Digital video: Wikis

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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.

Contents

History

Digital camera technology is directly related to and evolved from the same technology that recorded television images. In 1951, the first video tape recorder (VTR) captured live images from television cameras by converting the information into electrical impulses (digital) and saving the information onto magnetic tape. Bing Crosby laboratories (the research team funded by Crosby and headed by engineer John Mullin) created the first early VTR and by 1956, VTR technology was perfected (the VR1000 invented by Charles P. Ginsburg and the Ampex Corporation) and in common use by the television industry. Both television/video cameras and digital cameras use a CCD (Charged Coupled Device) to sense light color and intensity.

During the 1960s, NASA converted from using analog to digital signals with their space probes to map the surface of the moon (sending digital images back to earth). Computer technology was also advancing at this time and NASA used computers to enhance the images that the space probes were sending.

Digital imaging also had another government use at the time that being spy satellites. Government use of digital technology helped advance the science of digital imaging, however, the private sector also made significant contributions. Texas Instruments patented a film-less electronic camera in 1972, the first to do so. In August, 1981, Sony released the Sony Mavica electronic still camera, the camera which was the first commercial electronic camera. Images were recorded onto a mini disc and then put into a video reader that was connected to a television monitor or color printer. However, the early Mavica cannot be considered a true digital camera even though it started the digital camera revolution. It was a video camera that took video freeze-frames.

Since the mid-1970s, Kodak has invented several solid-state image sensors that "converted light to digital pictures" for professional and home consumer use. In 1986, Kodak scientists invented the world's first megapixel sensor, capable of recording 1.4 million pixels that could produce a 5x7-inch digital photo-quality print. In 1987, Kodak released seven products for recording, storing, manipulating, transmitting and printing electronic still video images. In 1990, Kodak developed the Photo CD system and proposed "the first worldwide standard for defining color in the digital environment of computers and computer peripherals." In 1991, Kodak released the first professional digital camera system (DCS), aimed at photojournalists. It was a Nikon F-3 camera equipped by Kodak with a 1.3 megapixel sensor.

The first digital cameras for the consumer-level market that worked with a home computer via a serial cable were the Apple QuickTake 100 camera (February 17 , 1994), the Kodak DC40 camera (March 28, 1995), the Casio QV-11 (with LCD monitor, late 1995), and Sony's Cyber-Shot Digital Still Camera (1996).

However, Kodak entered into an aggressive co-marketing campaign to promote the DC40 and to help introduce the idea of digital photography to the public. Kinko's and Microsoft both collaborated with Kodak to create digital image-making software workstations and kiosks which allowed customers to produce Photo CD Discs and photographs, and add digital images to documents. IBM collaborated with Kodak in making an internet-based network image exchange. Hewlett-Packard was the first company to make color inkjet printers that complemented the new digital camera images.

The marketing worked and today digital cameras are everywhere.

Overview of basic properties

Digital video comprises a series of orthogonal bitmap digital images displayed in rapid succession at a constant rate. In the context of video these images are called frames[1]. We measure the rate at which frames are displayed in frames per second (FPS).

Since every frame is an orthogonal bitmap digital image it comprises a raster of pixels. If it has a width of W pixels and a height of H pixels we say that the frame size is WxH.

Pixels have only one property, their color. The color of a pixel is represented by a fixed amount of bits. The more bits the more subtle variations of colors we can reproduce. This is called the color depth (CD) of the video.

An example video can have a duration (T) of 1 hour (3600sec), a frame size of 640x480 (WxH) at a color depth of 24bits and a frame rate of 25fps. This example video has the following properties:

  • pixels per frame = 640 * 480 = 307,200
  • bits per frame = 307,200 * 24 = 7,372,800 = 7.37Mbits
  • bit rate (BR) = 7.37 * 25 = 184.25Mbits/sec
  • video size (VS)[2] = 184Mbits/sec * 3600sec = 662,400Mbits = 82,800Mbytes = 82.8Gbytes

The most important properties are bit rate and video size. The formulas relating those two with all other properties are:

BR = W * H * CD * FPS
VS = BR * T = W * H * CD * FPS * T
  (units are:  BR in bits/sec,  W and H in pixels,  CD in bits,  VS in bits,  T in seconds)

while some secondary formulas are:

pixels_per_frame  = W * H
pixels_per_second = W * H * FPS
bits_per_frame    = W * H * CD
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Regarding Interlacing

In interlaced video each frame is composed of two halves of an image. The first half contains only the odd-numbered lines of a full frame. The second half contains only the even-numbered lines. Those halves are referred to individually as fields. Two consecutive fields compose a full frame. If an interlaced video has a frame rate of 15 frames per second the field rate is 30 fields per second. All the properties and formulas discussed here apply equally to interlaced video but one should be careful not to confuse the fields per second with the frames per second.

