HDTV (High-definition television)
by Chris Woodford. Last updated: January 28, 2021.
It's funny to
look back on ancient home appliances and laugh at
how crude and useless they seem today. Televisions from the 1940s and
1950s, with their polished wooden cases and porthole screens, seem
absurd to us now, fit only for museums; in their time, they were
cutting-edge technology—the very finest things money could buy. In
much the same way, the televisions we're all staring at today are
already starting to look a bit old hat, because there's always newer
and better stuff on the horizon. Back in the 1990s, HDTV
(high-definition television) was an
example of this "newer and better stuff"; today, it's quite
commonplace. But what makes it different from the TVs that came
before? And what will come next? Let's take a closer look!
Photo: HD isn't just about TVs. Most decent smartphones now boast high-definition screens, typically with Quad HD resolution (2560 × 1440 pixels) or better.
What is HDTV?
All televisions make their pictures the same way, building up one
large image from many small dots, squares, or rectangles called
pixels. The biggest single difference between HDTV and what
came before it (which is known as standard definition TV or SDTV) is
the sheer number of these pixels.
Having many more pixels in a screen of roughly the same size gives a
much more detailed, higher resolution image—just as drawing a
picture with a fine pencil makes for a more detailed image than if
you use a thick crayon. SDTV pictures are typically made
from 480 rows of pixels stacked on top of one another, with 640
columns in each row. HDTV, by comparison, typically uses either 720 or 1080
rows of pixels, so it's up to twice the resolution of traditional
SDTV. One early HD standard, HD Ready, introduced back in 2005, required a minimum resolution
of 720 rows. Today, most HDTVs are described as Full HD (FHD): they use 1080 rows of pixels and 1920 columns,
making roughly 2 million pixels (2 megapixels) altogether, compared to about 300,000
(0.3 megapixels) in an SDTV screen, or just over six times more. (For the
sake of comparison, our eyes contain 130 million light-detecting cells called rods and
cones so our vision is effectively 130 megapixels. Put that another way and it means the
images created on our retinas are at least 50 times more detailed than the images created by HDTV
and over 400 times more detailed than SDTV.)
Photo: More pixels: HDTV (1) gives about six times more pixels than SDTV (2). This is what six times more pixels looks like.
Another way in which HDTV differs from SDTV lies in the way the
pixels are painted on the screen. In SDTV and in earlier versions of
HDTV, odd-numbered rows were "painted" first and then
even-numbered rows were painted in between them, before the
odd-numbered rows were painted with the next frame (the next moving
picture in the sequence). This is called interlacing, and it
means you can fill the screen more quickly with an image than if you
painted every single row in turn (which is called progressive
scanning). It worked very well on old-style cathode-ray
televisions, and cruder LCD televisions that built pictures
more slowly than they do today, but it's not really necessary anymore
now there are better LCD technologies. For this reason, the best
HDTVs use progressive scanning instead, which means they draw
fast-action pictures (for example baseball games) both in more detail
and more smoothly. So when you see an HDTV described as 1080p, it
means it has 1080 rows of pixels and the picture is made by
progressive scanning; an HDTV labeled 720i has only 720 rows and uses
interlacing; a 720p has 720 rows and uses progressive scanning. (SDTV
would be technically described as 480i using the same jargon.)
Photo: Interlacing and progressive scanning: With old-style interlaced scanning (1), the red lines are scanned one after another from the top down. Then the blue lines are scanned in between the red lines. This helps to stop flicker. With progressive scanning (2), all the lines are scanned in order from the top to the bottom. HDTV generally uses progressive
scanning, though (like SDTV), it can use interlacing at higher frame rates.
It's a digital technology
Where SDTV was an old-style analog technology, HDTV is
fundamentally digital, which means all the advantages of digital broadcasting:
theoretically more reliable signals with less interference,
far more channels, and automatic tuning and retuning.
(If you're not sure about the difference, check out
our introduction to analog and digital.)
