Digital video

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Digital video is a type of video recording system that works by using a digital rather than an analog video signal. The terms camera, video camera, and camcorder are used interchangeably in this article.

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[edit] History

Starting in the late 70s to the early 80s, several types of video production equipment- such as time base correctors (TBC) and digital video effects (DVE) units (two of the latter being the Ampex ADO, and the NEC DVE)- were introduced that operated by taking a standard analog video input and digitizing it internally. This made it easier to either correct or enhance the video signal, as in the case of a TBC, or to manipulate and add effects to the video, in the case of a DVE unit. The digitized and processed video from these units would then be converted back to standard analog video.

Later on in the 1970s, manufacturers of professional video broadcast equipment, such as Bosch (through their Fernseh division), RCA, and Ampex developed prototype digital videotape recorders in their research and development labs. Bosch's machine used a modified 1" Type B transport, and recorded an early form of CCIR 601 digital video. None of these machines from these manufacturers were ever marketed commercially, however.

Digital video was first introduced commercially in 1986 with the Sony D-1 format, which recorded an uncompressed standard definition component video signal in digital form instead of the high-band analog forms that had been commonplace until then. Due to the expense, D-1 was used primarily by large television networks. It would eventually be replaced by cheaper systems using compressed data, most notably Sony's Digital Betacam, still heavily used as a field recording format by professional television producers.

Consumer digital video first appeared in the form of QuickTime, Apple Computer's architecture for time-based and streaming data formats, which appeared in crude form around 1990. Initial consumer-level content creation tools were crude, requiring an analog video source to be digitized to a computer-readable format. While low-quality at first, consumer digital video increased rapidly in quality, first with the introduction of playback standards such as MPEG-1 and MPEG-2 (adopted for use in television transmission and DVD media), and then the introduction of the DV tape format allowing recording direct to digital data and simplifying the editing process, allowing non-linear editing systems to be deployed wholly on desktop computers.

The latest development in the consumer digital video space is an Indian standard called HDVSL which is supported by the Indian Government and is likely to witness large scale commercial rollout in 2008. HDVSL works on a fundamentally different paradigm wherein the video is delivered as a push video digital stream in H.264 format independent of the two way data stream but on the same pipe. The standard in effect creates two data inpaths and one data outpath. The first data inpath and outpath are traditional data streams, typically ethernet or IP. The second data inpath carries a one way digital video stream with H.264 coding and typically originates from an edge server in the network. Since the video is not hauled over the internet but stabilised and often buffered at an edge server - it offers a significantly superior view experience, esp with HD content. Bottom of the pyramid subscribers only require a single SoC solution which decodes the video stream. The return path maybe through a mobile phone. Using HDVSL based systems, villagers in remote area are able to telephone a call centre operator who for example may find a video clip for them on Youtube which is then played on the television for the villager to view. This way there is no need for computer literacy or even basic literacy or for a computer in the home.

[edit] Technical overview

Digital video cameras come in two different image capture formats: interlaced and progressive scan. Interlaced cameras record the image in alternating sets of lines: the odd-numbered lines are scanned, and then the even-numbered lines are scanned, then the odd-numbered lines are scanned again, and so on. One set of odd or even lines is referred to as a "field", and a consecutive pairing of two fields of opposite parity is called a frame.

A progressive scanning digital video camera records each frame as distinct, with both fields being identical. Thus, interlaced video captures twice as many fields per second as progressive video does when both operate at the same number of frames per second. This is one of the reasons video has a “hyper-real” look, because it draws a different image 60 times per second, as opposed to film, which records 24 or 25 progressive frames per second.

Progressive scan camcorders such as the Panasonic DVX100 are generally more desirable because of the similarities they share with film. They both record frames progressively, which results in a crisper image. They can both shoot at 24 frames per second, which results in motion strobing (blurring of the subject when fast movement occurs). Thus, progressive scanning video cameras tend to be more expensive than their interlaced counterparts. (Note that even though the digital video format only allows for 29.97 interlaced frames per second [or 25 for PAL], 24 frames per second progressive video is possible by displaying identical fields for each frame, and displaying 3 fields of an identical image for certain frames. For a more detailed explanation, see the adamwilt.com link.)

Standard film stocks such as 16 mm and 35 mm record at 24 or 25 frames per second. For video, there are two frame rate standards: NTSC, and PAL, which shoot at 30/1.001 (about 29.97) frames per second and 25 frames per second, respectively.

