The plain man’s guide to the changeover to 625 line TV 

24 April 2017 tbs.pm/11721

From a discussion paper published by Associated TeleVision Limited in 1963

Because of the immense complexity of this subject an attempt is made in the following to simplify it as much as possible and to this end the form of question and answer has been adopted.

  1. Ⓠ Why is this country changing over to 625 lines?

(i) To give a better television standard with scope for technical improvement (better quality pictures, particularly on larger receiver screens). The 405 line standard is effectively played out, having been fully exploited.

(ii) To standardise on a common series of UHF TV wavelengths within Europe which greatly alleviates the mutual difficulties of planning national television services so that those of one country do not interfere with, and hence limit, the coverage of those of another.

(iii) To adopt a uniform line standard with Europe to facilitate the exchange of programmes without the degradation inherent in standards conversion.

(iv) To enable this country to maintain its position in the television broadcasting and general electronics fields, and to expand its export trade.

(v) To avoid wastage in development effort being applied to two systems (one for home and one for export).

  1. Ⓠ Who made the decision and is there any possibility of it being reversed?

Parliament made the decision on the recommendation of the Postmaster-General (the Minister concerned) on the advice of the Television Advisory Committee who in turn were advised by the Technical Sub-Committee and the Pilkington Committee. There is no possibility of the decision being reversed.

  1. Ⓠ What is the urgency of the changeover?

The B.B.C. is being pressed to open its second TV service and extend it to National coverage as quickly as possible. By Government decree the new service must be on 625 not 405 lines. By the same token the Government represented by the P.M.G. wish to change over the existing B.B.C. 1 and I.T.V. services to 625 lines as quickly as possible.

  1. Ⓠ Will the existing 405 line service be retained and if so, for how long?

A The existing 405 line service must be retained over the whole country to keep faith with the public until the 405 line only receivers in the hands of viewers at the moment can reasonably be said to have completed their normal life—say seven to ten years.

  1. Ⓠ Can existing 405 line only receivers be modified to receive a 625 line service?

No. It would be impracticable and entirely uneconomical. All new receivers currently being marketed by industry are capable of receiving both 405 and 625 lines, and are available optionally, with or without the UHF tuner, which can be added at a later date.

  1. Ⓠ It is announced that B.B.C.2 will be in UHF, i.e. Bands IV and V. At one time it was stated that there was room for another service, albeit with some limitations as to coverage, in the present VHF Bands I and III. Why could not B.B.C.2 be accommodated in this part of the spectrum?

There was enough room, i.e. spectrum space, for a 405 line service with near national coverage, but not for a 625 line service giving any significant coverage because of its wider channel width. Remember that 405 line service requires channels only 5 Mc/s in width while a 625 line service requires channels 8 Mc/s wide.

  1. Ⓠ Why can’t existing transmitting stations be converted from 405 to 625 lines right away?

Apart from the fact that nobody would be able to receive them on existing 405 line only sets, the channel widths in VHF have already been fixed at 5 Mc/s so the stations are too close together in frequency to carry the wider bandwidth required for 625 vision. They would ‘overlap’ and interfere with each other.

  1. Ⓠ How then is the changeover to be achieved?

Actually, a large number of possible plans have been considered by the TAC/TSC and narrowed down to three, with certain variations.

  1. Ⓠ What were these plans?

There was the ‘Sequential switching Plan’ evolved by the B.B.C. This involved duplication of the existing transmitting stations in VHF. This process, it was proposed, would be carried progressively through the country, area by area. That is, a new VHF station would be built in London, for example, using a different and wider channel from that now used by the present Crystal Palace transmitter. As soon as existing 405 line only sets were obsolete, the original Crystal Palace transmitter would be closed down and the new one would continue to carry the service on 625 lines on the new VHF channel. The frequency space thereby made available would then be used for another 625 line service elsewhere.

  1. Ⓠ This is all very well, but what about sets replaced in Birmingham at the 3rd or 4th or even 6th year of the London duplication period. Surely the 405 line Birmingham service would have to be continued until such sets in turn were seven years old?

No. The radio industry would make no more 405 line only sets after the start of the London duplication process. All sets sold to the public thereafter would be 405/625 line sets so that they would cope with the changeover whenever it came.

  1. Ⓠ Would this not greatly increase the cost of sets?

No, the industry reckoned the increased cost of a dual-standard VHF set would only be a few pounds.

The BBC’s experimental Colour Control Room in 1958

  1. Ⓠ This seems an eminently practical plan, can it be adopted?

