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Filed under: Post Production •Web Video •

PAR for the Course

Over the years, I have seen a lot of folklore and bad math employed to determine how to work with non-square pixels, resulting in a plethora of incorrect working practices. Therefore, in this article I’m going to spend a lot of time laying out the historical and mathematical basis for where these numbers came from. Hopefully this will provide you with a solid foundation on which you can build a new set of working practices.

PARs in CS4 = FUD

With the release of their Creative Suite 4 (aka CS4), Adobe introduced across their product line a “new” set of pixel aspect ratios for standard definition non-square-pixel media and compositions. This has caused widespread fear, uncertainty, and doubt, as many users suspect their old workflows have now flown out the window.

Relax. For one, these pixel aspect ratios are not new - they’ve been known for roughly a decade. They are also more correct than the ones everyone has been using to date. Finally, the changes required in workflows are nowhere near as drastic - nor affect as many users - as you might think. They just require a couple of decisions.

(If you’re more of an artist than an engineer, you can skip ahead a couple of pages - although you’ll miss out on some wonderfully geeky party conversation points, along with a deeper understanding of how to navigate the non square pixel minefield.)

From Analog to Digital

In the beginning, video and videotape were both analog: a series of voltages and blips that described not just the picture, but also all of the timing information necessary to determine where each field - and each horizontal line in that field - started and stopped. These include “blanking intervals” to cover when the electron beam painting the picture on an old CRT reset itself from the right edge of the screen back to the left as well as from the bottom of the screen back up to the top. If you’re interested in the geeky details, National Instruments has a good description here; the illustration below is a sample from their site:

An analog video signal contains picture information (the squiggly lines) plus timing information (the pulses). Image from the National Instruments Developer Zone.

The ITU-R BT.601 specification defines how this analog signal should be encoded digitally, and became the stone tablet for how standard definition digital video would work from then forward. The first digital video decks - D1 (component video) and D2 (composite video) - followed this spec, making it easy for them to emulate an analog video deck. This is why standard definition digital video is often known as “601” or “D1” format video.

The Origins of Madness

To understand the logic behind the 601 specification, it is important to know two things about product design engineers:

• Digital circuits are easier to design if they work like gears meshing together. It’s hard to design a circuit to, say, divide by 29.97; believe it or not, it’s easier to design a circuit that multiplies by 30,000 and then divides by 1001 (which happens to be the true frame rate of NTSC video - not 29.97). The take-away from this: Engineers prefer to define things as ratios between whole number, rather than as decimal values.

• Whenever you can use common parts across multiple devices, you can save money. This is even a concern with professional tape decks carrying five digit price tags. So if you can find a way to use the same part in more than one device, you go for it, even if it means standing on one foot and rubbing your stomach for what may otherwise seem like no apparent reason.

These two principles heavily shaped the 601 specification. To wit, it was decided to try to use as many common numbers (and therefore, parts) between NTSC and PAL as possible. For example, both formats employ a master clock of 13.5 MHz; everything that is needed is derived from this number. Also, both NTSC and PAL “D1” formats ended up with 720 pixels (samples of the master clock) per horizontal line. So how did we get from that starting point to that ending point?

A frame of NTSC video - when you look at the entire analog signal of image plus blanking intervals - contains 525 lines per frame (not 480 or 486), running at a frame rate of 30,000/1001 frames per second. Doing the math, a 13.5 MHz master clock divided by 525 lines divided by (30,000/1001) = 858 digital samples per line of video.

Similarly, PAL video contains 625 lines, running at a frame rate of 25 fps. 13.5 MHz divided by 625 divided by 25 = 864 samples per line of video.

But doesn’t D1 NTSC and PAL video have 720 samples per line, not 858 or 864? Yes - but “720” is just the image. The rest is the blanking interval and timing information between each line: namely, 138 samples in NTSC (720 + 138 = 858), and 144 samples in PAL (720 + 144 = 864). The following figure - which is straight from the 601 specification - lays it out:

Another way of visualizing the above diagram is like this:

How a video frame is stored on analog and 601-compliant digital videotape. The two squished images are the two fields of a frame (each consisting of alternating lines of the final frame); the black area is the blanking interval during which the electron beam in old-fashioned CRTs and television sets is traveling from right to left and from the bottom back to the top.

next page: deriving the pixel aspect ratios

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Outstanding.

Posted by Mark Spencer  on  07/02  at  09:49 PM

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Thanks Chris - Nice overview and history. Since almost everything I do now ends up on the web as “HD” video (even SD stuff) this is important stuff to understand.

Posted by stephen v2  on  07/03  at  08:24 AM

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>> I wouldn’t be surprised if other companies eventually followed Adobe’s lead and started using the correct values in their own applications. <<

And especially that’ll take a longer jurney, especially for ignorants like apple.

Even their bloody Quicktime Player Pro is capable of display the right apertures, even not with anamorphic flags nor anything.

Sorry for swearing and THANK YOU! for the brilliant 4 pages of information which unfortunately cannot be explained with a few phrases.

Posted by .(JavaScript must be enabled to view this email address)  on  07/07  at  09:16 AM

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Does this mean all frame aspect ratios for webvideos encoded from PAL or NTSC video (which i always encode with a square par) are no longer 1.33 or 1.78 and will become 1.37 and 1.82?

Posted by .(JavaScript must be enabled to view this email address)  on  07/09  at  09:00 AM

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Only if you don’t crop off the difference between production and clean aperture. (Personally, I think video should be cropped down to the action safe area before scaling down to web size; after all, this is all the active image area that the creator was expecting the viewer to see.)

Posted by Chris Meyer  on  07/09  at  09:23 AM

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Clean aperture for DV-NTSC is 704 x 480, at 10:11 pixel aspect ratio.  It is not 712.8 x 480.

Clean aperture for DV-PAL is 702 54/59 x 576, at 59:54 pixel aspect ratio.  (Premiere Pro truncates the fractional pixel: 702 x 576, at 128:117 pixel aspect ratio.)

Posted by .(JavaScript must be enabled to view this email address)  on  08/15  at  10:15 AM

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“Clean aperture for DV-NTSC is 704 x 480, at 10:11 pixel aspect ratio.  It is not 712.8 x 480.”

I beg to differ, but am completely sympathetic as to where the confusion comes from.

NTSC DV is a special case: it’s NTSC D1 with 6 horizontal lines missing, and it should be treated as such. Thus it has clean ap size of 712.8 x 480. And this isn’t my speculation or pontification; this is “The Word” from people a lot smarter and more respected than me.

However, there are other formats - such as ATSC digital standard definition - which are defined as having a true 480 lines. _These_ formats have different math, and 704x480 is the correct clean ap size for them. (So, I guess if you want to redefine what DV is for - not a budget version of D1, but instead a format onto itself for acquiring SD ATSC material - then you can think of DV-NTSC’s clean ap as being 704 x 480. It’s revisionist history, but works as a workflow. As long as you treat the underlying PAR as 10/11.)

I know - messy. Remember that DV was never intended to be professional production format; it was supposed to be the consumer replacement for Hi-8. We weren’t supposed to have to deal with all the ugliness that exists under its hood. But, here we are.

Posted by Chris Meyer  on  12/31  at  03:25 PM

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BTW, I’ve just read that FCP 7 now uses the new/correct PARs as well:

http://blogs.adobe.com/toddkopriva/2010/01/final-cut-pro-7-using-correcte.html

So, less tripping while round tripping! (Although it wasn’t as big of a problem as many feared in the first place.)

Posted by Chris Meyer  on  01/15  at  09:58 AM

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