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Glossary - R
RAID stands for Redundant Array of Independant(or Inexpensive) Disks. This is a technology of using multiple hard drives to achieve redundancy, or to achieve higher speeds. It has many uses in the computer world, but is also used in Digital video recording.
Random Access Memory. An electronic chip, usually known as "memory", holding digital information while there is powe rapplied to it. Its capacity is measured in kilobytes.
- Random interface
A term describing a camera that has a free running horizontal sync as opposed to a 2:1 interlace type which has a sync locked and therefore has both fields in a frame interlocked together accurately.
- Remote control
A transmission and receiving of signals for controlling remote devices such as pan and tilt units, lens functions, wash and wipe control and similar.
Former name of the EIA association. Some older video tests charts carry the name "RETMA Chart".
- RF Signal
Radio frequency signal that belongs to the region up to 300GHz.
A video coaxial cable with 75 Ohm impedance and much thicker diameter than the popular RG-59(of approximately 12mm). With RG-11 much longer distances can be achieved (at least twice the RG-59), but it is more expensive and harder to work with.
A coaxial cable designed with 50 Ohm impedance, therefore not suitable for CCTV. Very similar to RG-59, only slightly thinner.
A type of coaxial cable that is most common in use in small to medium size CCTV systems. It is designed with an impedance of 75 Ohms. It has an outer diameter of around 6mm and it is a good compromise between maximum distances achievable (up to 300m for monochrome signal, and 250m for colour) and good transmission.
Root Mean Square. All AC voltages are measured with multimeters which show the RMS value of the signal(not the peaks). For a sinewave signal such as the mains, the RMS value happens to be 1.41 times(square root of 2) below the peak values.
Read Only Memory. An electronic chip, containing digital information that does not disappear when power is turned off.
A format of digital communication where only two wires are required. It is also known as a serial data communication. The RS-232 standard defines a scheme for asynchronous communications, but it does not define how the data should be represented by the bits, ie. it does not define the overall message format and protocol. It is very often used in CCTV communications between keyboards and matrix switchers, or between matrix switchers and PTZ site drivers. The advantage of RS-232 over others is in its simplicity and use of only two wires.
This is an advanced format of digital communication when compared to RS-232. The basic difference is in the need for four wires instead of two as the communications is not single-ended as with RS-232, but differential. In simple terms, the signal transmitted is "read" at the receiving end as the difference between the two wires without common earth. So if there is noise induced along the line, it will be cancelled out. The RS-422 can drive lines of over a kilometre in length and distribute data up to 10 receivers.
This is an advanced format of digital communications compared to RS-422. The major improvement is in the number of receivers that can be driven with this format, and this is up to 32.