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Video Conversion

2. The Connection Between the Analog and the Digital

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 Revelation:

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.

How come?

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 analog equipment!

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.

2.1 What does it mean?

There are three basic sampling rates from which almost all modern digital video formats are derived:

13.5 MHz ITU-R BT.601 (aka CCIR 601 aka Rec. 601) non-square pixels for both 625/50 and 525/59.94 systems. This sampling rate was originally designed for digitizing component video signals. Now used extensively in almost all modern digital video gear.
14.75 MHz "Industry standard" square pixels for 625/50 systems. Originally designed for digitizing composite video signals.
12 + 3/11 MHz SMPTE 244M "industry standard" square pixels for 525/59.94 systems. Originally designed for digitizing composite video signals.

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 intervals.

  • ITU-R BT.601 defines a standard sampling rate for both 625/50 and 525/59.94 video signals: 13.5 MHz
  • 13.5 MHz will give you a total of 13,500,000 samples per second, but we are only interested in sampling the parts of the signal that actually contain image information. The parts of the signal spent in horizontal or vertical blanking are of no interest to us, and can be omitted.
  • 625/50 systems have a line length of 64 µs, of which 52 µs is the "active" part that contains actual image information. (The rest is reserved for horizontal blanking.)
    • 52 µs × 13.5 MHz = 702 samples (pixels) per scanline
    • In the vertical direction, there are 574 complete scanlines and 2 half lines. Even the half lines get digitized as if their "missing" other half belonged to the active picture, giving a total of 576 scanlines.
    • Thus, the active image area at 13.5 MHz sampling is 702×576 pixels. This is the actual area that forms the 4:3 (or anamorphic 16:9) frame.
  • 525/59.94 systems have a line length of 63+5/9 (63.555...) µs, of which 52+59/90 (52.6555...) µs is the "active" part that contains actual image information. (The rest is reserved for horizontal blanking.)
    • 52+59/90 µs × 13.5 MHz = 710.85 samples (pixels) per scanline. 
    • In the vertical direction, there are 484 complete scanlines and 2 half lines. As above, all of them get digitized and half lines will be treated as if their missing other half belonged to the active picture, giving a total of 486 scanlines.
    • Thus, the active image area at 13.5 MHz sampling is 710.85×486 pixels. This is the actual area that forms the 4:3 (or anamorphic 16:9) frame.
    • However, we cannot use partial pixels in any practical video work. Therefore, the number 710.85 needs to be rounded up to 711, and we get a 711×486 pixel frame instead.
    • 711 samples equals to 52+2/3 (52.666...) µs at 13.5 MHz, so the rounded-to-the-nearest-pixel active area is a little bit wider than it ideally ought to be. Fortunately, the difference of 0.0111... µs is (for all practical purposes) insignificant, and well within the tolerances of NTSC-M specifications.

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.

2.3 I am already lost!

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 section.

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Created: Dec 27, 2015 by 1
Edited: Dec 27, 2015 by 1


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