Digital video can be copied with no degradation in quality. No matter how many generations a digital source is copied, it will be as clear as the original first generation of digital footage.

Digital video can be processed and edited 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 and software. Using an NLE station, digital video can be manipulated to follow an order, or sequence, of video clips. Avid's software and hardware is almost synonymous with the professional NLE market, but Apple’s Final Cut Pro, Adobe Premiere, Sony Vegas and similar programs are also popular.

More and more, videos are edited on readily available, increasingly affordable hardware and software. Even large budget films, such as Cold Mountain, have been edited entirely on Final Cut Pro, Apple's non linear editing software.

Regardless of software, digital video is generally edited on a setup with ample disk space. Digital video applied with standard DV/DVCPRO compression takes up about 250 megabytes per minute or 13 gigabytes per hour.

Digital video has a significantly lower cost than 35 mm film, as the digital tapes can be erased and re-recorded multiple times, viewed on location without processing, and the tape stock itself is very inexpensive (about $3 for a 60 minute MiniDV tape, in bulk, as of December, 2005). By comparison, 35 mm film stock costs about $1000 per minute, including processing.

Digital video is used outside of movie making. 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.

Many types of video compression exist for serving digital video over the internet, and onto DVDs. Although digital technique allows for a wide variety of edit effects, most common is the hard cut and an editable video format like DV-video allows repeated cutting without loss of quality, because any compression across frames is lossless. While DV video is not compressed beyond its own codec while editing, the file sizes that result are not practical for delivery onto optical discs or over the internet, with codecs such as the Windows Media format, MPEG2, MPEG4, Real Media, the more recent H.264, and the Sorenson media codec. Probably the most widely used formats for delivering video over the internet are MPEG4 and Windows Media, while MPEG2 is used almost exclusively for DVDs, providing an exceptional image in minimal size but resulting in a high level of CPU consumption to decompress.

While still images can have any number of pixels the video community defines one standard for resolution after the other and notwithstanding the devices use incompatible resolutions and insist on their resolution and rescale a video several times from the sensor to the LCD. Anamorph still images are the result of technical limitations while anamorph videos can be result of standardization aberrations. As of 2007, 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. The highest speed is attained in industrial and scientific high speed cameras that are capable of filming 1024x1024 video at up to 1 million frames per second for brief periods of recording.

[edit] Interfaces and cables

Many interfaces have been designed specifically to handle the requirements of uncompressed digital video (at roughly 400 Mbit/s):

The following interface has been designed for carrying MPEG-Transport compressed video:

Compressed video is also carried using UDP-IP over Ethernet. Two approaches exist for this:

[edit] Storage formats

[edit] Encoding

All current formats, which are listed below, are PCM based.

  • CCIR 601 used for broadcast stations
  • MPEG-4 good for online distribution of large videos and video recorded to flash memory
  • MPEG-2 used for DVDs and Super-VCDs
  • MPEG-1 used for video CDs
  • H.261
  • H.263
  • H.264 also known as MPEG-4 Part 10, or as AVC
  • Theora standardized but still in development. used for video over the internet.

[edit] Tapes

  • Betacam, BetacamSP, Betacam SX, Betacam IMX, Digital Betacam, or DigiBeta — Commercial video systems by Sony, based on original Betamax technology
  • HDCAM was introduced by Sony as a high-definition alternative to DigiBeta.
  • D1, D2, D3, D5, D9 (also known as Digital-S) — various SMPTE commercial digital video standards
  • DV, MiniDV — used in most of today's videotape-based consumer camcorders; designed for high quality and easy editing; can also record high-definition data (HDV) in MPEG-2 format
  • DVCAM, DVCPRO — used in professional broadcast operations; similar to DV but generally considered more robust; though DV-compatible, these formats have better audio handling.
  • DVCPRO50, DVCPROHD support higher bandwidths as compared to Panasonic's DVCPRO.
  • Digital8 — DV-format data recorded on Hi8-compatible cassettes; largely a consumer format
  • MicroMV — MPEG-2-format data recorded on a very small, matchbook-sized cassette; obsolete
  • D-VHS — MPEG-2 format data recorded on a tape similar to S-VHS

[edit] Discs

[edit] See also

[edit] External links

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