effect causes momentary blocking. The delay of a second or so in the digital signal is going to delay millennial celebrations slightly.

Uh-Huh? Hördi-Gördi!

ONdigital tell us that MTV will be added to the selection of Primary channels some time in 1999. Sorry that the amusing advertising on Carlton and LWT has been taken off as 'unfair to satellite'!

[End of document of 25 November 1998, updated]

Minor updates (11/98)

We are now trying to obtain some more magic boxes for all those TVs and VCRs scattered about the house! Unfortunately, each requires its own subscription and there does not seem to be any discount for bulk at present... are multi-viewer households really going to do it this way, we wonder. (When we were setting up the paperwork for our second box a few days later, there was a hiccup because ONdigital's computer system did not seem to have provision for more than one box per customer - this was soon resolved after some hasty consultation).

DTT reception from Crystal Palace has now been reported from a location near Oxford. Accordingly, our own present lack of success (mentioned above) is likely to be a problem of very local topography (Headington Hill is on our line of sight). Investigations are being made and it is quite possible that something can be done: similarly, our Sandy Heath installation has scope for improvement. Perhaps once it has been possible to persuade the decoder to tune to these very weak signals it will work, at least intermittently.

[End of document of 30 November 1998, updated]

Minor updates (12/98)

The BBC-1 regional news programmes, which in England are currently replaced on DTT by a 'national' version, are to be given the appropriate opt-outs in line with the analogue service when necessary technical arrangements have been completed during 1999. Channel 5 and BBC Parliament (sound only) are now on DTT; First ONdigital will be starting soon.

There is a good deal of discussion in the online newsgroups and elsewhere about reception problems. Numerous complaints are attributed either to excessive signal strength, where the receiver can be swamped by the stronger accompanying analogue signals; or to inadequate signal strength, where interference (usually impulsive, either mains-borne or via the antenna system) is often disruptive. In unfortunate cases, overloading and impulsive interference can act at the same time. Experiments with attenuators, amplifiers, mains filters and improvements to antennae and feeders can often provide solutions and it is not really clear at this stage whether anomalies in the DTT signal itself (other than sheer low amplitude in the poorer service areas) are ever significant.

Our own experience with glitches and breaking-up has been the result of both high and low signal strengths from different transmitters; problems have been readily amenable to careful attention to the levels of extraneous signals, especially ignition interference. [Link to technical details here] Sometimes the frequency distribution of the unwanted signals (TV, impulsive and other) can be the reason for puzzlingly better reception from a weaker transmitter; overloading in masthead and distribution amplifiers can also be harder to recognise with DTT than with analogue.

The signal strength display of our current Philips set-top receivers (DTX 6370) has a quite sharp transition from 'good' to 'poor', with only a few dB range within the intervening 'satisfactory' zone. To allow for fading, a signal comfortably into 'good' is probably required for consistent reception. On the other hand, if no signal at all except a regular 'blip' into 'poor' occurs, take heart - a few dB more will produce results!

There have also been discussions about problems in the receiver software (e.g. lock-up requiring a switch-off to regain control, which we have experienced several times following interference break-up), although these will probably be dealt with in future upgrades. Unfortunately, much serviceable hardware is likely to be condemned whilst this is being sorted out.

[End of document of 21 December 1998, updated]

'The Scottish Experience', Hogmanay '98/99

Reception has been tried at a rural Perthshire site approximately 10 miles NE from Crieff and 11 miles NW from Perth. The terrain is quite hilly and conditions are irregular; adequate analogue reception with a fringe installation (18-bay multidirector - Aerialite GSP50 - about 40 feet high with Triax masthead amplifier) is available from Angus (29 miles away; includes BBC Scotland and Grampian Television, but not Channel 5) and a similar rig provides a rather poor and variable signal from Black Hill (41 miles away; includes Scottish Television and Channel 5). A third array provides Channel 5 from the Perth relay at a variable level, but normally better than from Black Hill; signals from nearer small relays at Crieff, Dunkeld Town and Methven are mostly poor to marginal, as are those from Craigkelly (32 miles away) and Durris (63 miles away).

