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Old October 14th, 2007, 08:06 PM   #1
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Vegas PARs?

This is just an idle enquiry, really, but does anybody know where the Vegas pixel aspect ratios are derived from?

Specifically, PAL DV in Vegas has a PAR of 1.0926, and PAL DV widescreen has a PAR of 1.4568. Most other sofware (and quite a lot of hardware) uses PARs of 1.0666 (or 1.07) and 1.4222 respectively. The correct PARs are actually 1.094 and 1.4587.
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Old October 14th, 2007, 08:27 PM   #2
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Have a look at A Guide to Picture Size, courtesy of the BBC, as they get into this issue in some detail.
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Old October 14th, 2007, 10:46 PM   #3
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Thanks, Mike.

Given that the actual reasons for Vegas's choice of PAR are merely referred to as having been discussed at length back in the Sonic Foundry days, rather than being expressly stated, I'm still none the wiser. I think that RBartlett's suggestion that Sonic Foundry had better engineers and mathematicians than the BBC is incorrect, at any rate. The BBC figures are right.

If you dig deep enough into the specs, the fact that horizontal blanking is 120.3 μs means that picture aspect ratio can legally vary between 1.3256 and 1.3450, anyway, so it follows that pixel aspect ratio might vary between 1.0876 and 1.1036, so Vegas isn't exactly wrong, unlike the de facto PAR that most equipment uses.

Like I said, this was really mostly an idle enquiry. I wondered if there was any important reason for Vegas's numbers that might impact on a real project.
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Old October 14th, 2007, 11:53 PM   #4
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Steve, I did a bit more searching and came up with 2 Sonic Foundry era posts that might help.
DV pixel aspect ratio problem and
Does Vegas Video have a DV Aspect Ratio Bug?
There were a lot more discussions but these were two that jumped out at me.
One thing I have learned over the years is to trust the Vegas developers math skills.
There was a discussion a few years back about why the 29.97 drop frame rate (nothing for you PAL folks to worry about) in After Effects didn't match that of Vegas.
It turned out that Adobe left it at that number while the Vegas folks took it to 4 decimal places :-)
In the second thread I mentioned above, a Sony rep talks about how Adobe rounds the (NTSC DV) PAR off to 0.9 while Vegas set it to 0.9091

As much as I'd like to continue this discussion, it's past midnight where I'm at which means it time for some sleep.
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Old October 15th, 2007, 03:13 AM   #5
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Right, Rec.601 apparently specifies the PAR for 625-line DV as 59:54 (which is what the Vegas guys use - 1.0926 {actually 1.09259259259...}).

The BBC guys, who aren't speaking specifically of DV, but rather PAL video, derive their aspect ratio the same way I did, from the PAL specification of 15625 Hz line frequency with a horizontal blanking period of 12 μs - giving a 52 μs active line time, or exactly 702 samples at 13.5 MHz (note that because all of the websites that are linked to work from the other direction, starting with the Rec.601 PAR and 13.5 MHz, they don't end up with exactly 702 pixels). A 4:3 576 line image with 702 horizontal samples has a PAR of 1.094 {actually 1.094017094017094017...}.

So, where does the Rec.601 PAR of 59:54 come from? I'm not entirely sure.

BUT... 14.75MHz is an alternative video sampling frequency which is widely used because it gives 767 samples per 52 μs line. This is very close to the 768 pixels that a square aspect ratio 576 line 4:3 picture has, and is commonly referred to as "square pixel format" (except the pixels aren't quite square of course, they have a PAR of 768:767 or 1.0013038).

14.75:13.5 is the same ratio as 59:54!

It seems the mystery is solved. The Rec.601 PAR is the aspect ratio of DV pixels compared to "square pixel format" pixels. The BBC guys' aspect ratio is the aspect ratio of pixels compared to true geometric squares. Neither of them is "wrong", they're just not talking about exactly the same thing.

Thanks for helping me solve the mystery, Mike!
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