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P
frames: One of the three types of frames used in the coded MPEG-2
signal. These contain only predictive information (not a whole picture)
generated by looking at the difference between the present frame and the
previous one. They contain much less data than the I frames and so help
towards the low data rates that can be achieved with the MPEG signal. To
see the original picture corresponding to a P frame a whole MPEG-2 GoP
has to be decoded. See also: B frames, I frames and MPEG.
PAL:
Phase alternate line. The television broadcast standard throughout
Europe (except in France and Eastern Europe, where SECAM is the
standard). This standard broadcasts 625 lines of resolution, nearly 20
percent more than the U.S. standard, NTSC, of 525. See also: NTSC and
SECAM.
Palette:
In 8-bit images or displays, only 256 different can be displayed at any
one time. This collection of 256 colors is called the palette. In 8-bit
environments, all screen elements must be painted with the colors
contained in the palette. The 256-color combination is not
fixed--palettes can and do change frequently. But at any one time, only
256 colors can be used to describe all the objects on the screen or
image.
Pan
and Scan: The
technique used to crop a widescreen picture to conventional 4:3
television ratio, while panning the original image to follow the
on-screen action.
Pan
and Scanner: One
who pans and scans, typically during a live event originating in a
widescreen format (16:9) but simulcast in 4:3.
Parallel:
One transmission path for each bit.
Parallel
cable: A
multi-conductor cable carrying simultaneous transmission of digital data
bits. Analogous to the rows of a marching band passing a review point.
Parallel
data:
Transmission of data bits in groups along a collection of wires (called
a bus). Analogous to the rows of a marching band passing a review point.
A typical parallel bus may accommodate transmission of one 8-, 16-, or
32-bit byte at a time.
Parallel
digital: A
digital video interface which uses twisted pair wiring and 25-pin D
connectors to convey the bits of a digital video signal in parallel.
There are various component and composite parallel digital video
formats.
Pre-read:
See: Read before write.
Progressive:
Short for progressive scanning. A system of video scanning whereby lines
of a picture are transmitted consecutively, such as in the computer
world.
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Protocol:
Set of syntax rules defining exchange of data including items such as
timing, format, sequencing, error checking, etc.
PSIP:
Program and system information protocol. A part of the ATSC digital
television specification that enables a DTV receiver to identify program
information from the station and use it to create easy-to-recognize
electronic program guides for the viewer at home. The PSIP generator
insert data related to channel selection and electronic program guides
into the
ATSC MPEG transport stream. See also: Electronic Program Guide.
Parity:
A method of verifying the accuracy of transmitted or recorded data. An
extra bit appended to an array of data as an accuracy check during
transmission. Parity may be even or odd. For odd parity, if the number
of 1's in the array is even, a 1 is added in the parity bit to make the
total odd. For even parity, if the number of 1's in the array is odd, a
1 is added in the parity bit to make the total even. The receiving
computer checks the parity bit and indicates a data error if the number
of 1s does not add up to the proper even or odd total.
PCM:
Pulse code modulation. A method by which sound is digitally recorded and
reproduced. Sounds are reproduced by modulating (changing) the playback
rate and amplitude of the sampled (stored) digital pulses (waves). This
enables the PCM sound to be reproduced with a varying pitch and
amplitude.
Picture:
A source image or reconstructed data for a single frame or two
interlaced fields. A picture consists of three rectangular matrices of
eight-bit numbers representing the luminance and two color difference
signals.
PID:
Packet identifier. The identifier for transport packets in MPEG-2
Transport Streams.
Pillarbox:
Describes a frame that the image fails to fill horizontally (a 4:3 image
on a 16:9 screen), in the same way that a letterbox describes a frame
that the image fails to fill vertically (a 16:9 image on a 4:3 screen)
See also: Letterbox and side panels.
Pixel:
A shortened version of "Picture cell" or "Picture
element." The name given to one sample of picture information.
Pixel can refer to an individual sample of R, G, B luminance or
chrominance, or sometimes to a collection of such samples if they are
co-sited and together produce one picture element.
Plant
native format:
A physical plant's highest video resolution. Point-to-multipoint: An
arrangement, either permanent or temporary, in which the same data flows
or is transferred from a single origin to multiple destinations; the
arrival of the data at all the destinations is expected to occur at the
same time or nominally the same time.
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