High-definition television (HDTV) is a digital television broadcasting system
with greater resolution than traditional television systems (NTSC, SECAM, PAL). HDTV is
digitally broadcast, because digital television (DTV) requires less bandwidth if sufficient
video compression is used. HDTV technology was introduced in the United States in the 1990s
by the Digital HDTV Grand Alliance, a group of television companies.
HDTV broadcast systems are defined threefold, by:
- The number of lines in the vertical display resolution.
- The scanning system: progressive scanning (p) or interlaced scanning (i).
Progressive scanning redraws an image frame (all of its lines) when
refreshing each image. Interlaced scanning redraws the image field
(every second line) per each image refresh operation, and then redraws the
remaining lines during a second refreshing. Interlaced scanning yields greater
image resolution if subject is not moving, but loses up to half of the resolution
and suffers "combing" artifacts when subject is moving.
- The number of frames per second or fields per second.
The 720p60 format is 1280 × 720 pixels, progressive encoding with 60 frames per second
(60 Hz). The 1080i50 format is 1920 × 1080 pixels, interlaced encoding with 50 fields
per second. Sometimes interlaced fields are called half-frames, but they are not, because
two fields of one frame are temporally shifted. Frame pulldown and segmented frames are
special techniques that allow transmitting full frames by means of interlaced video stream.
For commercial naming of the product, either the frame rate or the field rate is dropped,
e.g. a "1080i television set" label indicates only the image resolution. Often, the rate
is inferred from the context, usually assumed to be either 50 or 60, except for 1080p,
which denotes 1080p24, 1080p25, and 1080p30, but also 1080p50 and 1080p60 in the future.
A frame or field rate can also be specified without a resolution. For example 24p means 24
progressive scan frames per second and 50i means 25 interlaced frames per second, consisting
of 50 interlaced fields per second. Most HDTV systems support some standard resolutions
and frame or field rates.
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