Unfortunately, it is almost certain that it cannot now be adopted. The reason is that the spare channels in VHF required to form the ‘open squares on the chessboard’ providing manoeuvring room for the duplication process will no longer be available because the Government White Paper gave first priority for the use of free channels to provide a National TV Service for Scotland and Wales and to fill in the more important gaps in the service areas of the existing 405 line VHF services. It is remotely possible that additional VHF spectrum space (channels 14 and 15 at the top of Band III) might be freed by the defence services, who now occupy it to provide the necessary ‘open squares on the chessboard’. This has been done in several European NATO countries but the chances in the UK are very slim indeed. It is, therefore, safe to assume that this plan is no longer viable.

  1. Ⓠ What are the other plans?

Next, the ‘Switching Plan’ proposed by the I.T.A. which consisted in the erection of ‘shadow’ VHF 625 line stations in all areas with an ‘overnight’ changeover on an ‘appointed day’ at the end of seven years from the old to the new stations and the old to the new wider 625 line channels. This plan was fully described in the I.T.A’s publication ‘405-625 — A Plan for Changing to 625 Line while Retaining VHF Transmission’. This plan needs some supplementation by UHF channels to make up for the smaller number of 625 line channels which can be fitted into the VHF Band I and III.

  1. Ⓠ This also seems a sound plan, can it be done, but if not what are the objections?

It can be done but the following objections have been put forward:

(i) The introduction of 625 line operation for existing services would be delayed for seven years and this would not be in accordance with the Government’s policy to start the changeover of the TV standard in the UK to 625 lines as soon as possible.

(ii) It is feared that the dealers’ service staff all over the country could not cope with an ‘overnight’ changeover and that chaos would exist for many months. This would undoubtedly lead to very severe public criticism in Parliament and elsewhere.

(iii) Due to the re-allocation of channels in Bands I and III many new receiving aerials would be needed—again an overnight change—and all transmitters and receiving installations in the whole country would have to change over at the same time. Public lethargy would inevitably mean that many installations would not be ready, even if the capacity for modification was available—which is highly doubtful.

(iv) The I.T.V. contractors would be debarred from the transmission of colour TV on the present service for seven years, i.e. till the ‘appointed day’ (see also Q.28). As a result it must be assumed that the I.T.A. plan is a non-starter.

  1. Ⓠ What is the other plan?

The only surviving plan being seriously considered is the duplication of existing programmes (B.B.C.1 and I.T.V.) on 625 lines in UHF. Viewers who have acquired receivers to take B.B.C.2 — which will be UHF — will also be able to receive either B.B.C.1 or I.T.V. 1 in the duplicate transmission in the UHF band. At the end of the formal seven to ten year period the VHF, 405 line version of the present programmes could be closed down because all the old 405 line receivers not capable of receiving 625 lines would have been scrapped.

  1. Ⓠ What about receivers sold during the seven year period? Wouldn’t they have to be rather special? How about the cost?

Yes, in places where the existing 405 line services have not been duplicated on 625 lines in UHF, dual-standard, dual-band receivers would be necessary (that is VHF/UHF, 405/625 line) so that viewers could receive the old VHF, 405 line service until the new UHF 625 line service came along. As regards cost, the public would normally spend some £l,000m. on replacement of obsolete sets over the next ten years anyway, and it is estimated that the additional cost of special receivers might be some £150m. Actually this additional figure might be significantly reduced if duplication of the existing 405 line services on 625 lines on UHF followed closely on the spread of the B.B.C. second TV programme. For example, if the existing services duplicated in UHF became progressively available to say 60% of the viewers, many of that 60% would buy 625 line UHF-only receivers so that the number of ‘dual-standard, dual-band’ sets sold would be materially reduced with a corresponding reduction in the total cost to the public.

  1. Ⓠ What do the radio manufacturers say to this?

Naturally they do not wish to make receivers more complicated than necessary but have accepted that they will have to produce dual-standard receivers during the interim changeover period. Actually, two main types of receiver were marketed in 1962, some being on the ‘insurance’ basis as follows:—

(i) 405/625 line convertible receiver with VHF tuners only, requiring the addition of 625 line components including the UHF tuner,

(ii) Fully dual-standard 405/625 line receiver with or without UHF tuner which can be fitted later.

Receivers of the first type can clearly be modified to meet the needs of the new service, albeit at some cost. New receiving aerials would, of course, be required in any case for UHF costing from £2 upwards (dependent on location) plus erection cost.

  1. Ⓠ It would appear, therefore, that duplication of existing programmes in UHF is almost inevitable. On the face of it, it appears to be a simple solution. Has it any disadvantages?

