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Satellite Transmissions
Voice, video or data transmissions relayed from a sending earth station to a satellite and back to a receiving earth station. An electronics retransmission device serving as a repeater, normally placed in orbit around the earth in geostationary or low altitude orbit for the purpose of receiving and retransmitting electromagnetic signals. It normally receives signals from a single source and retransmits them over a wide geographic area, known as the satellite's "footprint."
Direct Broadcast Satellite (DBS) -- a satellite designed with sufficient power so that inexpensive earth stations, or downlinks, can be used for direct residential reception
. Ground Earth Station (GES) -- fixed facility where the satellites downlink. Fixed facility with which satellites "up-link or down-link' to exchange information. MATMO frequently uses the GES located in Southbury, Connecticut
Earth Station -- the ground equipment, including a dish and other electronics components needed to receive and/or transmit satellite telecommunications signals. An "uplink" is used for sending information to a satellite for distribution to various earth receiving stations, while a "downlink" is used to receive such information.
Uplink -- the path, or link, from a transmitting earth station to the satellite; frequently applied to a transmitting earth station.
Downlink -- the path, or link, from the satellite to earth stations which receive its signals. The term is frequently applied to a parabolic antenna that receives signals from a satellite. It is often referred to as a dish, a terminal, an earth station, or a TVRO (television receive only).
Television Receive Only (TVRO) -- an earth station capable of receiving satellite TV signals but not of transmitting them; a "downlink."
Dish -- a parabolic antenna that is the primary element of a satellite earth station, or downlink.
Mobile Earth Station (MES) -- a portable satellite dish that communicates with the (INMARSAT) satellite. Nicknamed the 'satellite phone"
INMARSAT -- an international global telecommunications satellite network established by government treaty in 1979, with 79 member countries. Land Earth Stations (fixed or portable, even to suitcase-sized) provide links between rural sites and telecom networks. Can provide low-bandwidth digital services anywhere on the earth's surface for as little as $1/minute.
Geostationary Orbit -- describes the orbit of a satellite whose position relative to the earth's surface is constant so it appears to hover over one spot on the earth's equator.
Low Altitude Satellites -- satellites that orbit the Earth at lower altitudes than the geosynchronous satellites and cannot maintain a constant position above the Earth. Thus, they are only accessible when they come into view of the receiving dish, two or three times a day for a few minutes at a time.
Very Small Aperture Terminal (VSAT) -- a type of satellite dish (1.8-2.4 meters in diameter). Used primarily for data transmissions (low speed to high speed). Can send and receive voice, data and video signals if enhanced. VSATs can transmit over wide areas by relaying to satellites in geosynchronous orbit.
Ku-Band Frequencies -- in the 11 to 14 GHz band used to send and receive signals to and from satellites. Being somewhat more narrow than C-Band transmissions, the dish needed to receive these signals is smaller; Ku-Band tends to be somewhat less expensive than C-Band for this reason.
Secure Environment for Information Systems in MEDicine (SEISMED) Send-Only Codec
A video codec able only to originate (transmit) communications signals, for use at the sending location in point-to-multipoint or broadcast applications when two-way codec communication between locations is not possible or required.

Sequential Couleur Avec Memoire (SECAM)
A color television signaling standard with 625 scan lines and 25 interlaced frames/second. Used in France, the Newly Independent States (NIS) of the former Soviet Union, and parts of the Middle East. See PAL, NTSC.

Serial Line Interface Protocol (SLIP)
an Internet access method.

Slow-scan "Still video."
A slow progression of freeze-frames (less than 1 or 2 per second). Compare full motion.

Slow-scan Video
A device that transmits and receives still video pictures over a narrow telecommunications channel, such as standard telephone lines.

Spatial Filtering
In video compression, removal of redundant information on a line by line basis by discarding diagonally-adjacent pixels. Also known as field subsampling, loop filtering, H.261 usage.

Spooling
As one image or data set is being reviewed, additional images can be received and stored for sequential review without "locking up" the computer.

Standards International Telecommunications Union (ITU-T)

Statistical Multiplexer
A device which combines a number of time-varying bit streams into a single bit stream for transmission. In DBS, may dynamically allocate bandwidth through control signals to MPEG2 video codecs.

Statistical Time Division Multiplex (STDM)
used by DBS and for mixed data requirements to improve network utilization.

Static Image
An image whose characteristics do not change. In the past, this was achieved by exposing film to light. Static images now are created with digital cameras and "freeze frame." Static images generally are of high resolution. The require less bandwidth and storage than video.

Still video
--
See
Slow-scan
 

Store-and-forward
Captured audio clips, video clips, still images, or data that is transmitted or received at a later time (sometimes no more than a minute). Email is a store-and-forward system. Enables dissynchronous communication, with the advantage of not needing concurrent participant involvement. Compare to real time.

Stored image management
See image management

Strategic Health Informatics Networks for Europe (SHINE)
S-video
Similar to component video, but closer to the RGB signaling required by monitors. Type of video signal used in Hi8, S-VHS and some laserdisc formats. It transmits luminance and color portions separately, using multiple wires. S-Video avoids composite video encoding, such as NTSC, and the resulting loss of picture quality. Also known as Y-C Video.

Switch
A high speed bridge that links devices on a network.

Switched line or network
A telecommunications option that operates like a dial-up phone line (which is, in fact, a switched line-as are ISDN, ATM, switched 56). There is often a usage charge for switched services, particularly for long distance connections such as phone lines. Compare to leased line, where the connection is continuously open and charges are usually on a flat, monthly rate.

Switched Virtual Circuit (SVC)
(frame relay use).

Sub-Quarter Common Intermediate Format (SQCIF)
Describes a subset of the type of coded video signals transmitted when using ITU-T Rec. H.261 and H.263 coding methods. See CIF.

Syndrome
A code transmitted along with data to enable correction of transmission- induced errors in the received data. See Forward Error Correction .

 


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