The HD Specification So what is HD? Hi-definition video is more than just a name to indicate an image has higher resolution than previous video forms, rather HD is a specific technical specification that all major hardware manufacturers and software developers have agreed upon for the future of film, TV, video and broadcasting. The HD specification dictates a number of areas of the HD format but most particularly and importantly it specifies the frame size and dimensions of the image. Existing Standard Definition (SD) has fixed dimensions of 720 pixels wide x 576 pixels high for PAL and 720x480 for NTSC. The HD spec allows for two different HD frame sizes; a smaller 1280x720 and a larger 1920x1080. The larger of these being nearly three times the size of SD. SD/HD frames These two size specs are commonly referred to as "720" and "1080" and of the two, the later, larger frame is quickly becoming the dominant standard for both HD TV's and for camera shooting formats. Currently only JVC outputs the smaller 720 size for HD - all other manufacturers, including Sony, Canon, and Panasonic output a 1080 image. There is another factor to the two frame size specifications; "progressive scan" and "interlaced" images. HD 1280x720 is a progressive scan image and this is often indicated by the use of a "p" next to the 720 (720p). A progressive scan image is one where each frame is composed of a single, solid picture. 1920x1080, by contrast, is, most often, an interlaced image (referred to as 1080i) whereby the image is made up of two interlaced fields at half second intervals which together make up the full frame rate. There is much debate over which type of image looks ?better? but the distinction is marginal. The overwhelming majority of HD equipment manufacturers; from cameras, to software, to TV?s, are focused on 1080. Also all TV display devices can scale very effectively to accommodate both so there are few issues regardless of which format you work in. The grail of course is 1080p which has the best of both worlds ? both large frame and progressive. This all sounds fine and dandy; HD is a much bigger picture and a much sharper image. There is of course a catch however and that?s the sheer amount of data a HD video file takes up and how much bandwidth it requires to be viewed without stuttering. Uncompressed HD at full 1920x1080 is just too monstrously huge to be captured, handled or even played back by anything but the most powerful, professional production systems. So, in order to make HD viable and efficient it needs to be compressed. HD Compression There are currently two major compressed HD video formats for consumers and prosumers widely available in a variety of cameras, from the very cheap to the relatively expensive. The first is and the second, and newest, is AVCHD. Short, obviously, for High Definition Video, was introduced in 2004 and offered the first viable HD format available in inexpensive cameras. In order to make the HD format more manageable, applies two very important and clever processes to the signal to reduce its size without overly reducing image quality.
The first of these is a stretching of the image known as anamorphic stretching. Anamorphic images are nothing new and have been common in cinema since the 1950?s, using curved lenses to squeeze a wider image onto a narrow frame of film. The HD spec defines the HD image as 1920 pixels wide. In this case the pixels are square and equally wide as they are high. The HDV format doesn?t record 1920 square pixels but rather records 1440 rectangular pixels with each pixel having a Pixel Aspect Ratio (PAR) of 1.333:1 (where each pixel is 1.333 times wider than it is high). This allows for an image visually the same width as 1920 but recorded with fewer actual pixels and thus reducing the amount of data required.The second process is in the compression of the video data itself. HDV uses the MPEG-2 codec (Compressor/DECompressor) which is the same type of compression used in DVD?s and digital broadcast signals. Video is a frame-based medium where by individual frames are shown so quickly your eye is fooled into seeing a singular moving image. MPEG-2 compresses the visual information by organizing these frames into groups known as a Group Of Pictures (GOP). In this way HDV doesn?t record all the individual frames but instead a complete frame and then a group of partial frames that refer back to that whole frame. By this grouping the amount of data required for the HD signal is again greatly reduced.
One of the downsides of MPEG-2 compression is that when editing the footage the video files have to be unpacked and the frames rebuilt on the fly by the editing software as you work. This makes HDV editing very system intensive and requires relatively powerful computers to work efficiently. That said with dual-core computer CPU?s now the standard any machine bought now or in the past year or two should be more than capable of editing HDV.
Short for Advanced Video Codec High Definition, AVCHD is the newest HD format to be released for inexpensive HD video production. Where tend to more expensive and professional in features, AVCHD is intended to become the new standard for HD for everyday home movie use.
AVCHD employs the same anamorphic stretching of HDV but specifically uses the H.264 codec which is able to greatly compress a video signal whilst still retaining a large degree of quality. Roughly speaking a H.264 is able to compress to half the size of 2 and yet still look visibly as good.As a result of this compression AVCHD files are remarkably small in file size by comparison to even HDV without sacrificing too much in quality. The downside of AVCHD is that the very large amount of compression means the computer has to work very hard to unpack the files to edit them. Moreover, since AVCHD is a brand new format there is as yet no editing software available that can read or work with AVCHD files. That?s said all major developers have announced they will support AVCHD in coming versions of their software and so we will soon see AVCHD in common use.
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