While being a workable solution, there are certain positive disadvantages. The first is that duplicating the existing services in the UHF band would in the long run lead to the loss of the VHF band for the main TV programmes. Furthermore, some serious doubts exist about the ability of UHF ever to give as good a coverage as VHF as reception problems with UHF are tricky. This could prove to be a very serious matter.

In other connotations, a number of conflicting views are held by various authorities dependent upon their relative involvements. Briefly these views may be summarised as follows:—

(i) The G.P.O. The G.P.O. favours UHF duplication because, taking into account all aspects of the problem, including the international planning difficulties that would be involved in any piece-meal rearrangement of the VHF bands, it appears to be the only viable method.

(ii) The B.B.C. The B.B.C. accept duplication but are opposed to its implementation concurrently with the spread of their second programme because they fear that the extra demand would overload the industry’s capacity for the production of UHF transmitters, new types of transmitting valves and hardwear generally and thus slow up the expansion of their new service. There appears to be a very real foundation for this fear. This would, of course, be doubly true if a second I.T.V. programme on 625 lines UHF were started as well. Also, the B.B.C. are reluctant to commit their own technical effort in carrying out the concurrent duplication.

(iii) The Radio Industry. The receiver manufacturers also accept duplication but would be opposed to its introduction for the existing programmes concurrently with B.B.C.2 as they are convinced that receiver sales would suffer severely. If, therefore, the extension of B.B.C.2 over the country were held up, so also would be the sales of new TV sets. On the other hand they would welcome a fourth programme as soon as possible.

(iv) The I.T.A. The I.T.A. view appears to be that if the changeover of the existing 405 line services must be brought about by the process of UHF duplication, then it should go ahead as soon as possible and be got over with, however much they, in common with others, may dislike giving up the established ‘pattern of coverage’ and obvious advantages of VHF transmission and reception. They also have regard to the long term issue that colour programmes may only be transmitted in a 625 line service and the B.B.C. will be providing colour in B.B.C.2 as soon as the choice of a common colour system for Europe is settled.

If the I.T.A. were called upon to develop a second service in the very near future they might wish to reconsider the position and they might well take the same view as the B.B.C., viz. that a new programme network should take priority over duplication.

(v) The Programme Contractors. The Programme Contractors tend not to favour UHF duplication because it would involve them in an additional payment to the I.T.A.—said to be in the order of £3.5 million per annum over and above the present rentals of £5.5 million for which they would get no extra returns, ever. In addition they would in some cases have to re-equip their studios with 625 line apparatus. Some contractors’ studios are already equipped but others are not, and the latter would be involved in considerable capital outlay.

  1. Ⓠ If there are to be two versions of B.B.C. and I.T.V. one 405 line and one 625 line, as appears to be the case with UHF duplication, how are they to be produced?

It can only be done by ‘standards conversion’. For example, programmes would be generated on 625 lines and standards converted ‘downhill’ to 405 lines to feed the existing 405 line network.

  1. Ⓠ Does the process of standards conversion introduce degradation in the converted picture?

Some degradation is inevitable but is thought that the results would be sufficiently good to be unnoticed by the average viewer, although the I.T.A. believe the degradation of the 405 line version would be ‘significant’ with existing types of converter. The B.B.C. have, however, recently demonstrated a new and enormously improved form of standards converter, which shows great promise.

  1. Ⓠ If conversion from 625 lines to 405 lines degrades the picture appreciably, would it not be better to generate pictures at 405 lines and convert ‘uphill’ to 625? Surely this would ensure that established 405 line viewers (which will be in the majority for many years) would continue to get as good a picture as they do now, thus representing the greatest good for the greatest number?

There is certainly some basis for such an argument, but on balance the ethics of condemning the mass of viewers to the reception of what may be a very lightly degraded picture, pale into insignificance compared with the opposite face of the coin. If ‘uphill’ conversion from 405 to 625 lines were adopted, the 625 line picture would be appreciably worse than the 405 and the whole operation would be brought into disrepute and the main point of altering the standard would be lost. The public would vociferously—and rightly—want to know why they had to spend money to get a worse picture.

Actually in the early days of 625 line working, this sorry expedient may have to be occasionally adopted in cases where programmes come from a regional area studio, from which a 625 line G.P.O. link has not been completed. This situation will no doubt be avoided as much as possible.

The planned BBC-2 UHF network main transmitters

  1. Ⓠ It would appear then that duplication of existing services in the UHF band is the only way in which they can be changed over to 625 lines. Is there any difficulty about this?

