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Digital video

From Academic Kids

Digital video is a type of video recording system that works by using a digital, rather than analog, representation of the video signal. Digital video is most often recorded on tape, then distributed on optical discs, usually DVDs. There are exceptions, such as camcorders that record directly to DVDs, and Digital8 camcorders which encode digital video on conventional analog tapes.

Contents

Introduction

Video cameras come in two different image capture formats: interlaced and progressive scan. Interlaced cameras scan an image in alternating sets of lines: the odd-numbered lines are scanned at one point in time, and then the even-numbered lines are scanned one clock tick later, then the odd-numbered lines are scanned again one clock tick later, etc. The set of odd-numbered lines or the set of even-numbered lines is referred to as a field, and a consecutive pairing of two fields of opposite parity is called a frame. Similarly, the set of all lines of a progressive scan image is also called a frame. Interlaced video captures twice as many fields per second as progressive video does when both operate at the same number of frames per second. Stills from interlaced video can show some artifacts, so deinterlacing is required in those cases. Progressive scan camcorders tend to be significantly more expensive than interlaced ones with the same frame rate and number of samples per frame.

"Standard" film stocks such as 16 mm and 35 mm record at 24 frames per second. In the U.S.A. digital video films at 29.97 (actually, more precisely, 30/1.001) "frames" per second (on the NTSC system); in Europe, on the PAL system, cameras film at 25 frames per second. In these cases, the term "frames per second" is not technically correct although it is commonly used. Digital video does not have frames on a length of film; instead it scans the fields of an image, and a full scan of each of those fields is considered a "frame." For instance, the NTSC Canon XL-1 has 480 lines; a scan of first the odd, then the even lines, provides a complete frame. The odd lines or the even lines taken alone, are called a field. The camera completes this process of scanning each field 29.97 times each second. The result of this is a temporal resolution of 60 (59.94) images per second. Each "frame" actually contains two images separated in time by approx 1/60 s.

Provided that the video is retained in the same format (not "recompressed", as often occurs when video is edited for distribution, or compressed with special "lossless" codecs), digital video is a "lossless" format. That is, unlike analog sources, copies can themselves be copied without degradation in quality; a 256th generation copy will be as clear as the original 1st generation footage provided that no frames have been dropped. On some capture cards or on some slower computers, the information being streamed in as the tape is rolling is coming in too fast for the computer to process, and the computer may drop a few frames. In this case the viewer typically will not notice anything visually, but the audio may "click" or "pop" briefly (for 1/30th of a second) which, oddly enough, typically will be noticed, especially in music. For this reason, it is important to process the video on equipment which can handle it.

Digital video can be processed on an NLE, or non-linear editing station, a device built exclusively to edit video and audio. These frequently can import from analog as well as digital sources, but are not intended to do anything other than edit videos. Digital video can also be edited on a personal computer which has the proper hardware (an IEEE 1394 (Firewire) card and a fairly fast processor, as well as abundant disk space) and software (Avid, Adobe Premiere, Final Cut Pro, iMovie, MGI Videowave, Video Toaster, etc.) Using a NLE station, digital video can be manipulated to follow an order, or sequence, of video clips. 98% of feature films, television shows, and commercials are edited digitally with Avid's hardware and software.

Digital video has significantly lower cost than 35 mm film, as the tapes can be viewed on location without processing, and can be reused on the spot. For instance, a take of a scene in 35 mm would require the full attention of at least the cinematographer and director, and if both of them were happy with the take it would be sent to print. But if there is a problem that they did not notice, the print of that take is useless, as the film stock cannot be reused. Digital video is a favorite of Independent film, as the cost is much lower. For instance, the cost of the total film stock for a feature film may easily be in the tens of thousands of dollars when using 35 mm, but could be as low as a few hundred dollars for digital video, even if the crew does not reuse any tapes. Digital video is also faster to work with in filming, as the results of a take can be viewed instantaneously. For this reason, George Lucas has been using digital film in filming the Star Wars sequels, with digital video assist.

Digital video is used outside movie making. In particular, digital television (including higher quality HDTV) started to spread in most developed countries in early 2000s. Digital video is also used in modern mobile phones and video conferencing systems. Digital video is also used for Internet distribution of media, including streaming video and peer-to-peer movie distribution.

There are many formats for digital video encoding and file containters supporting different levels of quality, resolution color depth and feature sets.

As of 2005, the highest resolution demonstrated for digital video generation is 33 megapixels (7680 x 4320) at 60 frames per second ("UHDV"), though this has only been demonstrated in special laboratory settings [1] (http://archive.avsforum.com/avs-vb/history/topic/409580-1.html). The highest speed is attained in industrial and scientific high-speed cameras that are capable of filming 1024x1024 video at up to 1 mln frames per second (for very short time, obviously).

Storage formats

Encoding

Tapes

See also

External links

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