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RAID:
Redundant array of independent disks. A grouping of standard disk
drives together with a RAID controller to create storage that acts as
one disk to provide performance beyond that available from individual
drives. Primarily designed for operation with computers RAIDs can offer
very high capacities, fast data transfer rates and much-increased
security of data. The latter is achieved through disk redundancy so that
disk errors or failures can be detected and corrected. A series of RAID
configurations is defined by levels and, being designed by computer
people, they start counting from zero. Different levels are suited to
different applications.
Level
0: No redundancy--benefits only of speed and capacity--generated by
combining a number of disks. Also known as "striping."
Level
1: Complete mirror system--two sets of disks both reading and
writing the same data. This has the benefits of level 0 plus the
security of full redundancy--but at twice the cost. Some performance
advantage can be gained in read because only one copy need be read, so
two reads can occur simultaneously.
Level
2: An array of nine disks. Each byte is recorded with one bit on
each of eight disks and a parity bit recorded to the ninth. This level
is rarely, if ever, used.
Level
3: An array of n+1 disks recording 512 byte sectors on each of the n
disks to create n x 512 "super sectors" + 1 x 512 parity
sector on the additional disk which is used to check the data. The
minimum unit of transfer is a whole superblock. This is most suitable
for systems in which large amounts of sequential data are
transferred--such as for audio and video. For these it is the most
efficient RAID level since it is never necessary to read/modify/write
the parity block. It is less suitable for database types of access in
which small amounts of data need to be transferred at random.
Level
4: The same as Level 3 but individual blocks can be transferred.
When data is written it is necessary to read the old data and parity
blocks before writing the new data as well as the updated parity block,
which reduces performance.
Level
5: The same as Level 4, but the role of parity the disk is rotated
for each block. In level 4 the parity disk receives excessive load for
writes and no load for reads. In Level 5 the load is balanced across the
disks. Soft RAID: A RAID system implemented by low level software in the
host system instead of a dedicated RAID controller. While saving on
hardware, operation consumes some of the host's power.
RAM:
Random access memory. A temporary, volatile memory into which data
can be written or from which data can be read by specifying an address.
Rate
conversion: 1. The process of converting from one digital sample
rate to another. The digital sample rate for the component digital video
format is 13.5 MHz. For the composite digital video format, it is either
14.3 MHz for NTSC or 17.7 MHz for PAL. 2. Often used incorrectly to
indicate both resampling of digital rates and encoding/decoding.
Read
before write: A feature of some videotape recorders that plays back
the video or audio signal off of tape before it reaches the record
heads, sends the signal to an external device for modification, and then
applies the modified signal to the record heads so that it can be
re-recorded onto the tape in its original position.
RealAudio:
Popular software for streaming audio and video over the Internet. made
by RealNetworks of Seattle, Washington.
Realtime:
Computation or processing done in the present to control physical
events occurring in the present. For example, when a digital effects
system operator moves a joystick and the video images on the monitor
appear to move simultaneously, the computations required to make the
images move are said to have occurred in realtime. See also: Rendering.
RealVideo:
Popular software for streaming audio and video over the Internet.
made by RealNetworks of Seattle, Washington. Rec. 601: See: ITU-R
BT.601-2.
Reclocking:
The process of clocking digital data with a regenerated clock.
Rendering:
The process of non-realtime drawing of a picture relying on computer
processing speed for graphics and compositing.
Resolution:
1. Detail. In digital video and audio, the number of bits (four, eight,
10, 12, etc.) determines the resolution of the digital signal. Four bits
yields a resolution of one in 16. Eight bits yields a resolution of one
in 256. Ten bits yields a resolution of one in 1,024. Eight bits is the
minimum acceptable for broadcast television.
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2.
A measure of the finest detail that can be seen, or resolved, in a
reproduced image. While influenced by the number of pixels in an
image (for high definition approximately 2,000 x 1,000, broadcast NTSC
TV 720 x 487, broadcast PAL TV 720 x 576), note that the pixel numbers
do not define ultimate resolution but merely the resolution of that part
of the equipment. The quality of lenses, display tubes, film process and
film scanners, etc., used to produce the image on the screen must all be
taken into account. This is why a live broadcast of the Super Bowl looks
better than a broadcast recorded and played off of VHS, while all are
NTSC or PAL.
Resolution
independent: Term used to describe the notion of equipment that can
operate at more than one resolution. Dedicated TV equipment is designed
to operate at a single resolution although some modern equipment,
especially that using the ITU-R 601 standard, can switch between the
specific formats and aspect ratios of 525/60 and 625/50. By their
nature, computers can handle files of any size, so when applied to
imaging, they are termed resolution independent. As the images get
bigger so the amount of processing, storage and data transfer demanded
increases--in proportion to the resulting file size. So, for a given
platform, the speed of operation slows. Other considerations when
changing image resolution may be reformatting disks, checking if the RAM
is sufficient to handle the required size of file, allowing extra time
for RAM/disk caching and how to show the picture on an appropriate
display.
Return
loss: A measure of the ratio of signal power transmitted into a
system to the power reflected or returned. It can be thought of as an
echo that is reflected back by impedance changes in the system. Any
variation in impedance from the source results in some returned signal.
Real-life cabling systems do not have perfect impedance structure and
matching, and therefore have a measurable return loss. Twisted pairs are
not completely uniform in impedance. Changes in twist, distance between
conductors, cabling handling, cable structure, length of link, patch
cord variation, varying copper diameter, dielectric composition and
thickness variations, and other factors all contribute to slight
variations in cable impedance. In addition, not all connecting hardware
components in a link may have equal impedance. At every connection point
there is the potential for a change in impedance. Each change in the
impedance of the link causes part of the signal to be reflected back to
the source. Return loss is a measure of all the reflected energy caused
by variations in impedance of a link relative to a source impedance of
100 ohms. Each impedance change contributes to signal loss (attenuation)
and directly causes return loss.
RGB:
The abbreviation for the red, green and blue signals, the primary colors
of light (and television). Cameras and telecines have red, blue and
green receptors, the TV screen has red, green and blue phosphors
illuminated by red, green and blue guns. Much of the picture monitoring
in a production center is in RGB. RGB is digitized with 4:4:4 sampling
which occupies 50 percent more data than 4:2:2.
Ringing:
An oscillatory transient on a signal occurring as a result of bandwidth
restrictions and/or phase distortions. A type of ringing causes ghosting
in the video picture.
RLE:
Run length encoding. A compression scheme. A run of pixels or bytes
of the same color or value are coded as a single value recording the
color or byte value and the number duplications in the run.
ROM:
Read only memory. A memory device that is programmed only once with
a permanent program or data that cannot be erased.
RP-125:
A SMPTE parallel component digital video recommended practice. Now SMPTE
125M. See: SMPTE 125M.
RS-232:
A standard, single-ended (unbalanced) interconnection scheme for serial
data communications.
RS-422:
A medium range (typically up to 300 m/1000 ft or more) balanced
serial data transmission standard. Data is sent using an ECL signal on
two twisted pairs for bi-directional operation. Full specification
includes 9-way D-type connectors and optional additional signal lines.
RS-422 is widely used for control links around production and post areas
for a range of equipment.
Run-length
coding: A system for compressing data. The principle is to store a
pixel value along with a message detailing the number of adjacent pixels
with that same value. This gives a very efficient way of storing large
areas of flat color and text but is not so efficient with pictures from
a camera, where the random nature of the information, including noise,
may actually mean that more data is produced than was needed for the
original picture.
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