Set-Top Boxes

Why does my STB put black bars on my TV?

If you've watched a widescreen-format DVD on your standard format analogue TV you will have noticed the black strips across the top and bottom of the picture. You will get the same thing when using an STB to view widescreen content.

Standard analogue TVs use a 4:3 (four by three) aspect ratio, which means it's a little wider than it is taller. For every four units of width our TV screen has three units of height. This is a hangover from when the film industry standardised on the size of film, and in 1941 the same standards were adopted for television broadcasting. In a bid to attract customers, cinemas later adopted widescreen formats, such as Cinemascope and Vistavision, but until recently TV remained on the old standard.

The most common theatre screen aspect ratio is 16:9 (16 by nine), and digital TV has once again adopted the same format. So, to show the wider picture on a "square" screen, the depth of the picture has to be reduced, leaving black bars top and bottom. This is called letterboxing.

A mixture of programs is being broadcast -- some still in 4:3, others in 16:9 format. Both picture shapes need to coexist for quite a while. Some set-top boxes give you the option of viewing widescreen pictures in this "letterbox" format or "centre cut" full screen format (with the 4:3 section "cut out" of the 16:9 picture).

Some television content is originally produced in 4:3 aspect ratio and will display on a widescreen TV with vertical black bars on each sides of the screen. This is called pillarboxing. Sometimes, on a widescreen, the letterbox and pillarbox effects can even be combined to give you a "postage stamp" effect.

Sometimes, particularly in retailers showcasing many TVs, you will see incorrectly displayed images which have been stretched or squashed horizontally to fit a 16:9 picture on a 4:3 display (giving you extra-tall, skinny people), and 4:3 pictures stretched out to fit widescreen monitors (very short, extremely stocky people). This is, of course, NOT how you are expected to view digital content in your home.

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