Reception would initially be limited to the densely populated areas and there would be many ‘blind spots’ even within these areas, but even so it is estimated that some 70% of the population would be covered. When it comes to achieving national coverage, however, a total of 64 main stations and probably between 500 and 1000 ‘gap-filling’ small stations will be required.

  1. Ⓠ Does this mean 64 main stations and from 500 to 1000 ‘gap-fillers’ per programme and is this not a very costly undertaking?

A Yes, 64 main stations are required for a single programme plus the ‘gap-fillers’ a very expensive proposition costing some £40m. The cost of transmitters for each subsequent programme would, however, be substantially less, perhaps £20m.; four or even more UHF transmitters could be accommodated in one building if proper provision were made in the first place. Similarly all services, e.g. electricity supply, roads, heating, lighting and indeed, staff, would be already available and to a great extent common to all equipment. Finally, only one mast would be required to carry all the aerials and as very high masts (1,250 ft. in some cases) are necessary, this represents a very costly item.

  1. Ⓠ Is the policy of covering the main centres of the country as quickly as possible with the new B.B.C. service in UHF and likewise duplicating the existing services in UHF generally acceptable?

The B.B.C. do not favour the immediate duplication of their existing service because as stated they wish to extend their new second service on a limited coverage basis, to as many highly populated areas as quickly as they can. The I.T.A., on the other hand, advocate the proper engineering of the duplicated services area by area. That is to say they wish to try as far as possible, to ‘reconstruct’ the existing VHF coverage in UHF by erecting the requisite main station and the more important ‘gap-fillers’ for one area before passing on to the next area so that they would not have to return at a much later date to finish the job, and once again disturb established patterns of public viewing.

  1. Ⓠ Why do the I.T.A. advocate this policy?

It is thought to be a tidier and more generally effective method of really ‘duplicating’ as far as possible the existing I.T.A. service areas and one suspects its effect of giving the fullest possible coverage in a progressively increasing number of areas is not without weight in that it would form a sounder basis for starting a second I.T.V. service in the future.

  1. Ⓠ Speaking of I.T.V.2 or, what is perhaps more immediately important, the duplication of I.T.V. 1 in UHF, will it be possible to give as good national coverage as at present given by the VHF service?

There have been grave doubts on this point particularly by the I.T.A. but the tests now progressing in this country, the UHF service in Germany, recent experience in U.S. and other information seems to suggest that results on UHF are better than originally thought might be the case. Consequently there is reason for moderate confidence that equally good coverage will ultimately be obtained, although a very much larger number of transmitting stations will of course be required.

What is rather disquieting, however, is the fact that even if equally good coverage be obtained on UHF its distribution will differ a good deal from VHF so it may not be possible to reproduce exactly or even within fairly wide limits the present VHF pattern of areas. The resultant differences and the increased number of ‘overlaps’ could cause a fair amount of local difficulty, both in terms of contractors’ areas, audience coverage, etc., and also in terms of viewer dissatisfaction. Viewer loyalties tend to grow up and people object to having to look at a different contractor’s programme after they have grown attached to the old one. If most items are networked this is clearly of less importance but experience has shown that it does happen.

Some examples of the difference between VHF and UHF coverage from 1969

  1. Ⓠ What can be done about this variation in coverage between existing VHF areas and the new UHF areas?

Probably nothing, it will just have to be swallowed by the public and contractors alike with the best grace they can command! It is reassuring to recall, however, that there is still bound to be a VHF service as at present for some 7 to 10 years.

  1. Ⓠ What about colour?

It has been laid down by the P.M.G. that colour transmission in this country shall only be carried out on 625 lines. This ipso facto means that colour can only be transmitted on UHF and at present is restricted to the B.B.C.2 service (as the I.T. A. have no such service). There is, of course, no actual bar to transmission of colour in the VHF bands, but as there is no room for a 625 line service in VHF this means that colour is barred until the VHF bands are re-engineered for 625 lines if this ever comes to pass.

  1. Ⓠ It seems, therefore, that the B.B.C. are free to go ahead on colour. Do they intend to do so?

Yes, but there are certain inevitable difficulties and delays. These arise from the fact that there are three systems of colour TV transmission (viz. the American NTSC, the French SECAM, and the German PAL) which have been thought by the European broadcasters to be worthy of consideration for the adoption as a European standard if agreement as to the choice can be reached among the various countries.

In order to try to achieve this result, certain international committees are at present studying the problem, to see if such agreement can in fact be reached.

