1. FEATURE Aspect Ratio
Aspect Ratio –
Isn’t This Simple?
Jacek Pawlowski
Aspect ratio is the ratio of image width to image height.
Although over decades, many different values were in use,
ranging from 1.19:1 to as much as 4:1, we practically meet
only two of them in digital TV - 1.33:1 (or 4:3) and 1.56:1 (or
16:9).
Standard definition TV to know that Pan & Scan not
(SDTV) is mostly transmitted necessarily shows the central
in 4:3 format but sometimes part of the original picture (as
you may find the widescreen show in figure 2).
SDTV signal which has 16:9 While coding digital signal, it
aspect ratio. The latter, despite is possible to include the coor-
having the same aspect ratio, dinates of the most important
is not equivalent to HDTV! This part of the screen. So we may
is illustrated in figure 1. As you see either a central part or a
can see, much more pixels part more to the left or more
are transmitted for the same to the right.
screen area in HDTV in com- It works automatically – we
parison with widescreen SDTV. do not feel it.
In other words, the widescreen However, if you do not believe
SDTV has the same aspect me, just put a 4:3 TV set over
ratio but much worse picture the 16:9 set and observe both
resolution than HDTV. Moreo- while tuned to the 16:9 signal.
ver, while for the 4:3 SDTV and Of course it will be much easier
16:9 HDTV a pixel is almost if both sets have exactly the
perfectly squared, the wide- same picture height.
screen SDTV uses horizontally We are waiting for your
stretched pixels. photographs of Pan & Scan in
How signals of different action!
aspect ratios are displayed
on our TV-set?
It depends on
the setting of
the set-top-box
(STB) and the
TV-set itself.
Let’s examine
Figure 1. Aspect ratios of SDTV, widescreen SDTV and HDTV. different cases.
The magnified portion of a screen shows the size and shape of the
pixels.
Figure 2. 16:9 signal seen on 4:3 TV-set.
16:9 signal to aspect ratio. So what we or Letterbox. Letterbox
see depends on the incom- results in black horizontal
on 4:3 TV-set ing signal and the setting of
the receiver.
bars above and below the
picture, while Pan & Scan
Traditional 4:3 TV-sets Figure 2 shows the 2 set- shows only part of the pic-
usually do not have any tings that we can make in ture (but without those
signal processing related our set-top-box: Pan & Scan annoying bars). It is worth
18 TELE-satellite & Broadband — 12-01/2008 — www.TELE-satellite.com
3. such signals. However, it hap-
pens that a widescreen cinema
movie is transmitted as the 4:3
signal. See figure 3. If a 4:3
signal already contains black
bars, we will see them on the
4:3 TV-set no matter what set-
ting we make in the satellite
receiver. You must have seen
this situation on analog TV
before. It is not quite fair of the
providers to serve such content
4:3 signal on 4:3 TV-set 4:3 signal on 16:9 TV-set
Why consider 4:3 on 4:3? Of course if the And now the trickier part. We have the
signal is really 4:3 – there is no problem. We modern flat screen one or not so modern CRT
see it properly on our traditional TV-sets. The type but with 16:9 aspect ratio and signal
majority of the satellite channels still transmit does not match it.
via digital TV but if the same
signal is still used for the ter-
restrial analog TV and retrans-
mitted digitally via satellite, we
may forgive them.
Figure 3. If a 4:3 signal already con-
tains black bars (after conversion
from a cinema movie), we will see
them on the 4:3 TV-set no matter
what setting we make.
20 TELE-satellite & Broadband — 12-01/2008 — www.TELE-satellite.com
4. I tried to show the various + Zoom (not shown on the to time if the incoming
pictures we can produce by figure). But the picture quality signal changes. One gen-
playing with the receiver and will be worse. That is because eral rule that can be said
TV-set settings. we first produce a small picture is: use as little as possi-
to enlarge it later in the TV-
While the name of the func- set. It is always better to pro-
tions may differ from model to duce the bigger picture in the
model, basically every set-top- receiver itself.
box has 2 options: Letterbox
and Pan & Scan.
Some STB’s have the Full
Screen mode in which picture
is transformed in such a way
as to fit the whole screen; it is
squeezed horizontally (stream
aspect 16:9; TV aspect 4:3), or
it is squeezed vertically (stream
aspect 4:3; TV aspect 16:9).
Since this option is still not very
often used, I did not prepare a
drawing showing this.
Most commonly, a TV-set Figure 4. Playing 4:3 signal on
can play the signal without any 16:9 TV-set.
processing (the second column
in figure 4) or stretch or zoom
it (the third column in figure 4).
Most of the users will probably
choose the Letterbox setting
in the receiver and non-linear Conclusion
stretch in the TV-set. The pic- So, how should we configure
ture will be distorted but mainly our receiver and TV-set? There
on the left and right edges not is not one perfect setting suit-
in the center. able for all signals. Moreover it
also depends whether the black
However this will not be opti- (or grey) bars annoy you or not.
mal if the 4:3 signal is a con- For example, you may accept
version of a widescreen movie them if the TV-set is really big
– see the bottom part of the and your preference is to have
drawing. In such situation, the the picture as close to the origi- ble the processing of a TV- Fortunately, the HDTV sig-
most optimal setting is Pan & nal as possible. After all, who set. Let the satellite receiver nals and HD TV-sets have the
Scan in the receiver and normal likes to see a bloated and fat produce the picture as close matching 16:9 aspect ratio.
mode in the TV-set. Miss World? Well, some women, to what you want to see as Once all the channels switch
maybe... possible. In this way, you will to HDTV, this problem will
Apparently the similar effect We have to change the STB achieve better quality of the not exist any more. Let’s be
can we get by using Letterbox and TV-set settings from time picture. patient.
www.TELE-satellite.com — 12-01/2008 — TELE-satellite & Broadband 21