DTT transmissions have not yet begun from Angus nor from any of the relays; however, it proved possible to obtain reception from Black Hill, our current DTT DX record of 41 miles, although serious disturbances occurred during poor weather conditions. Two of the six multiplexes are above the existing analogue passband (Group B), but re-rigging does not seem worthwhile because STV (channel 47 multiplex) is in-band and the other programmes would be better taken from Angus when available (the existing Group C/D array will be correct). We believe that a GaAsFET head amplifier would probably provide a clean enough signal for reliable DTT reception of STV from Black Hill to complement the Grampian bouquet from Angus. We shall report the outcome when Angus is on-air [described below - link here].

It is difficult to resist a small digression: reasonable Band III reception from Durris used to be possible in Edinburgh using an indoor 3-element array feeding a standard 1950s triode cascode receiver, a distance of 81 miles.

[End of document, updated to 3 January 1999]

Minor updates (1/99) - an upbeat 'rant'

We were fortunate, in response to an ONdigital telephone poll on 16 January, to be able to say that our own experiences of DTT are extremely positive and that we enjoy essentially perfect reception, the installation procedure was straightforward and ONdigital's service and documentation have been exemplary. The discussions above are prompted primarily by a natural curiosity about pushing the boundaries of a new technology rather than by any dissatisfaction as consumers.

The online newsgroup discussions now contain numerous tales of woe from people experiencing reception problems and some local retailers we have spoken to seem less than euphoric about DTT even in a well-served area such as Oxford. This is quite worrying, despite making allowance for the inevitable 'Victor Meldrew' customers who will drone on about unfit goods and so forth. Some problems arise from the use of inadequate or faulty antenna installations; but there are evidently more general issues concerning poor coverage and interference, although things might well improve as receiver design develops. Also, the present strategy of fitting as many channels as possible into a small number of multiplexes, whilst providing a welcome breadth of choice, is a calculated risk against a possible compromise in quality.

It would be unfortunate if a deadlock arose in the future because of insufficient satisfactory DTT coverage, as a result of having to fit in with the present analogue system, delaying the analogue switch-off which would enable the reallocation of spectrum space to provide a better DTT service. We hope that most new adopters are pleased with their decision and enjoy sharing the pioneering spirit with those (in Britain) at the start of the Alexandra Palace service, the first ITV service, the launches of Telstar and Early Bird, the first UHF/625 service, the first colour service and the live pictures from the Moon and planets.

[End of document, updated to 18 January 1999]

Minor updates (2/99)

The Philips software update available over the air from mid-January seems to have cured the 'locking-up' problem and has made subtitles available, although the software still needs some improvement. We are trying to find out whether a service menu is available to enable adjustments to be made to the RGB drive (some receivers require higher or lower RGB levels than the SCART standard) and to the signal threshold (or bit error rate threshold) for 'add channels', to force the acquisition of weak out-of-area multiplexes: during tropospheric lifts, the Crystal Palace signals just limp into the 'poor' zone (despite adverse local topography) and would probably produce occasional pictures.

[End of document, updated to 15 February 1999]

Scotland Revisited (8/99)

This is a follow-up to the 'Scottish Experience' in wildest Perthshire [described above - link here]. Since Hogmanay, Angus DTT has come on-air (Postcode Predictor: "uncertain coverage ... viewing quality cannot be guaranteed"). Meanwhile, the ageing Triax masthead amplifier has lost its lid and is letting in the elements; analogue reception has deteriorated perceptibly. Despite this, a vintage Philips box found all six multiplexes and perfect reception was possible on all DTT channels - Channel 5 snow-free at last! Unfortunately, reception from Black Hill was unusually poor - probably temporarily, although a deterioration of the antenna system is quite possible - consequently our December DTT record could not be repeated.

