The case for 405 line VHF colour

As things now stand at the time of going to press, Government policy is to broadcast colour transmissions using a 625-line standard, to be radiated by the BBC on an Ultra High Frequency in Bands IV and V.

This decision will effectively preclude the 47,000,000 viewers who normally watch the Independent Television channels from receiving colour transmissions on their normal channels and means that the programmes will reach the minimum audience at the maximum, indeed astronomical, cost.

These questions and answers by general manager, john mcmillan attempt to discuss a possible alternative in terms comprehensible to the non-technical reader.

The technology of the subject is such that any discussion of colour must additionally cover the frequency bands to be used for transmission and the line standard. These things are closely connected.

1 There is much talk regarding the relative quality of 625 and 405-line television pictures. What determines the quality of a picture?

The quality of a television picture is fundamentally determined by the amount of ‘information’ transmitted, and received. More ‘information’ – better pictures.

The ‘information’ in its turn is limited by the frequency bandwidth of the video signal. More bandwidth – better pictures.

The line standard adopted affects the bandwidth. More lines – more bandwidth.

The more bandwidth used, the fewer transmitters can be accommodated in a particular slice of the available frequency spectrum allocated to television broadcasting.

2 What line standards are in actual use today?

In order of theoretical ‘goodness’ and somewhat simplified, they are as follows:

3 We seem to be right at the bottom of the league table. Surely a change to a ‘better’ standard is most desirable?

Not so. It so happens that the British Standard as specified by Messrs. Schoenberg and Blumlein of EMI some 36 years ago was a singularly good choice in all respects. It is a fact that it is possible to transmit enough information to give a first class picture on a 3 Mc/s video bandwidth.

4 What about the actual line structure? Does not 405 lines give a coarse-grained picture?

Viewed at the proper viewing distance, the line structure of any standard is virtually invisible.

5 So you think 405 lines is an adequate standard?

Yes. If a 405-line receiver has a good aerial and is properly tuned it gives a very good picture. Above all things, a 405 lines system is already in being and gives excellent coverage with the minimum number of transmitters.

6 Why did the Government of the time elect to move to a 625-line standard?

Largely due to a desire to operate on a common European Standard and so facilitate interchange of programmes.

This was a nice idea rather than a useful practical facility. It is no longer a nice idea because Europe has now decided on two different methods for colour.

7 BBC 2 transmission is often heavily criticised. What are the facts?

Of the frequency bands available under international agreement for TV broadcasting two Bands. I and III are VHF and two Bands, IV and V are UHF.

The transmission and reception characteristics are quite different.

On VHF the BBC attains 99.5 per cent coverage of the country with 30 main stations and 62 major fill-in stations.

Similar figures for 97 per cent coverage by the ITA are 32 main stations and 30 fill-in stations. For UHF and an estimated coverage of 95 per cent some 64 main stations and 250 major fill-in stations are required.

To extend this to 97 per cent a further 1,000 minor fill-in stations are estimated to be necessary. Further extension to a higher coverage figure is considered to be economically impractical.

At 97 per cent coverage over 1,500,000 people will be without television on UHF due to local screening difficulties.

Present official policy is to abandon the present economic Bands I and III and move all television broadcasting to Bands IV and V. Surely a most unsound scheme requiring a huge increase in cost for an inferior result.

8 If the VHF bands are so effective and economical, why does the Government not convert them to 625 lines?

Because the additional Channel bandwidth required for 625 lines (8 Mc/s) restricts the number of transmitters which can be accommodated in the space available to a level which would not give the high percentage national coverage deemed to be necessary for mass viewing.

9 So if we are to adopt 625 lines for colour we will positively have to go to UHF and accept the costs and consequences?

Yes. However, it must be re-emphasised that on all counts reception in lay hands is much worse on these bands. They have never been a real success in any country (USA, UK. Germany).

10 Is there any way out of this?

There is indeed. We could transmit colour on 405 lines on the existing VHF channels and forget about Bands IV and V except for additional programme services if, indeed, this country feels it can afford them.

11 Are there any technical difficulties?

None at all. On the contrary, certain features of the 405 lines transmission characteristic (positive modulation, amplitude modulated sound) are peculiarly suited to the transmission of colour. This is by far the cheapest, quickest, and most efficient way of getting colour TV off to a flying start with an immense potential audience on both BBC and ITV channels.

12 It is being said that the compatible picture in monochrome is inferior on 405 lines. Is this true?

Practically speaking, no. All things are. however, relative. It is true that compared with 625 lines the black-and-white compatible picture is, technically, slightly inferior as indeed is the case in normal black-and-white transmissions.

This, however, is a third order effect and would quite certainly not be noticed by the normal viewer.

All compatible black-and-white pictures from a colour channel are slightly inferior to those from a monochrome channel.

This is the inevitable price of introducing colour. It is of no practical importance.

13 Is it true that this country will not be able to sell colour receivers overseas unless we adopt 625 lines UHF standards for Great Britain?

That allegation is false. A receiver designed and originally manufactured for VHF 405-line colour and UHF 625-line black-and-white can be produced at minor additional expense in the factory for any other system. Our export prices would still be competitive. Incidentally, one of the main arguments some years ago for the adoption of a UHF 625-line black-and-white standard in this country was the same export argument. It was adopted for BBC 2 but there is no evidence of large export results.

14 If the existing BBC 1 and ITV VHF networks were converted to colour as you suggest what would happen to BBC 2?

It would stay in black-and-white … at least for the time being. Thus viewers would have two reliable colour programmes and one black-and-white instead of the present plan which provides for one unreliable colour service and two black-and-white.