| AC-3:
See Dolby Digital.
ADC
(A-D, A/D, A-to-D): Analog to Digital Conversion. Also
referred to as digitization or quantization. The conversion of an
analog signal into the digital data representation of that
signal--normally for subsequent use in a digital machine. For TV,
samples of audio and video are taken, the accuracy of the process
depending on both the sampling frequency and the resolution of the
analog amplitude information--how many bits are used to describe the
analog levels. For TV pictures eight or 10-bits are normally used; for
sound, 16 or 20-bits are common, and 24-bits are being introduced. The
ITU-R 601 standard defines the sampling of video components based on
13.5 MHz, and AES/EBU defines sampling of 44.1 and 48 kHz for audio.
For pictures, the samples are called pixels, each containing data for
brightness and color. See also: Binary, Bit.
AES:
Audio Engineering Society that promotes standards in the
professional audio industry. International Headquarters--60 East 42nd
Street, Room 2520, New York, New York 10165-2520. Tel: 212-661-8528.
Fax: 212-682-0477. Email: HQ@aes.org. Internet: www.aes.org.
AES/EBU:
Informal name for a digital audio standard established jointly by the
AES (Audio Engineering Society) and EBU (European Broadcasting Union)
organizations. The sampling frequencies for this standard vary
depending on the format being used; the sampling frequency for D1 and D2
audio tracks is 48 kHz.
AIF
(Audio Interchange File): An audio file format developed
by Apple Computer to store high quality sampled sound and musical
instrument information. The AIF files are a popular format for
transferring between the Macintosh
and the PC. See also: AU, WAV
Algorithm:
A formula or set of steps used to simplify, modify, or predict data.
Complex algorithms are used to selectively reduce the high digital
audio and video data rates. These algorithms utilize physiologists'
knowledge of hearing and eyesight. For example, we can resolve fine
detail in a still scene, but our eye cannot resolve the same detail in
a moving scene. Using knowledge of these limitations, algorithms are
formulated to selectively reduce the data rate without affecting the
viewing experience. See also: Compression, MPEG.
Aliasing:
Defects or distortion in a television picture. In analog video,
aliasing is typically caused by interference between two frequencies
such as the luminance and chrominance frequencies or the chrominance
and field scanning frequencies. It appears as moiré or herringbone
patterns, straight lines that become wavy, or rainbow colors. In
digital video, aliasing is caused by insufficient sampling or poor
filtering of the digital video. Defects are typically seen as jagged
edges on diagonal lines and twinkling or
brightening (beating) in picture detail.
Alpha
channel: A relative transparency value. Alpha values facilitate
the layering of media object on top of each other. In a four digit
digital sampling structure (4:2:2:4) the alpha channel is represented
by the last digit.
Analog:
1. An adjective describing any signal that varies continuously as
opposed to a digital signal, which contains discrete levels. 2. A
system or device
that operates primarily on analog signals.
Anti-aliasing:
The smoothing and removing of aliasing effects by filtering and
other techniques. Most, but not all, DVEs and character generators contain
anti-aliasing facilities.
Archive:
Off-line storage of video/audio onto backup tapes, floppy disks, optical
disks, etc.
Artifacts:
Undesirable elements or defects in a video picture. These may
occur naturally in the video process and must be eliminated in order
to achieve a high-quality picture. Most common in analog are cross
color and cross luminance. Most common in digital are macroblocks,
which resemble pixelation of the video image.
ASCII:
American Standard Code for Information Interchange. A standard
code for transmitting data, consisting of 128 letters, numerals,
symbols, and special codes each of which is represented by a unique
binary number.
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ASIC:
Application specific integrated circuit. An integrated circuit
designed for special rather than general applications.
Aspect
ratio: The ratio of television picture width to height. In NTSC
and PAL video, the present standard is 4:3. In widescreen video, it is
typically 16:9, however, 14:9 has
been used as a transition.
Asynchronous:
Lacking synchronization. In video, a signal is asynchronous when its
timing differs from that of the system reference signal. A foreign
video signal is asynchronous before it is treated by a local frame
synchronizer.
ATM:
Asynchronous Transfer Mode. A data transmission scheme using
self-routing packets of 53 bytes, 48 of which are available for user
data. Typically 25, 155, and 622 Mbps--the latter of which could be
used to carry non-compressed ITU-R 601 video as a data file.
ATSC:
Advanced Television Systems Committee. Formed to establish
technical standards for advanced television systems, including digital
high definition television (HDTV). 1750 K Street NW, Suite 800,
Washington, DC 20006. Tel: 202-828-3130. Fax: 202-828-3131. Email: atsc@atsc.org.
Internet: www.atsc.org. ATSC Formats are 18 voluntary video formats,
known as Table 3. The U.S. digital television transmission standard
using MPEG-2 compression and the audio surround-sound compressed with
Dolby Digital (AC-3). So that a wide variety of source material,
including that from computers, can be best accommodated, two line
standards are included--each operating at 24, 30, and 60 Hz. The
Consumer Electronics Manufacturers Association (CEMA) has said that
all receivers will be capable of operating with all of the formats.
All
pixels are square and pixel sampling rates vary, but all are around 75
MHz. There is a Transport Layer that packages video, audio and
auxiliary data and allows their mix to be dynamically varied--opening
the door to new services and forms of programming (e.g., many channels
of stereo audio, distribution of computer software, or very high
resolution images). The data is compressed to 19.39 Mbits per second
and delivered using a 6 MHz bandwidth channel. HD and SD assignments
are per ATSC announcement on February 20, 1998. Note that 1,088 lines
are actually coded in order to satisfy the MPEG-2 requirement that the
coded vertical size be a multiple of 16 (progressive scan)
or 32 (interlaced scan). See also: HD0, HD1, HD2, MPEG-2, HDTV.
Attached:
A physical channel of a digital picture manipulator is attached to
a logical channel of a controller if the physical channel is
successfully acquired by the controller. A physical channel may be
attached to only one logical
channel of one controller at a time.
ATV:
Advanced television. Digital television, including standard, enhanced and
high-definition versions.
AU
(also SND): Interchangeable audio file formats used in
the Sun Sparcstation, Nest and Silicon Graphics (SGI) computers.
Essentially a raw audio data format proceeded by an identifying
header. The .au file is cross-platform compatible.
See also: AIF, WAV.
Autotiming:
Capability of some digital video equipment to automatically adjust
input video timing to match a reference video input. Eliminates the need
for manual timing adjustments.
AVI:
Audio video interleaving. The Microsoft Video for Windows file format
for combining video and audio into a single block in time such as a
1/30th second video frame. In this file format, blocks of audio data
are woven into a
stream of video frames. ASF is intended to supersede AVI.
AVO:
Audiovisual object. In MPEG-4, audiovisual objects (also AV objects)
are the individual media objects of a scene--such as video objects,
images, and 3D objects. AVOs have a time dimension and a local
coordinate system for manipulating the AVO are positioned in a scene
by transforming the object's local coordinate system into a common,
global scene coordinate
system.
Axis:
Relating to digital picture manipulation, the X axis is a horizontal
line across the center of the screen, the Y axis is a vertical line,
and the Z axis is in the third dimension, perpendicular to the X and Y
axes, and indicates depth
and distance.
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