HD is the new SD

Or Why is screen resolution worse than in 1947?

Andrew Rowland | 14 January 2017 | 0 comments

I investigate the resolution of TV images since the onset of broadcasting. Resolution is a combination of the number of lines transmitted and the size of the screen, so that the larger the screen, the coarser the image, if the number of lines stays the same. As screens have grown over the decades, we have progressed from 405 to 625 lines, then to HD and UHD because of this effect.

I find that a modern 42 inch screen and an HD programme has the same resolution as you would have enjoyed if you had had a 12 in. TV in 1947, the year the BBC recommenced broadcasting television after the war. With the majority of new sets sold in the UK now in the 40″–60″ range, HD is already only a necessary substitute for SD, and SD itself is nearly unwatchable on the largest of these screens, with a resolution way below anything we have seen in past. I ask why broadcasters are being so slow to move to HD only (and UHD) transmissions.


During my lifetime, I have lived through successive changes in broadcast TV screen resolution, from the 405 line black-and-white, to 625 lines in the sixties and the advent of HD at 720 and 1080 lines. Each of these changes has been brought about because of increasing screen sizes. The larger the screen, the easier it is to see the line structure, so the more blurry the picture looks. The extra resolution is necessary to make the picture look sharp again.

Reading recently that screen size has doubled over the ten years to 2013 [1], I questioned my assumption that the increases in resolution compensated for the increase in screen size. After all, the much-trumpeted onset of HD was not as revolutionary as the advertisers would have had you believe. When we went from 405 lines to 625 in 1964, that was an increase in vertical resolution of approximately 150%, i.e. half as many lines again. Going from that to ‘HD ready’ 720 lines was only 125%, and from 720 to 1080 is 150%, the same as the 1964 jump. Full HD is just 185% of the resolution that broadcast television started with in the 1930’s. All those claims that HD (high definition) screens have four times as many pixels as standard (SD) ones are of course true, but only because the resolution increases in both the vertical and horizontal directions. The actual resolution in any one direction is much more modest.

And today’s TVs are huge compared with those of the past, with sets between 40″ and 60″ accounting for 65% of all sales [2]. So I decided to look at some facts and figures. I wanted to compare screen resolution – the number of lines making up the image – with typical screen sizes over the years. That raises the question of what is ‘typical’. Does it mean average? Well, in 2013 the average screen size was, according to the BBC report I just mentioned, 33 inches. (In 2000 it had been 21″.) But how do you calculate average? Just on sales? In 2012 the category that sold most in the EU was the 40″–50″ range, with 34% of the market, and 50″–60″ close behind with 31%. But the average screen size was much lower because of the smaller screens also sold [2]. If you compare Germany, Denmark, the UK and Poland, the UK actually has the smallest average screen size in sales, but also has the greatest number of televisions sold. We buy more sets than any other country in Europe, and all those smaller ones intended for bedrooms, kitchens and so on pull the average down. Denmark, on the other hand, has one of the lowest figures for sets sold but also has the largest average screen size because consumers are just buying that big living room TV. And of course, the sets sold in any particular year will not reflect accurately the installed base of TVs, some of which will be old (and presumably smaller).

In the end, I decided to pick a number of screen sizes that were common for living room TVs at a number of points in the past, and see what their resolutions were in lines per inch. I certainly don’t have historical sales figures so my examples are a fairly subjective choice, based partly on cost when I could find some evidence [3]. I also chose a few years using key points in the evolution of television. All this is for illustration only, so take it all with a certain pinch of salt, but it does illustrate the trends clearly enough.

I ignored the horizontal resolution because the width of screens has changed as we went from the old 4:3 aspect ratio to widescreen 16:9. But I couldn’t ignore aspect ratio entirely, because screen sizes are measured across the diagonal. That made sense when screens were round or nearly so, as early ones were, and I suppose the industry hung onto a measurement that flattered the sets. So I calculated the height of the screen from the horizontal and divided by the number of scan lines to get the resolution. But the complexities don’t quite end there. Although a set may be nominally 625 lines, say, that doesn’t mean that the image is 625 lines high. In fact a 405 line image had only 377 visible scan lines, and a 625 line one only 576 lines. The missing lines were simply blanks (at first: later they were used to encode teletext) to allow the scanning dot to return to the top left of the screen once it had reached the bottom – the fly-back interval. So I used the number of visible scan lines in my calculations. Another thing is that the quoted size of CRT tubes was often overstated because the edges of the screen were masked off to keep the edges of the image neat. I have ignored this, however. Neither issue affects modern digital television standards.

Finally, an apology for using inches. I am afraid the English-speaking world still tends to refer to screen sizes in this outdated unit, and I thought showing centimetres as well would clutter the results table too much.