  1. Ⓠ This seems a rather formidable and lengthy business, why is it necessary?

A By adopting this very formal procedure, complete order is maintained in the broadcasting scene and the resultant standards are accepted throughout Europe including the Soviet Union. This means that interchange of programmes between European countries either direct or on video-tape is enormously simplified and the quality vastly improved. It would, in fact, represent a major step forward in the world television scene.

  1. Ⓠ It certainly seems worth sticking to the proper procedure, but how long will it take?

It is hard to give a specific answer but the earliest date by which agreement could be reached is about mid-1964 and it might be as late as 1966.

As soon as a decision on the system is made, however, receiver manufacturers can finalise set design although it may take something like a year to . complete design and production preparations before sets are commercially available to the public. Thence, although colour transmission by the B.B.C. on the International standard for Western Europe may start at an earlier date, a real service, with sets on sale to the public may not start till late 1966.

The present experimental colour transmissions of the B.B.C. have been proceeding on all three systems but of course they must ultimately be confined to the system decided upon by international agreement, if this can be reached.

  1. Ⓠ May one ask how much a colour receiver will cost?

Well, obviously prices cannot yet be fixed but taking away the number you first thought of, anything from £300 to £400 in the first instance, dependent on the maker’s ideas. When manufacture gets into its mass-production stride, it is anticipated that the retail price may come down to something like three times the cost of a black-and-white only receiver.

The writer desires to acknowledge with thanks the assistance of the Chief Engineer of the Independent Television Authority, the Director of the British Radio Equipment Manufacturers’ Association and many others in the compilation of this paper.


DEFINITION OF VARIOUS TERMS USED:

  1. 405-line TV

The TV picture on a receiver screen is made up of a series of lines traced by a rapidly moving luminous spot from L to R on the screen. A complete screenful of lines is termed a ‘frame’. In U.K. the present picture is made up of 25 complete frames per second, each frame containing 405 lines.

  1. 625-line TV

This is the European standard (except France which uses 819 lines) and the one to which the U.K. TV service is to be changed. In this standard there are still 25 complete frames per second but each frame contains 625 lines. France will also use 625 lines for any new service.

  1. VHF — Very High Frequency

This is part of the radio frequency spectrum allocated to the broadcast transmission of TV and FM sound. It is divided into Band I (41-68 Mc/s), Band II (87.5-100 Mc/s) and Band III (174-216 Mc/s). Bands I and III are allocated for TV transmission, Band II being used for FM sound. (VHF Frequency-modulated Sound service.) These spectrum allocations are international but their limits differ slightly in various countries according to national circumstances. The figures above apply to the U.K.

  1. UHF — Ultra High Frequency

A further part of the radio spectrum of higher frequency allocated for TV broadcasting. It is divided into Bands IV and V (470-582 and 614-854 Mc/s respectively in the U.K.) providing 44 channels for 625 line TV services.

  1. Mc/s — Megacycles per second

A ‘megacycle’ is one million cycles of alternating current.

  1. Channel Width

The number of Mc/s covered by the channel to accommodate vision signals plus accompanying sound signals and spaces or ‘guard-bands’ to prevent mutual interference between the signals in the channel, or between signals in adjacent channels. For 405 line transmission the channel bandwidth is 5 Mc/s for 625 line, 8 Mc/s in the U.K.

  1. Video Bandwidth

The band of frequencies required to transmit the vision signal. For 405 lines it is 3.0 Mc/s, for 625 lines 5.5 Mc/s in the U.K.

A Transdiffusion Presentation

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4 responses to this article

Paul Mason 26 April 2017 at 4:02 am

All history now in the digital age. “ITV 2” was a long time coming, delayed by the economic troubles of the 1970s although our third TV which we got in 1973 had a fourth channel button. ITV2 emerged in November 1982/as Channel 4 or S4C in Wales.

Kif Bowden-Smith 26 April 2017 at 1:31 pm

History indeed as you say but TBS regards itself as a history site! 🙂 So spot on for interest to us media historians!

Paul Mason 30 April 2017 at 10:34 am

Of course Kif, but what I need to emphasise to whippersnappers is the anticipation and frustration involved in arriving at a FOURTH TV channel in a world where we have hundreds of channels round the clock. We went 625 in 1973, and digital in 2003. The Mason household was always the last to catch on!

Alan Keeling 9 May 2017 at 3:44 pm

In the days of 405 lines, as well as receiving ATV (Midlands), we could also receive Granada from Winter Hill, but as soon as we had a colour TV set, out went Granada & in came a rather faint Anglia image.

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