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Sengstack
Known Participant
August 15, 2010
Answered

Export HD to NTSC Widescreen -- not truly 16:9

  • August 15, 2010
  • 3 replies
  • 42909 views

If you export HD footage, 1080x1920 for example, to NTSC Widescreen, you end up with 720x480 with rectangular pixels that have a 1.2121 pixel aspect ratio. That means, the display is effectively 873x480 pixels, which is a a screen aspect ratio of about 16.4:9 instead of 16:9. What you end up with is the original video image squashed between thin, vertical black bars on the right and left sides (not the thick black bars you get when you display a 4:3 video in a 16:9 display).

If you open the video in an NTSC Widescreen Sequence, you'll see those black bars. The video image does not fill the screen.

This is easily fixed: change the Scale Width value to 102.3 (uncheck Uniform Scale). That puts the image portion of the clip into the proper aspect ratio (and shoves the black bars off the left and right sides of the screen). But I'd rather change the scale of all the clips in a project.

Since NTSC has to be 720x480 pixels, if you want to display something at a 16:9 ratio, the pixel aspect ratio should be 1.185 instead of 1.2121.

Perhaps an Adobe engineer can explain why exporting HD to NTSC Widescreen creates a video with black vertical bars and how a 720x480 (with rectangular pixels with a 1.2121 aspect ratio) clip can be considered as having a 16:9 aspect ratio.

Thanks,

Jeff Sengstack

    This topic has been closed for replies.
    Correct answer FastFeet

    Will do. Thanks for the chortle.


    Hi Jeff,

    I just run accross your post. It is old, but I recently was running in the same problems as you. It was hard to get things straight for me, but finally I found a German paper that explained everything just to the point.

    16:9 = 1,777...

    HD uses square pixels (PAR=1.0) and if you check the two most common HD video sizes and take your calculator 1920:1080=1,777... and 1280:720=1,777.... So both of them are 16:9 formats. (Exactly we would calculate 1920 (pixel) x 1.0 (PAR) : 1280 (pixel) x 1.0 (PAR))

    Now in SD we have an analog NTSC/PAL an a DV NTSC/PAL. To make it easy first, lets look at DV. The default resolution for DV NTSC is defined as 720×480, but DV doesn't us square pixel. It uses rectangular pixel. The PAR for analog(!) 16:9(!) NTSC is about 1,2154. So to convert DV pixel into the square pixel world you need to calculate 720 (pixel) x 1,2154 (PAR) = 875 square pixel.

    Now we are ready for the surprise: 875:480 = 1,822916...!!! DV NTSC (as PAL) are just not exact 16:9!

    This means you can't transform an HD frame into an SD DV frame without cropping or padding with black bars.

    Old Premiere and AE actually stretched the frame a little to make the transform fit. In AME you really need to choose one of three options: you can have the HD frame cropped, the SD frame padded with black or the SD frame stretched, which would distort the picture a little bit, but not much. If you don't want to have the picuedistorted or have black bars you need to crop off 28 (27) pixels from your HD surce footage. This is why often 14 pixels are each cropped of the top and the bottom. You can do this in AME using the crop tool and set the output "scale to fit".

    So far this explains your problem and shows you the possible solutions. To understand why DV NTSC and PAL are not 16:9 you have to dig a little bit deeper into the analog world. This would take bit longer. For a rough and simple explanation consider that analog TV (NTSC/PAL) has no pixels. It's analog. So ITU-R BT.601 defines how to get analog video into the digital world. The defined sampling frequency results in a NTSC frame of 711×486. Considering the PAR of 1,2154 you get 864:486=1,777.... So analog (anamorphic 16:9) PAL is really 16:9. Analog NTSC video convertd to DV is padded with black bars to fill to 720.

    This all concerns analog to digital SD-TV. The black bars were not shown on CRT TVs due to the overscan area. Nowadays LCD TVs and computer monitors can show the full 720 pixel, but still use overscan. Also video that is recorded digitally is normally recorded with the full 720 pixel. So we have video that is not in 16:9. Therefore you need to handle the transforming either with bars, cropping or distorting.

    Hope this helped you Jeff.

    Marcus

    3 replies

    Wil Renczes
    Adobe Employee
    Wil Renczes
    Adobe Employee
    August 17, 2010

    Long thread.  If I can distill it down correctly, the OP question is essentially asking 'why do I get letter boxing?'

