Digital video

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Digital video is a representation of moving visual images in the form of encoded (3)digital data. This is in contrast to analog video, which represents moving visual images with analog signals. (4)Digital video comprises a series digital images displayed in rapid succession. In contrast, one of the key analog video methods, motion picture film, uses a series of photographs which are projected in rapid succession. 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, at about 30 frames per second, and PAL at 25 frames per second.

Digital video was first introduced commercially in 1986 with the Sony D1 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.(5)

Digital video can be copied with no degradation in quality. In contrast, when analog sources are copied, they experience generation loss. (6)Digital video can also be stored on hard disks or streamed over the Internet to end users who watch content on a desktop computer screen or a digital Smart TV. In everyday practice, digital video content such as(7) TV shows and movies also includes a digital audio soundtrack.

History

Starting in the late 1970s to the early 1980s, several types of video production equipment that were digital in their internal workings were introduced, such as time base correctors (TBC)[a] and digital video effects (DVE) units. They operated by taking a standard analog composite 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 information was then converted back to standard analog video for output.(8)

Later on in the 1970s, manufacturers of professional video broadcast equipment, such as Bosch(9) (through their Fernseh division), RCA, and Ampex developed prototype digital videotape recorders (VTR) 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. Ampex’s prototype digital video recorder used a modified 2″ Quadruplex(10) VTR (an Ampex AVR-3), but fitted with custom digital video electronics, and a special “octaplex” 8-head headwheel (regular analog 2″ Quad machines only used 4 heads). The audio on Ampex’s prototype digital machine, nicknamed by its developers as “Annie”, still recorded the audio in analog as linear tracks on the tape, like 2″ Quad. None of these machines from these manufacturers were ever marketed commercially, however.(13)

Digital video was first introduced commercially in 1986 with the Sony D1 format, which recorded an uncompressed (12)standard definition component video(14) signal in digital form instead of the high-band analog forms that had been commonplace until then. Due to its expense, and the requirement of component video connections using 3 cables (such as YPbPr or RGB component video) to and from a D1 VTR that most television (15)facilities were not wired for (composite NTSC or PAL video using one cable was the norm for most of them at that time), D1 was used primarily by large (17)television networks and other component-video capable video studios.(11)(16)

In 1988, Sony and Ampex co-developed and released the D2 digital videocassette format, which recorded video digitally without compression in ITU-601 format, much like D1. But D2 had the major difference of encoding the video in composite form to the NTSC standard, thereby only requiring single-cable composite video connections to and from a D2 VCR, making it a perfect fit for the majority of television facilities at the time. This made D2 quite a successful format in the television broadcast industry throughout the late ’80s and the ’90s. D2 was also widely used in that era as the master tape format for mastering laserdiscs (prior to D2, most laserdiscs were mastered using analog 1″ Type C videotape).(1)

D1 & D2 would eventually be replaced by cheaper systems using video compression, most notably Sony’s Digital Betacam (still heavily used as an electronic field production (EFP) recording format by professional television producers) that were introduced into the network’s television studios. Other examples of digital video formats utilizing compression were Ampex’s DCT (the first to employ such when introduced in 1992), the industry-standard DV and MiniDV (and its professional variations, Sony’s DVCAM and Panasonic’s DVCPRO), and Betacam SX, a lower-cost variant of Digital Betacam using MPEG-2 compression.(2)

One of the first digital video products to run on personal computers was PACo: The PICS Animation Compiler from The Company of Science & Art in Providence, RI, which was developed starting in 1990 and first shipped in May 1991. PACo could stream unlimited-length video with synchronized sound from a single file (with the “.CAV” file extension) on CD-ROM. Creation required a Mac; playback was possible on Macs, PCs, and Sun Sparcstations. In 1992, Bernard Luskin, Philips Interactive Media, and Eric Doctorow, Paramount Worldwide Video, successfully put the first fifty videos in digital MPEG 1 on CD, developed the packaging and launched movies on CD, leading to advancing versions of MPEG, and to DVD.(3)

QuickTime, Apple Computer‘s architecture for time-based and streaming data formats appeared in June, 1991. Initial consumer-level content creation tools were crude, (5)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 recordings in the format to be transferred direct to digital video files (containing the same video data recorded on the transferred DV tape) on an editing computer and simplifying the editing process, allowing non-linear editing systems (NLE) to be deployed cheaply and widely on desktop computers with no external playback/recording equipment needed, save for the computer simply requiring a FireWire port to interface to the DV-format camera or VCR. The widespread adoption of digital video has also drastically reduced the bandwidth needed for a high-definition video signal (with HDV and AVCHD, as well as several commercial variants such as DVCPRO-HD, all using less bandwidth than a standard definition analog signal) and tapeless camcorders based on flash memory and often a variant of MPEG-4.

Overview(6)

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. 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 number of bits. The more bits the more subtle variations of colors can be reproduced. 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 640×480 (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
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  • video size (VS)[3] = 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 bit/s, 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