Year Lines Vert. Res. (lines) Typical screen size Ratio Vert height (inch) Resolution (lpi)
1936 405 377 9 4:3 5.4 69.81 World's first regular high-definition television service
1941 525 483 12 4:3 7.2 67.08 in US
1947 405 377 12 4:3 7.2 52.36
1964 405 377 21 4:3 12.6 29.92
1964 625 576 21 4:3 12.6 45.71 BBC2 started
2000 625 576 26 4:3 15.6 36.92
2000 625 576 26 16:9 12.7 34.91 SD widescreen
2006 720 720 28 16:9 13.7 52.55 HD ready
2008 720 720 32 16:9 15.7 45.86
2012 1080 1080 40 16:9 19.6 55.10 Full HD
2012 1080 1080 50 16:9 24.5 44.08
2015 1080 1080 60 16:9 29.4 36.73
2016 2160 2160 60 16:9 29.4 73.47 UHD / 4K
2018 2160 2160 85 16:9 41.7 51.80

So let’s start with the advent of television in 1936, when the BBC started broadcasting from Alexandra Palace, which was heralded as the ‘world's first regular high-definition television service’ [4]. It might not have been high definition by today’s standards, but compared to the 30 lines of John Logie Baird’s first mechanical system, or even the 180 lines of the German broadcasts started a year earlier, it was high indeed.

According to sales catalogues of the time, the rich might expect to own a 9″ or 10″ TV, which were very expensive. Sets of 12″, 14″ and even 15″ were also available, but were exorbitant, costing in the region of a third of an average annual income. A 9″ screen would have been about 5.4 inches high, which, with an image consisting of of 377 scan lines, gives a resolution of just under 70 lines per inch (lpi). That screen would have been very sharp, but also very small. This was not something to watch as a family, but to pull your chair right up to and peer at like you would read a book.

When broadcasting recommenced after the war in 1947, it was with the same 405 line standard. A 12″ screen (my example for 1947) would have had a resolution of over 50 lpi, which was still very good. But by the mid-sixties, the writing was on the wall for 405 lines. A typical 19″ TV would have had a resolution of only 33 lpi, so when BBC2 started in 1964, it used the European 625 line standard¹, which restored the resolution for that 19″ set to around 50 lpi. BBC1 and ITV followed, and by November 1969 all three channels were broadcasting at least some programmes in colour [5], which America had had since 1951! That standard is what we now know as SD (standard definition).

It is interesting to note that the US chose the 525 line standard in 1941, so their pictures were sharper than ours up to 1964, and there wasn’t the same incentive to upgrade it, so stayed with 525 lines right through the introduction of colour until HD appeared, and indeed, still uses it for SD. France, however, adopted 819 lines (737 visible) for its TV in 1948, which gave near HD quality, though only in black and white, of course [6].

The next milestone was the changeover to widescreen, with an aspect ratio of 16:9, which was a better match to the cinema’s wide format. At first, widescreen pictures were carried on the old analogue 625 line system, but with the picture squashed in the horizontal direction so that when the TV set stretched it out again, it looked right. This is called ‘anamorphic’ and is also the way cinema film records wide pictures onto standard negative sizes, using a cylindrical shaped lens. However, as the same number of dots were being spread over a longer distance, the resolution actually decreased in the horizontal direction. Because the wider sets increased the diagonal measurement, a 26″ widescreen was the same height as a 21″ 4:3 screen. The changeover was spread over a number of years, but I have chosen 2000 because it was the year advertisements were no longer accepted by the broadcasters in 4:3 format.

It was only with the advent of digital television that any further increases in resolution were possible. TV (and radio) take up space in the airwaves, known as their bandwidth. In the UK, stations are spaced 8MHz apart, which limits the amount of information that can be sent without adjacent stations overlapping one another and causing interference. Trying to increase the number of lines beyond 652 just wasn’t going to work, but by converting the signals to digital and then using data compression techniques, more than one programme could be sent in the same bandwidth as an old analogue station. Finally multi-channel viewing was upon us. It also made possible the sending of HD broadcasts, and here two standards are used: 720p and 1080i. The ‘p’ stands for progressive, meaning that the 720 lines of each frame are sent one after the other, fifty times a second, giving a flicker-free picture. The ‘i’ means interlaced. Here, only half of each frame is sent: first the odd numbered lines, then for the next frame the even-numbered lines. Thus, although a 1080 line picture has half as many lines again as 720p, it actually only sends 540 for each field, so that a full frame is sent every 25th of a second. That is the same system as the analogue signals used to use. The horizontal resolution is greater, however – 1920 pixels compared to 1280 – so the bandwidth requirements are very similar. In practice, it is pretty hard to tell them apart visually, but the progressive mode is best for motion, such as sports, where the quick refresh rate helps. 1080i can show more detail, and is responsible for those stunning images in Planet Earth 2, for instance, but rapid motion can lead to some smearing of the image – though modern sets can be quite good at minimising that effect. German broadcasters tend to plump for 720p² and the UK’s for 1080i. There are currently no broadcasts of 1080p on either terrestrial or satellite because of the bandwidth that would be required³. For that you must resort to BluRay or an online service like Netflix.