Properties of compressed video

The above are accurate for uncompressed video. Because of the relatively high bit rate of uncompressed video, video compression is extensively used. In the case of compressed video each frame requires a small percentage of the original bits. Assuming a compression algorithm that shrinks the input data by a factor of CF, the bit rate and video size would equal to:

BR = W * H * CD * FPS / CF
VS = BR * T / CF

Please note that it is not necessary that all frames are equally compressed by a factor of CF. In practice they are not so CF is the average factor of compression for all the frames taken together.

The above equation for the bit rate can be rewritten by combining the compression factor and the color depth like this:

BR = W * H * ( CD / CF ) * FPS 

The value (CD / CF) represents the average bits per pixel (BPP). As an example, if we have a color depth of 12bits/pixel and an algorithm that compresses at 40x, then BPP equals 0.3 (12/40). So in the case of compressed video the formula for bit rate is:

BR = W * H * BPP * FPS 

In fact the same formula is valid for uncompressed video because in that case one can assume that the "compression" factor is 1 and that the average bits per pixel equal the color depth.

More on bit rate and BPP

As is obvious by its definition bit rate is a measure of the rate of information content of the digital video stream. In the case of uncompressed video, bit rate corresponds directly to the quality of the video (remember that bit rate is proportional to every property that affects the video quality). Bit rate is an important property when transmitting video because the transmission link must be capable of supporting that bit rate. Bit rate is also important when dealing with the storage of video because, as shown above, the video size is proportional to the bit rate and the duration. Bit rate of uncompressed video is too high for most practical applications. Video compression is used to greatly reduce the bit rate.

BPP is a measure of the efficiency of compression. A true-color video with no compression at all may have a BPP of 24 bits/pixel. Chroma subsampling can reduce the BPP to 16 or 12 bits/pixel. Applying jpeg compression on every frame can reduce the BPP to 8 or even 1 bits/pixel. Applying video compression algorithms like MPEG1, MPEG2 or MPEG4 allows for fractional BPP values.

Constant bit rate versus variable bit rate

As noted above BPP represents the average bits per pixel. There are compression algorithms that keep the BPP almost constant throughout the entire duration of the video. In this case we also get video output with a constant bit rate (CBR). This CBR video is suitable for real-time, non-buffered, fixed bandwidth video streaming (e.g. in videoconferencing).

Noting that not all frames can be compressed at the same level because quality is more severely impacted for scenes of high complexity some algorithms try to constantly adjust the BPP. They keep it high while compressing complex scenes and low for less demanding scenes. This way one gets the best quality at the smallest average bit rate (and the smallest file size accordingly). Of course when using this method the bit rate is variable because it tracks the variations of the BPP.

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.

Progressive scan camcorders 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 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.

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 Apple's Final Cut Pro.

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.[citation needed]

Digital video has a significantly lower cost than 35 mm film, as the digital tapes can be erased and re-recorded multiple times. Although the quality of images can degrade minimally each time a section of digital video tape is viewed or re-recorded, as is the case with MiniDv tapes, the tape stock itself is very inexpensive — about $3 for a 60 minute MiniDV tape, in bulk, as of December, 2005. Digital video also allows footage to be viewed on location without the expensive chemical processing required by film. By comparison, 35 mm film stock costs about $1000 per minute, including processing.[citation needed]

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.

Poster frame

A poster frame or preview frame is a selected frame of the video used as a thumbnail[3] .

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:

Storage formats

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.

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

Discs

See also

References

  1. ^ In fact the still images correspond to frames only in the case of progressive scan video. In interlaced video they correspond to fields. See section about interlacing for clarification
  2. ^ we use the term video size instead of just size in order to avoid confusion with the frame size
  3. ^ Delivering a reliable Flash video experience, http://www.adobe.com/devnet/flash/articles/flash_cs3_video_techniques_ch12.pdf, retrieved 14/1/2010

External links


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