It's easy to see how old-style, cathode-ray
tube SDTV evolved from the very earliest TV technology developed by
people like John Logie-Baird, Philo T. Farnsworth, and Vladimir
Zworykin (see our main article on television
for more about that). SDTV involves electron
beams sweeping across a screen controlled by electromagnets, so it's
absolutely an analog technology; HDTV is completely different in that
it receives a digitally transmitted signal and converts that back to
a picture you see on the screen.
How do you pack more pixels in the same space?
HDTV is about doing more with less—putting "more picture" in
roughly the same space, but how do you do that exactly? In a
cathode-ray tube TV, the size of the pixels is ultimately determined
by how precisely we can point and steer an electron beam and whether
we can draw and refresh a picture quickly enough to make it look like
a smoothly moving image. Even if you could double the number of lines
on a TV, if you couldn't draw and refresh all those lines quickly
enough, you'd simply end up with a more detailed but more jerky
The same problem applied when cathode-ray tubes gave way to other
technologies such as LCD and
plasma, but for different reasons. In
these TVs, there's no scanning electron beam. Instead, each pixel is
made by an individual cell on the screen switched on or off by a
transistor a (tiny electronic switch), so the size of a pixel is
essentially determined by how small you can make those cells and how
quickly you can switch them on or off. Again, making the pixels
smaller is no help if you can't switch them fast enough to make a
smoothly moving image.
Advantages and disadvantages of HDTV
Picture quality (or resolution, if you prefer) is obviously the
biggest advantage of HDTVs, but that's not the first thing you
notice. If you compare HDTVs with the old-style TVs that were commonplace about 20 years ago,
you can see straightaway that they're much more rectangular. You can see
that in the numbers as well. An old-style TV with a 704 x 480 picture
has a screen about 1.5 times wider than it is tall (just divide 704
by 480). But for an HDTV with a 1920 x 1080 screen, the ratio
works out at 1.78 (or 16:9), which is much more like a movie screen.
That's no accident: the 16:9 ratio was chosen specifically so people
could watch movies properly on their TVs. (If you try to watch a
widescreen movie on an SDTV screen, you either get part of the
picture sliced off as it's zoomed in to fill your squarer screen or
you have to suffer a smaller picture with black bars at the top and
bottom to preserve the wider picture—like watching a movie through a
letterbox.) The relationship between the width and the depth of a TV
picture is called the aspect ratio; in short, HDTV has a bigger
aspect ratio than SDTV.
Photo: Aspect ratio: HDTV (1) gives a more rectangular picture than SDTV (2).
What about the drawbacks? One is the existence of rival systems
and standards. Typically, HDTV can mean either 720p, 1080p, or 1080i,
and it's not just about the television set (the receiver) itself but
about all the kit that generates the picture at the TV station and
gets it to your home, including the TV camera and the transmitter
equipment and everything else along the way. In other words, you
might have a situation where the signal is 1080i or 1080p but the TV
in your home is 720p, or the signal is 1080i but the set is 1080p, in
which case the set either doesn't accept the signal or has to convert
it appropriately, which might degrade its quality. This problem has
largely disappeared now more people have converged on 1080p as the
standard version of HDTV, which is also known as Full HD (FHD).
However, HDTVs don't just take their signals from incoming cable or
satellite lines; most people also feed in signals from things like
DVD players, Blu-Ray players, games consoles, or laptops. Although
decent HDTVs can easily switch between all these sorts of input, the
quality of the picture you get out is obviously only ever going to be
as good as the quality of the signal you feed in. Moreover, old
programs and movies broadcast on TV may still be in SDTV format so
they'll simply be scaled up to fit an HDTV screen (by processes such
as interpolation), often making the picture look worse than it would
look on an older "tube" TV. It's worth bearing this in mind when
you fork out for a new television: if you're in the habit of watching
a lot of old, duff stuff, don't suddenly expect it to look magically
new and terrific.