Returning to our investigation, let’s see what HD resolution looks like on today’s screens. With 720p, our 1947 standard of around 50 lpi is achieved with a 28″ screen. Hardly earth shattering, considering that 32″ was far more the norm. What about the larger 40″ and above panels that are sold today? Well, with 1080 lines, a 40″ panel has a very respectable vertical resolution of 55 lpi, and even a 50″ one still scores 46 lpi. But go to 60″ and resolution is heading towards the same territory as we saw in the mid-sixties when we switched to 625 lines. In other words, on larger screens, HD is already beginning to look like SD. And SD channels manage at most 20 lpi on a 60″ panel, worse even than the pre-war German 180 line system on a 12″ screen.

On that reckoning, HD is the new SD, and with screens far in excess of 60″ available now, HD itself is looking due for replacement.

But before we look at the successor to HD, does my claim that modern HD is as blurry as TV in the 1940s actually hold water? What I am saying is that if you look at one inch of a full HD broadcast on a 42″ screen and count the lines, you will see the same number of lines as you would have done on a 405 line 12″ TV in 1947 (or any other year for that matter), i.e. just over 52 lines. You would normally view the picture from far enough away not to see the line structure or pixels – and the apparent sharpness will depend on how far away you are. Modern recommendations state that you should aim for a 30° to 40° viewing angle, so you would sit a great deal nearer to a 12″ screen than a 60″ one, and the large screen would look much sharper because you are not as close. However, I don’t think people did sit correspondingly closer in yesteryear. If you were on your own you could pull the chair nearer, but if not, the rest of the family would also want to see. So the picture was just smaller than we are used to today. That is why old film titles fill the frame – they had to be as large as possible so TV viewers could read them. And people today are surely buying large screens so that they fill more of the visual field and the viewer feels more immersed in the action. After all, living rooms haven’t doubled in size in the last ten years! On the other hand, we have seen lines per inch deteriorate to the mid-30s before anything was done about it in the past, and not everyone is sensitive to screen differences. While I can usually walk into a room when the TV is on and tell whether it is tuned to an HD or SD station, many people have told me that they can’t tell the difference between HD and SD. It makes me grateful for my sharp eyesight!


So do modern pictures really look worse than in 1947? No. Resolution is not only thing that affects how sharp the picture looks. Contrast also plays a rôle; modern sets have a greater difference between the darkest black and brightest white. And they do colour. But above all, the screens are larger. While programme and film makers in the past had to frame faces tightly so we could see the actors’ expressions, now they have the freedom to pull back if they want to, and titling no longer needs to fill the frame.

To be fair, subjectively, HD on a 42″ screen can look stunning. And yet... SD on that same screen looks well below par, and on much larger screens, say 65″, HD does look like SD—and SD is unwatchable. Which brings us to…

We are now seeing the introduction of Ultra-HD, also called UHD or ‘4K’ – the latter because the width is getting on for 4000 pixels: 3840 to be precise. This represents the biggest jump in resolution we have yet seen, a full 200% over HD. The sharpness of this standard is phenomenal, and as the last line of my table shows, you need a screen of at least 85″ to approach that old 50 lpi resolution. With decent sized UHD TVs now below the £600 mark they have become a mainstream purchase.

So why are so many channels still SD and the BBC still unable to even contribute regional programming in HD on BBC1? It is as if they are urging viewers to abandon their services.


Notes

¹ The other motivation for adopting the 625-line standard was to ease sharing of programme material and to enlarge the market for manufacturers of TVs and broadcast equipment (though important differences between the national standards did still exist).

² This applies to the public broadcasters, i.e. ARD and ZDF. The private ones like ProSieben and RTL use 1080i on satellite, but to receive them you have to subscribe to the HD+ service.

References

³ In English. Germany has started 1080p50 broadcasts on terrestrial television, so those full HD channels are better quality than the 1080i50 services available on satellite and cable [7].

References

[1] http://www.bbc.co.uk/news/technology-24171226

[2] European Television Market 2007 – 2012, Topten International Services, Zurich. Numbers based on 24 of the 28 EU states (90% of the market).

[3] http://www.tvhistory.tv/tv-prices.htm

[4] Burns, R. W., Television: An international history of the formative years. (1998). IEE History of Technology Series, 22. London: IEE, p. 576. ISBN 0-85296-914-7. Referenced by https://en.wikipedia.org/wiki/History_of_television#cite_note-Burns576-63

[5] http://downloads.bbc.co.uk/historyofthebbc/1960s.pdf

[6] https://en.wikipedia.org/wiki/Analog_high-definition_television_system#French_819-line_system

[7] HDTV-Perspektiven: Debatte um 1080p/50

© Andrew Rowland 2017

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