Frame Dimensions, Number of Lines, and Resolution
A video frame is composed of lines. In digital video, each line is sampled to create a
number of pixels (samples) per line. The more lines per frame, the higher the image
resolution. The more pixels per line, the higher the resolution of each line.
Number of Lines
NTSC uses 525 lines, whereas PAL uses 625. In analog video, many lines are not actually
used for picture information, so the total numbers relevant for the picture are
somewhat smaller: 486 lines for NTSC and 576 lines for PAL. HD formats defined by the
ATSC have either 1080 or 720 active picture lines per frame.
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Pixels (Samples) per Line
In digital video formats, each line is sampled a number of times. In an attempt to create
a single digital VTR that could digitize and record both NTSC and PAL signals, the
ITU-R BT. 601 specification uses 720 samples per line for both NTSC and PAL video.
Therefore, a digital NTSC video frame is 720 pixels x 486 lines, and a PAL video frame
is 720 pixels x 576 lines.
HD video with 1080 lines uses 1920 pixels per line (1920 x 1080). 720-line HD video uses
1280 pixels per line (1280 x 720). Both of these formats have an aspect ratio of 16:9.
Common video frame sizes are shown in the table below.
Used for web distribution or offline
An early standard for analog-to-digital
video editing, and an ATSC video
NTSC DV and DVD image dimensions.
Also part of the ATSC video
NTSC SD video dimensions used for
professional digital formats such as
Digital Betacam, D-1, and D-5.
PAL SD video dimensions used for
digital formats such as Digital
Betacam, D-1, and D-5, as well as
DVD and DV.
An HD video format, capable of
higher frame rates in exchange for
smaller image dimensions.
An HD video format with very
Some 720p formats (such as
DVCPRO HD and HDV) subsample
1280 pixels to 960 to minimize the
Some 1080-line formats (such as HDV
and DVCPRO HD) subsample 1920
pixels to 1440 or even 1280 to
minimize the data rate.
In most video devices, only 704 or 708 pixels are actually used for picture information.
Pixel Aspect Ratio
A pixel usually refers to a physical picture element that emanates light on a video
display. But a pixel is also a term for a sample of light intensity—a piece of data for
storing luma or chroma values. When stored on tape or on hard disk, the intensity of a
pixel has no inherent shape, height, or width; it is merely a data value. For example, one
pixel may have a value of 255, while another may have a value of 150. The value of each
pixel determines the intensity of a corresponding point on a video display. In an ideal
world, all pixels would be captured and displayed as squares (equal height and width),
but this is not always the case.
The ITU-R BT. 601 specification makes it possible to transmit either NTSC or PAL
information in a single signal. To achieve this goal, both NTSC and PAL video lines are
sampled 720 times. In both NTSC and PAL, the frame displayed has an aspect ratio of
4:3, yet neither 720 x 486 nor 720 x 576 constitutes a 4:3 ratio. The solution to this
problem is to display the pixels (the samples of light intensity) taller-than-wide, or
wider-than-tall, so that they fit into a 4:3 frame. This results in the concept of
“rectangular pixels”—pixels that must be stretched or squeezed to fit in the 4:3 frame.
Most SD video devices actually use 704 or 708 pixels for picture information but stretch
these pixels to 720 when recording to tape.
None of this was obvious in the days of linear editing, when video was simply copied from
one tape to another, because the video equipment always compensated automatically.
However, as people began using computers to work with video, digital video captured to
the computer looked distorted (squashed vertically or stretched horizontally) because the
computer displayed the pixels as squares, without compensating.