As expected, DTT ITV-1 from Angus is Grampian Television and S2 replaces ITV-2 but, although the Scottish versions of BBC-1 and BBC Choice are available, the BBC-2 DTT feed seems to be from England. There are five SDN test channels (2, 3, 6, 10, 11) and GMTV-2. Of the non-premium channels, BBC-1, BBC-2, Grampian, S2, BBC Choice, BBC News 24, BBC Knowledge, Sky One, Carlton Cinema and SDN-11 were in widescreen or 'postage stamp' format when running through the menu on Friday 6 August.

[End of document, updated to 8 August 1999]

Minor updates (9/99)

We managed to 'catch' an experimental software update today (29 September 1999) which has enabled the reception of Digital Teletext (DTX) on our original Philips boxes in Oxford. The BBC Text service is impressive, albeit a little slow (roll on those TiVo boxes which might be able to store the whole set of text pages for quick access!); the ITC Teletext channel is currently blank. A quirk with the selection of subtitles has been corrected in this version of the software, although the signal strength indicator screen does not now seem to work properly and the 'RGB' output setting reverts to 'composite' unexpectedly.

After ten months' daily viewing of DTT, we are still very pleased with it. New channels are gradually being organised and we hope that additional spectrum space will become available in the future - overcrowding in some of the existing multiplexes is not a good thing. Considering the technological leap required to create a new broadcasting system in such a short time, the odd delays in promised features such as DTX are of little significance. We hope that the geographical coverage problems can be largely overcome before very long; it is unfortunate that a nationwide single-frequency network was impossible. Last but not least for some, the ability to pick up the box and travel is excellent, rivalling the flexibility of satellite and without the strategically-positioned dish on a garden chair...

[End of document, updated to 29 September 1999]

Minor updates (11/99)

Today (8 November 1999) the 'official' software update to enable Digital Teletext reception with Philips receivers has been launched. Performance seems quite similar to the previous experimental version; the quirks in that seem to have been eliminated (although the signal strength indicator is rather limited) and it is very good to have RGB drive again. We understand that updates for the other receiver brands are in the final stages of development. The BBC and ITC text services are working quite well - each has its strengths and weaknesses and there are some interesting experiments in evidence.

After some evaluation we are sorry to note that the unexpected reversions from 'RGB' to 'composite' output continue to occur (e.g. select TV and STANDBY). It should be borne in mind that the new software release is really a 'long-term trial' - it is also still possible to 'crash' the receiver when in text mode. On the positive side, the text services accompanying BBC Knowledge and BBC Parliament are much appreciated.

[End of document, updated to 10 November 1999]

Minor updates (2/00)

To create more space in the attic we have been moving some antennae to external locations. Although the local high-power transmitter is only 2·8 miles away we now have the luxury of a wideband Antiference XG10 (12-bay multidirector, 25 feet high and largely screened from the road by the roof) which produces more than enough signal to enable purely passive distribution amongst several receivers. During a heavy snowfall it was necessary to take 5 dB of attenuation out to remove 'blocking' on the weaker multiplexes: the performance of full-wave multidirector arrays of the type with insulating inserts between the inner ends of the elements and the boom is known to deteriorate when wet, although most of the loss (say 10 dB) probably occurred in the transmission path because rain has never been an issue. The 5 dB had to be restored later because of a reduced immunity to interference. It is slightly worrying to find such a narrow fade range, although we have not encountered major difficulties with DTT reception from Hannington (32 miles) which also varies by at least 5 dB depending on propagation conditions.

[End of document, updated to 23 February 2000]

Minor updates (3/00)

A new software update (2 March 2000) has tidied up various minor quirks in our Philips DTX 6370 receivers. The signal strength meter is now working correctly. Well done! Alas, the 'RGB' problem continues - there are reports of it occurring with other brands of receiver too. It is evidently a software problem, as it did not exist with early releases.

