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  1. 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
  2. 2. www.TELE-satellite.com — 12-01/2008 — TELE-satellite & Broadband 19
  3. 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. 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