HD Advisor Nineteen Eighty-Four

Posted Fri Nov 19, 2010 at 11:00 AM PST by

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Answers by Joshua Zyber

DVD Pixel Aspect Ratios

Q: In HD Advisor 82, you answered a question about aspect ratios and said the following: "NTSC DVD has a resolution of 720x480 pixels, but those pixels are not square. Depending on the type of content on the disc, those same pixels can be used to create either a 4:3 image or a 16:9 image. In the case of the latter, the movie picture is authored in a vertically "squished" fashion, which a 16:9 HDTV will stretch horizontally after-the-fact to correct the picture geometry."

This information would be true if you were talking about the MiniDV format, but this is not true of the DVD format. On an NTSC DVD, the video is encoded with SQUARE pixels, and the resolution is different depending on the selected aspect ratio. For a 4:3 DVD, the resolution is 640x480 while a 16:9 DVD has a resolution of 720x404. (To be truly 16:9, the vertical resolution should be 405, but alas, it is not.) You can confirm this easily using a program called bbDemux. Give it a VOB file from a (non-copy-protected) DVD and it will spit out the de-multiplexed MPEG-2 file. Open that with QuickTime to check resolution.

A: Unfortunately, I believe that you've been mislead by the process you've used to check the resolution. NTSC DVD, whether 4:3 or 16:9, has a fixed resolution of 720x480 pixels. If these pixels were square, that would yield an aspect ratio of 1.5:1. However, DVD pixels are not square. DVD video is defined in two terms: Pixel Aspect Ratio (PAR) and Display Aspect Ratio (DAR). The PAR tells you the shape of the pixels themselves, while the DAR tells you the shape of the final image displayed on screen.

For 4:3 imagery, the pixels in a DVD have a PAR of 10:11. For 16:9 content, the PAR is 40:33. PAL DVD has a fixed resolution of 720x576 pixels. The PARs are 59:54 and 118:81 for 4:3 and 16:9 display respectively.

When output from a DVD player and viewed on a CRT television or monitor (remember that DVDs were originally designed with CRT display in mind), the pixels are stretched to the appropriate PAR to achieve the desired DAR.

Fixed pixel display devices such as LCD computer monitors or digital HDTVs use square pixels. To watch a DVD on these, the original non-square pixels must be scaled to a 1:1 PAR for playback. Thus, a 4:3 image is scaled to 640x480 square pixels, and a 16:9 image is scaled to 720x404 pixels. (From there, the pictures may be upconverted to the screen's native resolution, such as 1920x1080 for a 1080p TV.)

When you go through the process of demuxing a DVD on your computer and then playing the file in QuickTime, the QuickTime readout is only telling you the resolution after the image has been scaled for computer playback. It doesn't tell you the original resolution as the data was originally stored on the DVD.

Uncompressed Audio in Theaters

Q: As per my understanding and research, all the movie soundtracks in theatres are encoded on the CD-ROM (for DTS) at a 1.5 Mb/s bit rate. Since there is so much data during the sound mixing, it is compressed for convenience. I just want to understand why theatres don't get their soundtracks encoded on a higher capacity disc, such as a Blu-ray, so that we may get to hear much more clarity.

A: What you say is correct (or close to correct) as it pertains to traditional 35mm theaters. However, modern digital projection theaters and IMAX theaters utilize uncompressed PCM audio for all movie soundtracks.

In 35mm film-based projection houses, movie soundtracks are indeed compressed into any of three standard digital formats: Dolby Digital, DTS, or SDDS. Note that the versions of Dolby Digital and DTS used in theaters are different than the versions we use at home. The theatrical version of Dolby Digital is recorded onto the film print itself in between the sprocket holes, and runs at a fixed bit rate of 320 kb/s. The theatrical version of DTS is authored onto a CD-Rom that runs in sync with the film print, and is encoded at a fixed bit rate of 882 kb/s.

Why such low bit rates? Both systems were developed in the early 1990s, long before the advent of Blu-ray. Dolby Digital is limited by the small physical space that it must be squeezed into on the film print. And DTS is limited by the storage capacity of the CD-Roms.

Could either of these formats be updated at this point? I would say that's doubtful. The old equipment is standardized in far too many theaters to change now. Also, as theaters transition to digital projection and uncompressed PCM sound, there's little call to invest in the development of updates to the old formats.

Expert Feedback

This week, the Advisor received some feedback on an earlier column from Steve Venuti, president of the HDMI Licensing organization. I will therefore defer to a certified expert in the subject.

The PS3 and HDMI 1.4

Feedback: I ran across a Q&A that you had written and wanted to provide you with a little more information. Each version of the HDMI standard contains options rather than requirements. So, a product can implement one feature and not another and be in full compliance with the HDMI specification. The PS3, as an example, can implement 3-D capabilities and not offer Audio Return Channel or HDMI Ethernet Channel, and be fully compliant.

One other point of confusion that we constantly hear is how a legacy device (which obviously does not have a chip designed after the launch of the 1.4 version of the specification), can implement a feature based on the 1.4 standard. The confusion here is between the relationship of features and the requirements for new silicon. So, for example, the PS3 cannot implement the HDMI Ethernet Channel via a firmware upgrade since the silicon itself is not designed to handle this feature. However, 3-D capabilities CAN be handled by some silicon out in the field, so the PS3 can do a firmware upgrade and essentially bring that legacy silicon into compliance with the 3-D feature of the specification. There is not a 1-1 relationship between silicon and the features of a particular version. Sometimes new silicon is required and sometime the new feature can be implemented via a firmware upgrade.

Sony's PS3 in this case could be marketed as a 1.4 product with 3-D capability. We no longer allow manufacturers to just say blanket 1.4 compliance, since it could also intimate that the device is capable of other features associated with the 1.4 specification.

Check back soon for another round of answers. Keep those questions coming.

Joshua Zyber's opinions are his own and do not necessarily reflect those of this site, its owners or employees.

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