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Video Conversion
NOTICEThis is an archive of the "A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions" located at: http://lipas.uwasa.fi/~f76998/video/conversion/ for my reference.

A Quick Guide to Digital Video Resolution and Aspect Ratio Conversions

Contents

  1. Introduction
  2. The Connection Between the Analog and the Digital
  3. A Conversion Table for Digital Video Formats
  4. Frequently Argued Questions
  5. Related Links

Recent updates

15-Jan-2008

  • Link-rot fixes. (Thanks, Jeff!)

25-Feb-2006

28-Feb-2004

  • Added the 544×576 resolution (as per DVB specifications) to the 625/50 table

22-Feb-2004

  • Due to popular demand, I have now finished a major revamp of the conversion table as regarding to the 525/59.94 systems. Formerly, all 525/59.94 calculations were based on rounding up to 711 pixels (52.666... µs) in 13.5 MHz modes, then considering that figure the exact 4:3 resolution. Now, all 525/59.94 numbers are based on the exact active line length of 52+59/90 (52.6555...) µs, which more accurately follows the analog standards. Note: Even though the 525/59.94 numbers in the table have now (seemingly) changed in a big way, their practical approximations are still very close to the old values, so the change does not have all that many practical effects. For example, the old pixel aspect ratio value for 525/59.94 13.5 MHz pixels was 72/79 (0,91139...), now it is 4320/4739 (0,91158...) You are unlikely to see any difference even if you used the old values.
  • I am sending my greetings to Andreas Dittrich and Chris Meyer who offered some insightful comments and persuaded me to go forward with this change. They are the ones you may now thank (or curse) for these new, more pedantically exact, more cumbersome fractional numbers! :)
  • There is still some academical controversy over the 575 vs 576, 485 vs 486 active lines geometry issue. I may revisit the subject some time in the future.
  • The table has now been divided into two sections – one listing the common 525/59.94 sampling grids and the other listing the common 625/50 sampling grids.
  • Nothing has been changed on the 625/50 side. Nada. Zilch.

20-Feb-2004

14-Feb-2004

  • "Link rot" fixes. Some little changes in wording here and there.

26-Jun-2003

  • Typos in the conversion table. The sampling matrix width for some resolutions was listed as 52.33333 µs instead of 53.33333 µs.

11-Feb-2003

  • Some "link rot" fixes. Thanks, Andy and Olafs!

26-Jun-2002

  • The conversion table erroneously labeled the pixel aspect ratio column as if the values were in y/x format, while they actually were in x/y format. The heading of the table has been corrected. (Thanks, Colin!) The calculations given below the table were correct all the time and have not been modified.

13-Apr-2002

  • There was an unfortunate error in Section 4.7: the correct 525-line resolution is (of course) 720×526 + 2/3, not 720×533.25! Sorry. It has been corrected now.

11-Apr-2002

10-Apr-2002

  • Enhanced the table by adding sampling matrix widths in microseconds.
  • Various little touch-ups all over the document.

9-Apr-2002

  • All references to 525/60 have been changed to 525/59.94 to be more pedantic.
  • The table now calls 625/50 systems CCIR and 525/59.94 systems EIA (well, it is not the perfect solution, but probably better than using mere numbers or separately listing each and every letter designation for every broadcast standard in the world)
  • Some H.261, H.263 resolutions added to the conversion table.
  • Clarified the explanation of digitizing half lines.
  • Added some links to the Related links section.
  • Added some introductory links.
  • Replaced Section 4.6 with a new one

6-Apr-2002

  • Initial publication date

Abstract

Despite of ever-growing number of people working with digital video formats daily, there is still a great deal of confusion regarding how their image geometry and aspect ratios actually work. This document tries to shed some light on these issues.

Feel free to e-mail me any comments, corrections, suggestions, additions or opinions. Should you come across a broken link, please let me know so I can fix it.

Acknowledgements

My warm thanks go to Colin Browell, Andy Furniss, Ole Hansen, and Paul Keinanen, and Olafs who have provided valuable comments and feedback concerning this page.

Linking to this document

You are free to link to this document. If you do so, please use the URL <http://www.iki.fi/znark/video/conversion/>. This ensures that the link will always work, regardless of the actual physical location of this site.


1. Introduction

There is a fair number of mind-blowing, scary oddities and secrets in the world of digital video.

One of the very first a beginner will usually encounter is the fact that in digitized video data, pixels are often not considered "square" in their form. In most real-world digital video applications pixels have a width/height ratio – or aspect ratio, as it is more conveniently called – that can be something completely different from 1/1!

The second great revelation usually comes when one runs into the concept of anamorphic 16:9 video for the very first time. If it was initially hard to grasp the idea of pixels changing their shape when displayed in different environments, this one is even more baffling: the very same pixel resolution you have only just learned to associate with 4:3 displays can now suddenly represent another, totally different image geometry. In other words, the pixels have changed their shape again!

Unfortunately, these two are often the only things most ordinary people will ever learn about digital video and aspect ratios.

1.1 The dirty little secret revealed

Tutorials and manuals usually tend to keep very quiet and secretive about the finer technical details of digital video, particularly when it comes to the topic of (pixel) aspect ratios and image geometry.

Even if converting (resampling) video clips to other resolutions is discussed, the accompanying explanation is usually troublingly simplistic and vague – often inaccurate and misleading – and sometimes the suggested methods are just plain wrong. It is not uncommon that the examples only deal with arbitrarily chosen ("x pixels by y pixels") frame dimensions and use ideal frame aspect ratios such as 16:9 or 4:3 as the basis for calculations – not the actual pixel aspect ratios – which is usually a good indicator that the writer may not actually take the real image geometry into account at all.

It is almost as if the whole aspect ratio issue was considered some sort of dirty little secret of the video industry; black magic you could not even begin to explain to mere mortals in reasonable terms. This is a shame. In this case, there is really more to it than meets the eye. Confusing people with incomplete and watered-down explanations does not do any good to the industry. 

Now that you have read this far, it is time to reward your effort with The Third Big Revelation about aspect ratios and frame sizes - the one that is usually left unsaid:

Not a single one of the commonly used digital video resolutions exactly represents the actual 4:3 or 16:9 image frame.

Shocking, isn't it? 768×576, 720×576, 704×576, 720×480, 704×480, 640×480... none of them is exactly 4:3 or 16:9; not even the ones you may conventionally think as "square-pixel" resolutions.

So there. Now you finally know the truth. Let's find out what it actually means.

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Edited: Dec 27, 2015 by


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