Photo: Interpolation: Programs made for SDTV look fine on SDTVs but fuzzy on HDTVs:
if bigger sets use exactly the same picture signal, the bigger you make the screen, the more area each pixel in
the signal has to cover and the fuzzier it looks. In this example, to
make a crude 704 x 480 picture (1) display on a 1920 x 1080 screen (2), we
have to scale it up by interpolation so that each pixel in the signal occupies about four pixels on the screen. Or, to put it another way, your TV is showing only a quarter of the
detail that it can.
Where next? Will televisions keep on improving, giving us ever
more pixels and ever-better pictures? Manufacturers have moved
on from basic 1080p HDTV (Full HD) to what's called ultra-high definition (UHD), currently available in two flavors known as 4K UHD (3840 × 2160) and 8K UHD (7680 × 4320), both using progressive
scanning. (Strictly speaking, 4K and UHD are slightly different things—4K means 4000 horizontal
pixels while UHD means double the pixel dimensions of Full HD—but the terms are often used synonymously.)
Is this any more than a marketing gimmick? Are people actually going to want ever higher screen resolution?
Artwork: How many more pixels do you get for your money? This artwork compares the pixel dimensions of common SDTV and HDTV formats: SDTV (yellow, 640 × 480), Full HD/HDTV 1080p (orange, 1920 × 1080),
4K UHD (blue, 3840 × 2160), and 8K UHD (red, 7680 × 4320).
If all were the same size, you can imagine how much more detail would be packed into the higher-resolution screens.
You can see that there's a big difference between Full HD and 4K.
Lesser versions of HD, such as 720p, come in between the orange and yellow rectangles.
Decent smartphones typically now have Quad HD (2560 × 1440) or Quad HD+ (~3000 × 1440)
The situation is much the same as it is with digital cameras.
There's a limit to the amount of detail our eyes can process and
there are practical limits on how much resolution we really need
introduced by things like the bandwidths of Internet connections. In
the case of digital cameras, manufacturers have long liked to boast
about new models with ever more "megapixels," largely as a
marketing trick. In practice, that doesn't necessarily mean that the
images are better (a camera with more megapixels might still have a
smaller and poorer image sensor), that your eyes can tell the
difference, or that a super-high-resolution photo is always going to
be viewed that way (if you upload a photo to your favorite social
media site, it will end up scaled down to a few tens or hundreds of
thousands of pixels, wasting much of the detail you originally
captured—but you don't care about that if you're viewing the image
on a tiny cellphone.
Photo: An LG Ultra-HD (UHD) TV.
Photo by courtesy of Kārlis Dambrāns
published on Flickr
under a Creative Commons (CC BY 2.0) Licence.
Similar considerations apply to TVs. Just because you have an
HDTV, it doesn't follow that you will always be viewing
high-definition material on it. Maybe you'll be sitting too far away
from it to appreciate the extra level of detail? Or perhaps the
screen itself isn't big enough to let you appreciate the difference
between 4K and 1080p? Or maybe you do quite a lot of your viewing
using IPTV or streaming videos from YouTube, so the quality of your
Internet connection—how much data you can download per second—is
also going to play a factor in the quality of what you see on your
screen. For example, I rent and stream a lot of movies online,
but, although I have an HD screen, I generally opt for the standard
resolution (SD) versions, because I don't really notice the difference and
it's a lot cheaper (I can rent four SD movies for the price of three HD ones).
It's also worth noting that some online streaming services that claim to offer "HDTV" actually
deliver just 720p in practice, even if you're watching on a much higher resolution (4K)
screen. And if you've got a slow Internet connection, you might well be reduced to watching in lower-resolution
SD, automatically, whether you like it or not.
In short, just because bigger, better, faster, and neater is available, it
doesn't follow that people either want, need, or are automatically
going to use it. Having said that,
shipment statistics show a clear peak in sales of Full HD sets in 2013/2014, with a
decline of about 25 percent since then as 4K sets have become increasingly popular;
worldwide 4K sales have increased by over 10 times between 2014 and 2019, to the current figure of around 110 million units a year. The shift to 4K and 8K has begun!