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2008.11.04

It has already been more than one two years since the introduction of digital cameras with HDMI output. HDMI output is the digital way to output video compliant with HDTV standards. Previously digital cameras used A/V analog cables to output SD (Standard Definition) video, either using NTSC (America) or Pal (Europe). Actually, many digital cameras still only output SD.

The particular thing about HDTV is that there are a large number of variations which vary in terms of resolution, frame-rate and interlace vs progressive. The highest quality standard is 1080p which represents a resolution of 1920×1080, roughly 2 megapixels. Most modern LCD televisions support this resolution, although this was not true 2 years ago. Instead, they used to support 1080i which is an interlaced version. It turns out that 1080i has the same pixel resolution as 1080p except that it is delivered in two fields, each containing every other scan line.

For still images, both 1080p and 1080i are therefore theoretically equivalent, except for one small historical detail called overscan. Briefly, the effect of overscan is that the entire image received by the television is not entirely displayed. While this worked out for analog television, LCD televisions also implement this for compatibility reasons. The twisted part of this story is that this has a detrimental effect on image quality which is particularly visible when viewing still images.

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The way overscan works on LCD televisions is that the TV takes a 1080p or 1080i signal and scales the central portion of that signal so that it fills the display itself. In other words, the display gets 1920×1080 pixels, drops some of them to get a smaller image and then it scales it back to 1920×1080! This is done by a digital filter which softens details and can introduce artifacts. To compensate for the softness many display then apply a sharpening filter.

To avoid this problem, certain televisions like Sharp Aquos 1080p LCDs support something called a dot-by-dot mode. In dot-by-dot mode, no overscan is performed and therefore all1920x1080 pixels are displayed using the television’s 1920×1080 pixels. This perfect match produces a much crisper image. Unfortunately, there is a catch: Only 1080p and 720p are supported. Now, if you research the specifications of every HDMI outputting digital camera to date, you will find out that none supports 1080p yet. What a shame! So, while a modern LCD television can show us a very high quality image, we won’t be seeing this quality from a digital camera just yet.

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