Digital video standards do not live outside the realm of
analog world. On the contrary, all commonly used modern (SDTV) digital
video formats have a well-defined relationship with their
counterparts in analog video standards. You could really say they have their roots in analog soil.
And now, my friend, we are rapidly closing to The Fourth Big
It is really the analog
video standards that define the image geometry and pixel aspect
ratio in digital formats.
Even if you did all of your video work solely in digital
domain, those pesky old analog video standards still define the
shape of your images and pixels.
From the video industry's point of view, the current (SDTV, as opposed to
HDTV which is another kettle of fish) digital
video formats - those that actually get used in practical real-life
applications such as DVD, DV, VCD, SVCD, digital television etc.
- are all about interoperability. At the advent of digital video
- late 1970's, when committee work was started on CCIR 601 (later to become ITU-R BT.601) - there was already a vast
catalog of analog video material in formats defined solely by
analog standards. What is more, enormous amounts of money had been poured
in analog studio equipment such as cameras, video switchers, proc
amps, tape decks and other tools of trade. What a waste it would
have been if the "next generation" digital video
formats were designed in a such way they had absolutely nothing
in common with old analog formats, and required ditching all the
It was clear from the beginning that the industry wanted a
smooth, well-defined transition path between the current analog
systems and the brave new digital world without running into too
many compatibility issues. It was also considered necessary to be
able to freely mix and match digital and analog equipment. The
result was that the digital (SDTV) video formats we now use are based on
the concept of digitizing old, analog video signals, thus
interlocking to the analog video standards.
This connection between the digital and analog domains is
permanent. Some of the fundamental features of digital video,
such as image geometry, are actually defined in the analog
standards. Even if we go all-digital, the relationship is still
there, as long as we use either ITU-R BT.601 pixels or "industry
standard" square pixels.
There are three basic sampling rates from which almost all
modern digital video formats are derived:
Let's see how this works out with 13.5 MHz and both 525/59.94
and 625/50 systems:
If you have the B/W (luminance) part of a component video signal in
a coaxial cable, you can plug in an A/D converter and start
metering (sampling) the voltage level in the cable at regular
It also works the same way for square-pixel sampling rates.
You will just get a different number of horizontal samples. The
calculations are left as an exercise to the reader.
If you did not understand a word of the above, you might want
to take a look at the following introductory links:
Also see the Related Links
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