High-Definition Picture Formats

HD content comes in two main formats with a rarer but special third one, what are they and what are the differences?

The Formats

The two main high-definition formats that you will commonly encounter and that will be used for HDTV for the foreseeable future are 720p and 1080i. There is also a third format 1080p that will be available from next generation games consoles and DVD systems. The formats are specified as follows:

The differences between the three formats above are their resolutions, measured in pixels and their scanning mode; progressive or interlaced. Interlaced and progressive scan are two different ways capturing, transporting and displaying video content.

Common Picture ResolutionsCommon Picture Resolutions

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Interlaced Scanning

This system was developed in the early days of television to provide a good picture quality (many picture lines) without using too much bandwidth (broadcasting capacity).

Interlaced ScanningInterlaced Scan Pattern

An interlaced video frame is made built up from two scans, the first displaying every other picture line and the second filling the gaps left by the first. When displayed on cathode ray tube (CRT) the system relies on the afterglow of the phosphor inside the tube and human's persistence of vision to hide the fact that only half the image is being updated at a time. By using this 'trick' a sufficiently high scan rate can be used to avoid the image flickering while using half the bandwidth a sequentially or (progressively) scanned image at the same scan rate would have needed.

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Progressive Scan

This system doesn't split the video frame in to two separate fields as interlacing does, the image in drawn sequentially from top to bottom, in order, in a single scan.

Progressive ScanningProgressive Scan Pattern

This system is ideal for use with modern display technologies like LCDs and plasmas which are inherently progressive devices, drawing their pictures by updating sequential lines from top to bottom of the screen.

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HDTV Formats

The two main HDTV broadcast formats; 720p and 1080i exist to cater for needs of different programme material.

Interlacing can cause visible flickering or jagged images when displaying motion, resulting in a reduction in perceived vertical resolution as well as image quality. For fast moving content such as sport, a progressively scanned image is better, so it is likely most sports content will be broadcast in 720p, which provides 50/60 frames a second (Hz) of 1280x720 pixel resolution.

Less fast-moving content can be safely interlaced, and the bandwidth saving this produces can be used to deliver a higher resolution 1080 line image in the case of 1080i, making it suitable for other types of programming. The 1080i format delivers 50/60 higher resolution (1920x1080px) fields which combine to produce half the frame rate at 25 or 30 frames per second.

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Progressive Displays

Since display devices like LCDs and plasmas are inherently progressive devices, this makes them natively unsuitable for displaying 1080i or any other interlaced content.

In order to view interlaced content on a progressive display it must be de-interlaced first. De-interlacing is a complex process that be done several different ways, but when done properly results very good progressive image converstion. Done too simply, quickly or cheaply can produce poor results, so when purchasing a progressive display it can be well worth viewing interlaced as well as progressive content to discover the quality you can expect from a display's de-interlacing hardware.

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How is a 720p picture displayed on a television with a native resolution of 1366 x 768 pixels?

1366 x 768 pixels is a commonly found non-standard display resolution derived from the computing world where XGA resolution is 1024 x 768 pixels. Increasing the width to a widescreen aspect ratio gives a corresponding horizontal resolution of 1366 pixels.

Internally most televisions won't simply display the input data pixel for pixel. In the case of 1366 pixel displays showing a 720p, the signal processor inside the television re-samples (or overscans) the input to a higher internal resolution which is then mapped to the available display pixels.

How noticeable the re-sizing will be will depend on the quality of the internal electronics and how critical you are as a viewer. Although you may be tempted to search for a native 720p display, this re-sampling is quite common where non-native resolution signals need to be displayed, for example down-sampling 1080i/p content or up-sampling standard-definition signals.

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Hitherto largely unmentioned is the third of the mainstream high-definition formats, 1080p also unofficially known and/or marketed as 'full-HD'. This format, as its name suggests, boast the same 1080 line resolution (1920x1080px) of the 1080i format but delivered progressively, hence 1080p.

In light of the choice that must be made between the increased frames per second 720p delivers and improved resolution of 1080i respectively, 1080p is unlikely to be used to broadcast HDTV in the near future due to its increased bandwidth requirement. Additionally it is also not part of the European HD Ready requirements.

1080p content will be available from other sources however, including the next-generation DVD formats; HD-DVD and Blu-ray, and also from next-generation games consoles such as the Xbox and Playstation 3. The quality improvement 1080p will provide over the other high-definition formats will be much smaller than the upgrade from standard to high-definition, but it may be marketed as much more and is likely to be demanded by consumers wanting the very best high-definition content.

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