CCTV Glossary
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Rise time. The time taken for a signal to make a transition from one state to another; usually
measured between the 10% and 90% completion points of the transition. Shorter or faster rise
times require more bandwidth in a transmission channel.
RMS. Root Mean Square. A measure of effective (as opposed to peak) voltage of an AC waveform.
For a sine wave it is 0.707 times the peak voltage. For any periodic waveform, it is the square root
of the average of the squares of the values through one cycle.
ROM. Read only memory. An electronic chip, containing digital information that does not disappear
when power is turned off.
Routing Switcher. An electronic device that routes a user-supplied signal (audio, video, etc.) from
any input to any user-selected output. This is a broadcast term for matrix switchers, as we know
them in CCTV.
RS-125. - A SMPTE parallel component digital video standard.
RS-170. - A document prepared by the Electronics Industries Association describing recommended
practices for NTSC color television signals in the United States.
RS-232. - 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, i.e., 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 its simplicity and use of only two wires.
RS-422. - 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 to up to 10 receivers.
RS-485. - 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.
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