Standard-definition television (SDTV, SD, often shortened to standard
definition) is a television system which uses a resolution that is not
considered to be either high or enhanced definition. SDTV and
high-definition television (HDTV) are the two categories of display
formats for digital television (DTV) transmissions. "Standard" refers
to the fact that it was the prevailing specification for broadcast
(and later, cable) television in the mid- to late-20th century.The two
common SDTV signal types are 576i, with 576 interlaced lines of
resolution, derived from the European-developed PAL and SECAM systems,
and 480i based on the American NTSC system. Common SDTV refresh rates
are 25, 29.97 and 30 frames per second.[a] Both systems use a 4:3
aspect ratio.Standards that support digital SDTV broadcast include
DVB, ATSC, and ISDB. The last two were originally developed for HDTV,
but are also used for their ability to deliver multiple SD video and
audio streams via multiplexing. In North America, digital SDTV is
broadcast in the same 4:3 aspect ratio as NTSC signals, with
widescreen content often being center cut. However, the aspect ratio
of widescreen content may be preserved in a 4:3 frame through
letterboxing. In other parts of the world that used the PAL or SECAM
color systems, digital standard-definition television is now usually
shown with a 16:9 aspect ratio, with the transition occurring between
the mid-1990s and mid-2000s depending on region. Older programs with a
4:3 aspect ratio are broadcast with a flag that switches the display
to 4:3.[b] Standard-definition television Biography, NetWorth, Height, Age, Weight, Family, Married, Son, Daughter