Digital video

   

Digital video is a type of video system that works by using a digital representation of the brightness and colour of each pixel of the image. Black and white digital video is also possible.

Introduction

Digital Camcorders come in two different data formats: interlaced and progressive scan. Interlaced cameras scan an image by alternating lines: the odd-numbered lines are scanned, and then the even-numbered, for each frame. For this reason interlaced cameras really only capture half the information in a given scene, but due to persistence of vision, viewers typically do not notice any difference unless the object being filmed is moving quickly, which causes vertical edges to appear jagged. Interlaced still photos have to be processed in a program like Adobe Photoshop to de-interlace them. This process throws away either the odd or even scanlines of the image then interpolates between the remaining lines to fill the gaps left by removing halp the lines.

Progressive scan cameras scan the entire picture for each frame. This produces a finer image than interlaced scan cameras but typically costs a few hundred dollars more. Interlaced and progressive MiniDV cameras can use the same kind of tape. One disadvantage of progressive video is that it has only half of the temporal resolution of interlaced footage. This can make fast moving action seem "steppy".

"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 "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 seperated 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 or Firewire card and a fairly fast processor, as well as abundant disk space) and software (Adobe Premiere, iMovie, MGI Videowave, Final Cut Pro, etc.)

Digital video has significantly lower cost than 35mm 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 35mm 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 35mm, 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 used exclusively 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 2004, the highest resolution for a digital video camera is 8 megapixels (3840 x 2160) at 30 frames per second ("QuadHDTV"). 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).

Comprehensive online resource guide on digital video can be found at http://www.digital-video-recording.info which links to many thousands of digital video related sites.

Storage format specifications

Encoding

Tapes

See also


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