Plans are now being considered to bring the powers of the weaker multiplexes on the Oxford, Crystal Palace, Hannington and Sandy Heath transmitters up to provide comparable service areas for all six from each transmitter.

It is still possible to say that the best DTT reception is comparable with the best analogue (bear in mind that much analogue material passes through digital stages in the transmission chain now), although the less said about some of the heavily-compressed, statistically-multiplexed material the better. Some of the discussions in the online newsgroups are very reminiscent of the 'vinyl versus CD' discussions about sound recording. With vision by Bang and Olufsen and sound by Quad, we are generally happy bunnies.

[End of document, updated to 28 March 2000]

Minor updates (9/00)

Today two of the Oxford multiplexes ('A' and 'D') were reallocated to different (lower) frequencies, and these plus 'C' enjoyed a power increase ('2' having been increased about three weeks ago), as the concluding part of the 'Crystal Palace 5' equalisation project to improve the coverage from the Crystal Palace, Hannington, Hemel Hempstead, Oxford and Sandy Heath transmitters. Reports from the edges of the Oxford service area show a substantial improvement in the worst multiplexes, especially 'D' which was underpowered on channel 67.

Although during the preceding testing phase an additional multiplex often registered on the signal strength meter, the switch-over today was abrupt, the old multiplexes having both disappeared. Since OnDigital's letters explaining how to acquire the new multiplexes only arrived here this morning, their help-line is probably very busy tonight!

[End of document, updated to 15 September 2000]

Minor updates (11/00)

Today we were questioned in a second ONdigital telephone poll about the quality of reception, our experiences of Customer Services and whether we would continue to subscribe in the future. The reception issues included the perception of picture and sound quality and any interference to them, the proximity of mobile telephone masts and other radio and TV transmitters, the local topography and the presence of possible obstacles and reflectors in the neighbourhood. Several questions were focused on the specific symptoms of substandard DTT reception and it was clear that pains are being taken to identify the causes of any problems which viewers might have 'in the wild'.

As previously, it was possible to affirm that (with the exception of the long-standing 'RGB/composite configuration bug') we are very happy with ONdigital and with DTT in general - we haven't needed to contact Customer Services since registration two years ago - and that there was no present likelihood of our "defecting to Sky Digital", one major reason being Sky's insistence that reception must be confined to one specific location (this is not the case with Sky Analogue, which we still use whilst it survives).

Department of Kraazy Exsperimentz

It is interesting to try to pinpoint why some viewers prefer analogue pictures to (correctly-rendered) digital ones. Common complaints seem to be that the digital process makes pictures 'plasticky' and 'lifeless'. We need to exclude the obvious artifacts of over-compression, blocking, motion aliasing and posterisation which sometimes occur, as there is evidently something else to this. One approach is to try modifying a digital picture to try to identify the "analogue look". This can be done by adding a small amount of noise - barely sufficient to be described as 'grain' - and by trying a little video peaking (one or two judicious small capacitors in the luminance drive circuitry serve well). Varying gamma, the 'transfer characteristic', can also have a marked effect on preferences (easier to deal with using a monochrome set).

Direct 'A-B' comparisons of good analogue, normal digital and 'tweaked' digital suggest that, for some people, it is the presence of low-level noise and a peaky video response which elicits a preference. A long time ago, we conducted some similar experiments to compare off-air 405-line, positively-modulated and 625-line, negatively-modulated pictures and vice versa, with the tentative conclusion that viewer habituation was a major factor. Without wishing to become too partisan in this discussion, it does seem that one rapidly becomes accustomed to a particular set-up and might then regard a different one as odd, if not worse - the same is certainly true in audio reproduction. The truth is that there is still a very wide credibility gap between any television system and the source object, as is only to be expected - look into any studio monitor - the era of "hi-fi vision" is still some way off, alas.

Jon Godwin

[End of document, updated to 18 November 2000]


[End of document, updated to 15 December 2000 (amended)]