    Essentially, the problem comes down to the fact that your source doesn't match the destination PAR.  Currently, if the source & destination PARs don't match, you get letter boxing.  Why? From a technical standpoint of 'do no harm', it's bad practice to drop pixels, and the render pipeline is trying to preserve the aspect ratio of your footage.  What you're effectively asking for is a option in AME to 'fit to fill (distort)'.

    Currently, as a workaround, if you plan to export to a DV preset, if you nest your HD sequence into a DV sequence, then export the nested version, now your source sequence will match the destination, and that will circumvent the need to letterbox.

    Cheers

    able123
    able123
    Inspiring
    August 17, 2010

    Wil,  which is basically putting the HD into DV and scaling it down to fit the 16:9 DV..correct ?

    Wil Renczes
    Adobe Employee
    Wil Renczes
    Adobe Employee
    August 18, 2010

    robodog2 wrote:

    Wil,  which is basically putting the HD into DV and scaling it down to fit the 16:9 DV..correct ?

    Yep, exactly. Minor nuisance, I know, but at least you know exactly what you're getting for output, since you now control the panning. 

    B
    Bill Engeler
    Inspiring
    August 15, 2010

    Adobe does it the right way. A PAR of 1.185 will give a slightly distorted image in DV widescreen. This was fixed in CS4.  It can be very confusing, but the root of the matter is that a DV frame is not really 4:3 or 16:9 - there is some padding at the sides (18 pixels) that wasn't intended to be part of the visible frame. HD footage doesn't have these extra pixels, so when you go from HD to SD, you see these little pillars on the side. You can change the PAR is you want, but the result is slighly distorted - fatter - than reality. These links can explain it much better than I can:

    http://www.bbc.co.uk/commissioning/tvbranding/picturesize.shtml

    http://help.adobe.com/en_US/AfterEffects/9.0/WS3878526689cb91655866c1103906c6dea-7f3aa.html

    http://www.lynda.com/home/Player.aspx?lpk4=40550

    S
    SengstackAuthor
    Known Participant
    August 15, 2010

    Bill,

    I checked out the three sites you listed and none explained this issue clearly. They touch on PARs and resolution, but don't explain P Pro's export behavior (two are on After Effects and they don't discuss exporting to other formats). Perhaps you can clarify some points:

    1) The DV frame. When working in NTSC DV (widescreen or standard definition), the pixels we see in the Program Monitor in P Pro or any NLE are 720x480. The easy way to note that is to look at the Motion effect, Position parameter. The center point is 360/240. So, are you saying that P Pro truncates two 9 pixel wide vertical rectangles from each side when it displays the video in the Program and Source Monitors? Are you saying that the actual DV frame is 738x480?

    2) HD to NTSC Widescreen export. If P Pro is able to truncate those 9-pixel wide vertical lines to create a clearn display, why doesn't it truncate them when displaying HD footage that has been exported to NTSC Widescreen? Or, conversely, why doesn't P Pro have an option to export HD to NTSC such that the 720x480 image area is all that ends up in the AVI file (and not include the two vertical, 9-pixel wide bars)?

    3) P Pro displays those 720x480 pixels with a .91 pixel aspect ratio in standard definition and 1.2121 in widescreen. Effectively NTSC SD has a resolution of 655.2 x 480 and NTSC widescreen is 827.7x480. So the SD screen aspect ratio is not 4:3, it is 4.095 to 3. And NTSC widescreen is not truly 16:9, it is 16.36 to 9. Can you clarify that?

    4) You said you can change the PAR in the export settings. I tried that and it did not work. I used the Uncompressed Microsoft AVI format with Video set to 720x480, Progressive, and with an Aspect of 1.185 to 1 (I chose 1.185 but the display in the Video tab of the Export Settings dialog box shows only whole numbers). The resulting video had a pixel aspect ratio of 1.0 and an data rate of 26.6 MB/sec -- about 7 times the data rate for standard MS AVI. In my NTSC Widescreen Sequence, the clip did not fill the screen and had black bars at the top and bottom. So...How do you export with a PAR of something other than the standard PARs in the Export presets?

