same resolution but difference in display w.r.t. height and width

[Pravz]

Guys,

I have 2 video files both having the same resolution 720x400.
But when played on vlc or on my dvd player, the actual height and width of the stream displayed on monitor/tv are different.
Both do not have any black bars in the video.
So in-spite having the same resolution, they appear differently w.r.t. height and weight.

Any explanation for it ?

[jagabo]

DAR = PAR * SAR

DAR = Display Aspect Ratio, the final shape of the picture
PAR = Pixel Aspect Ratio, the shape of individual pixels
SAR = Storage Aspect Ratio, the frame dimensions

For example 16:9 NTSC DVD, PAR=40:33, SAR=704x480

DAR = PAR * SAR
DAR = 40:33 * 704:480
DAR = 40/33 * 704/480
DAR = 1.2121 * 1.4667
DAR = 1.7778 (16:9)

4:3 NTSC DVD: PAR=10:11, SAR=704x480
DAR = PAR * SAR
DAR = 10:11 * 704:480
DAR = 10/11 * 704/480
DAR = 0.9091 * 4.6667
DAR = 1.333 (4:3)

There is some argument over whether commercial DVDs contain the 4:3 or 16:9 image in the center 704x480 portion of the frame (with 8 pixels of padding at the left and right edges, or in the full 720x480 frame. In any case, the difference is only about 2 percent. If you want to assume the full 720x480 frame contains the 4:3 or 16:9 frame use pixel aspect ratios of 8:9 and 32:27 respectively.

[Anonymous344]

I am sorry to dig up this thread, but it is not very old, and the information jagabo posted above relates to a matter that I have found confusing for a little while, so I thought there was no point posting the same values again. Now, I fully understand the 704x480 and 720x480 argument to which jagabo referred and the purpose of nominal analogue blanking. My question relates to PAR.

A 704x480 16:9 DVD with a PAR of 5760/4739 means on playback the image is stretched to 856. So far, so good!

Another 16:9 DVD with the same 704x480 image padded to 720x480 obviously cannot also be stretched to 856 on playback, as the image would look slightly wrong, so is it true to say that this DVD would be resized to 720x(5760/4739)=875 on playback? Is this really how A DVD player resizes such an image?

Thanks for any help.

[jagabo]

DVD only supports two display aspect ratios: 4:3 and 16:9. As I understand it, with 720x480 frame sizes, there is a flag deep within the MPEG 2 data that indicates whether the 4:3 or 16:9 picture is contained within the 720x480 frame or a 704x480 subsection. I haven't found any tools that can report this flag. DVD players can use this flag to determine how to output the picture. I wouldn't be surprised if many don't bother and simply treat all DVDs the same. Nobody will notice the ~2 percent DAR error.

Note that standalone DVD players typically decompress the frame to the storage aspect ratio, then use their display chips to adjust the timing of the output to compensate for the DAR (for analog output). They don't digitally scale to 853x480 or 640x480.

[Anonymous344]

Thank you for the information, Jagabo. It is really interesting. I already knew that there were only two DARs; it is just PAR that I am unsure about. This flag in the MPEG-2 stream must basically be the PAR then. I wonder how free encoders set these flags, and whether they always use 5760/4739 for NTSC 16:9.

It seems I need to learn a bit about how DVD players work too. Is the calculation above correct though, may I ask?

[jagabo]

There are no PAR flags on DVD, only DAR. Even in th wider MPEG 2 spec there are only 3 DARs (4:3, 16:9, 2.21:1) and one PAR (1:1). Where are you getting 5760:4739?

[Anonymous344]

I found it as the figure for strict ITU-R BT.601 in a number of places online. For example, this is one of them:

http://forum.doom9.org/showthread.php?t=125909

If there are no PAR flags for DVD, then I do not understand the nature of this flag that is deep in the MPEG-2 that indicates whether the 4:3 or 16:9 picture is contained within the 720x480 frame or a 704x480 subsection. This is what I am trying to understand. Also, if there are no PAR flags/data in the MPEG-2 stream then I do not understand where ratios such as 40:33 or 32:37 come from, unless they are theoretical and used for calculations only.

[jagabo]

DVD doesn't use PAR flags since there are none in the MPEG 2 data. DVDs only have DAR flags. The player just does whatever is necessary to make the picture the indicated DAR. The 720/704 flag simply tells the player whether the (4:3 or 16:9) image is contained in the full 720 pixel width of the frame or the 704 subsection. I suspect many players always assume the full 720 pixel wide image is the 16:9 or 4:3 picture. On those players that do pay attention to the flag I suspect they just decode to 720x480 and then ignore the first and last 8 pixels of each scanline when putting out the scaled picture.

Yes, the PAR flags used when making MPEG files are theoretical. They are not saved in the MPEG data stream (PAR 1:1 excepted).

[Anonymous344]

Thanks! I believe I understand now.