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P
 PAL.  Phase Alternate (sometimes alternating) Line.  Describes the colour phase change in
a PAL colour signal.  PAL is the European colour encoding TV system featuring 625 lines per
frame, 50 fields per second and a 4.43361875 MHz sub-carrier (4.43 MHz).  Used mainly in Europe, China,
Malaysia, Australia, New Zealand, the Middle East and parts of Africa.
Pan and tilt head (P/T head).  A motorized unit permitting vertical and horizontal
positioning of a camera and lens combination.  Usually AC motors (24 VAC, 110 VAC,
& 240 VAC) of fixed speed, are used in such P/T heads, but increasingly DC variable speed
motors are being installed.
Pan unit.  A motorized unit permitting horizontal positioning of a camera.
PCM.   Pulse Coded Modulation.  A technique used in analogue to digital conversion.
Peak-to-peak (pp).  The amplitude (voltage) difference between the most positive and the
most negative excursions (peaks) of an electrical signal.
Peak White Inverter.  An electronic circuit that turns white parts (highlights) of a picture, above
a set threshold, to black or a shade of grey.
Pedestal (or set-up).  Where in a video waveform, at the start of the active line period, the level
steps up by up to 100mV, to differentiate between blanking level and black level.  It is not often
found in CCTV waveforms nowadays.
Phase Locked Loop (PLL).  A circuit containing an oscillator whose output phase or frequency
locks onto and tracks the phase or frequency of a reference input signal.  To produce the
locked condition, the circuit then detects any phase difference between the two signals and
generates an error voltage which can be used independently or to correct its own oscillator
phase or frequency.
Phot.  A photometric light unit for very strong illumination levels.  One phot is equal to
10,000 lux.
Photodiode.  A type of semiconductor device in which a PN junction diode acts as a
photosensor.
Photo-effect.  Also known as photoelectric-effect.  This refers to a phenomenon of ejection of
electrons from a metal whose surface is exposed to light.
Photon.  A representative of the quantum nature of light.  It is considered as the smallest unit
of light.
Photopic vision.  The range of light intensities, from 10⁵ lux down to nearly  10^-2 lux,
detectable by the human eye.
Pigtail.  A length of optical fibre with one "bare end" and the other end terminated in a
connector.
Pinhole lens.  A fixed focal length lens, for viewing through a very small aperture, used in
discrete surveillance situations.  The lens normally has no focusing control but may offer a
choice of iris functions.
PIP.  Picture in Picture
Pixel.  Derived from PIcture ELement.  Usually refers to the CCD chip unit picture cell.  It
comprises a photosensor plus its associated control gates.
Pixel or picture element.  The smallest visual unit that is handled in a raster file, generally a
single cell in a grid of numbers describing an image.
Photo multiplier.  A highly light-sensitive device.  Advantages are its fast response, good
signal-to-noise ratio and wide dynamic range.  Disadvantages are fragility (vacuum tube),
high voltage and sensitivity to interference.
Polarizing filter.  An optical filter that transmits light in only one direction (perpendicular to
the light path), out of 360° possible.  The effect is such that it can eliminate some unwanted
bright areas or reflections, such as when looking through a glass window or the surface of
water.  In photography, polarizing filters are used very often to darken a blue sky.
POTS.  Plain Old Telephone Service: the only name recognized around the world for basic
analogue telephone service. Telephone System takes the lowest 4kHz of bandwidth on twisted
pair wiring. Any service sharing a line with Telephone System must either use frequencies above
Telephone System or convert Telephone System to digital and interleave with other data signals.
P-picture (or P frame).  Prediction-coded picture.  An MPEG term to describe a picture
that is coded using motion-compensated prediction from the past reference picture.
Preset positioning.  A function of a pan and tilt unit, including the zoom lens, where a
number of certain viewing positions can be stored in the system's memory (usually this is
in the telemetry receiver) and can be recalled when required, either upon an alarm trigger,
programmed or manual recall.
Primary colours.  A small group of colours light that, when combined, can produce a broad
spectrum of other colours.  In television, red, green and blue are the primary colours from
which all other colours in the picture are derived.  White light is a summation of 11% blue,
30% red and 59% green.
Principal point.  One of the two points that each lens has along the optical axis.  The
principal point closer to the imaging device (CCD chip in our case) is used as a reference
point when measuring the focal length of a lens.
PROM.  Programmable read only memory.  A ROM that can be programmed by the
equipment manufacturer (rather than the PROM manufacturer).
Protocol.  A specific set of rules, procedures or conventions relating to format and timing of
data transmission between two devices.  A standard procedure that two data devices must
accept and use to be able to understand each other.  The protocols for data communications
cover such things as framing, error handling, transparency and line control.
PSTN.  Public Switched Telephone Network.
PSU.  Power Supply Unit.
PTZ camera. Pan, Tilt and Zoom camera.
PTZ site driver (or telemetry receiver or decoder).  An electronic device, usually located
with the camera equipment, which receives telemetry control signals in order to operate pan,
tilt, zoom and focus functions.
Pulse.  A current or voltage that changes abruptly from one value to another and back to the
original value in a finite length of time.  Used to describe one particular variation in a series
of wave motions.
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