    Thanks,

    Jeff Sengstack

    able123
    able123
    Inspiring
    August 17, 2010

    With all the back and forth on this, I still don't understand what's going on in P Pro, "under-the-hood," such that when converting from HD to NTSC Widesreen, vertical black bars show up on the left and right edges of the visible area of the AVI clip. No one has yet addressed this transcoding process. If we see 720 columns (I'll use "columns" to avoid confusion with horizontal "lines") of image information in DV whether SD or Widescreen (we don't see any vertical black bars even though they are part of the clip), how come we don't see 720 columns of image information in a clip after converting it from HD to NTSC Widescreen? Why do we see only 704 columns of image information plus two visible, 8-pixel wide, vertical black bars on each side? I follow-up on this question at the end of this note.

    But first:

    As for displaying DV NTSC SD or Widescreen, I think the following sums up what's been posted here:

    DV (SD or Widescreen, NTSC or PAL) has only 704 vertical columns of image area (plus 8 columns off each of the right and left edges that are not supposed to be visible -- 720 columns total).

    To display those columns in a standard definition, 4:3 frame aspect ratio, P Pro shrinks the width of the columns by multiplying them by 10/11: 704 * 10/11 = 640). The excess 8 vertical columns on a each side (now slightly narrower as a result of the width reduction) are out of sight and out of mind (even if you scale the clip to less than 100%, P Pro truncates those black bars from the visible area of the clip). So what we see on TV or in our P Pro monitors is an image that is 640 units wide and 480 units tall (a 4:3 frame aspect ratio). P Pro still considers the visible area of the clip as being 720 pixels wide (as evidenced in the Motion effect). And on NTSC or PAL TV sets, it's still 720 pixels wide, because TV set pixels are non-square (and again, the 8 extra vertical columns on the left and right sides are not visible on TVs sets or computer monitors).

    When displaying those 704 columns in widescreen (16:9 frame aspect ratio), P Pro expands the width of the columns by multiplying them by 40/33 (1.212.....) as in: 704 * 40/33 = 853.3. So the resulting visible area of the clip is 853.3 units by 480 units (a 16:9 frame aspect ratio). And again, those 8 columns on each side (now a bit wider due to the width expansion) are not visible.


    So, back to the question at the top of this note: Why doesn't P Pro take HD clips, which start their lives in a 16:9 frame aspect ratio, and convert them to NTSC Widescreen such that the resulting clip has 704 columns of visible image information (and 16 non-visible black columns)? In that way, 1920 vertical columns would be interpolated to 704 vertical columns and those columns would be 480 pixels tall. And as part of the conversion process, 8 extra, non-visible vertical columns would be added to each side of the resulting clip.

    That, to me, is the crux of this discussion. Since P Pro, other NLEs, video playback software like Media Player, and TV sets all "hide" the two sets of 8 pixel-wide vertical columns (black bars) when displaying NTSC SD or Widescreen, why doesn't P Pro transcode HD to NTSC Widescreen such that those 8 extra lines are not visible in the resulting file?

    Jeff


    I guess you're exporting frames and opening in photoshop to measure exactly how many pixels are left and right of frame? which is square pixels..psd...

    I think the bottom line is that what you see is the result of un-exact measurements...relating to par, dar etc.

    And a lot of different cameras, who basically translated the japaneese company "standards" of HD ( blue ray etc) according to their not exact language translation...meaning there's a lot of leeway about application of the standard...You would think that translation of science would be more exact but it isn't.

    If I go from metric to feet and inches, or nautical miles to statute miles, there are discrepancies ...as the math just isnt perfect.  There is some slop in the math, no matter what you do, and the accuracy of math is sometimes in the translation of the different "systems".

    and this is made more obvious as you translate more than 2 things....like maybe now you are dealing with 3 things..

    Its a fascinating subject though...thanks for sticking it out and trying to get a definitive answer !

    E
    excited_Genie16B8
    Legend
    August 15, 2010

    Since NTSC has to be 720x480 pixels, if you want to display something at a 16:9 ratio, the pixel aspect ratio should be 1.185 instead of 1.2121.

    Dude, I came up with the same answer.  Despite some offered explanations, I find it hard to ignore the math here.  16 divided by 9 = 1.777.  Multiply that by 480 and you get a square pixel "widescreen" resolution of of 853.333.  Divide that by the 720 you're limited to and we end up with the correct PAR of 1.185.

    I don't get why that's not recognized.

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