720P question

[John Jay]

as I understand it, 720P has dimension 720 x 1280 with a PAR - pixel aspect ratio of 1.0

what if in post the PAR was forced to 1.33 - would a HDTV display the resultant video as vertically squashed? ie 2.35:1?

or is the PAR ignored by HDTV when 720p is detected?

[Steve Crisdale]

Originally Posted by John Jay

as I understand it, 720P has dimension 720 x 1280 with a PAR - pixel aspect ratio of 1.0

what if in post the PAR was forced to 1.33 - would a HDTV display the resultant video as vertically squashed? ie 2.35:1?

or is the PAR ignored by HDTV when 720p is detected?

I haven't done it... I'm sure not many people have; because... like, Why would you want to?!! Then again, I'd bet that you'd get a squished result - because it'll still be 720 video, with an out-of-compliance PAR. It may not look so good though!!

If you're hoping to achieve a Cinemascope type proportion rather than 16:9, then you'll have better results by cropping in an NLE to that size.

[John Jay]

Hi Steve

if you have HDTV could you try it for me? - much appreciated

also try 720x 960 with PAR 1.33 to see if it displays correctly as 16:9

reason: small files sizes etc

[John Mitchell]

Originally Posted by John Jay

Hi Steve

if you have HDTV could you try it for me? - much appreciated

also try 720x 960 with PAR 1.33 to see if it displays correctly as 16:9

reason: small files sizes etc

John - don't think this would work, except in certain very specific situations:
1.You need a HTPC with suitable output to playback file streams like this or perhaps a Divx stream on DVD (not sure cause I've never played with Divx).
2. You would need a way of manually stretching the pixels out horizontally then reducing the scale of the overall image. Only a custom scaler could achieve this because:
a: Normally all PAR does in SD is stretch the picture horizontally.
b: 16x9 and 4x3 PAR are set by a flag in the SD picture stream (on either DVD or digital TV) - to the best of my knowledge there is no similar flag in the HD stream; it's always square pixel -
c: my Sony projector will not accept any kind of pixel aspect commands on an HDTV source - it simply says not applicable.
d: I've tested this on my DVDO iScan and I cannot trick it into believing the 720P stream is anything else but square pixel. Perhaps a more advanced scaler might offer more processing as this is purely passthru on HDTV.

Seems an awful lot of mucking around for little gain.

[John Jay]

Hi John

Just to point out that the Sony & Canon flavours of HD all use a luma of 1440 and PAR of 1.33, Pana HD uses an even bigger PAR. So PAR must be recognised otherwise footage from those cameras would not display correctly on a HDTV

It may seem strange to you; but I have a potential requirement to display 720P with a non unity PAR which is why I am asking HDTV owners to see if it is possible, thanks.

[John Mitchell]

Originally Posted by John Jay

Hi John

Just to point out that the Sony & Canon flavours of HD all use a luma of 1440 and PAR of 1.33, Pana HD uses an even bigger PAR. So PAR must be recognised otherwise footage from those cameras would not display correctly on a HDTV

It may seem strange to you; but I have a potential requirement to display 720P with a non unity PAR which is why I am asking HDTV owners to see if it is possible, thanks.

Hi John...

What you say about the Sony and the Panny is true, but I thought that conversion is done in the camera, not the display device. Aren't they stored on tape in square pixel AR (not 1440 but 1920 x 1080i)? That's a custom scaler in the camera head not normally found on display devices. Even if they were stored on tape that way, it would be once again, the deck providing the custom scaling, not the display device...

I'm not disputing that you have a requirement - I was pointing out the technical difficulty of what you propose, and that you might have to source a custom scaler to do it (and therefore is that worth it?). I will try and run a test from my laptop and DVDOiScan, BUT the problem there is my videocard will probably not support those frame sizes. Can you tell me what you are proposing as your playback source?

I think for the most accurate answer to your question you may have to post on a forum for HTPC's - those guys use D-Scaler and other customisable software including Direct Show filters to achieve a decent HDTV picture. Mostly free software that takes quite a lot of trial and error to set up properly. They could probably tell you off the bat whether its possible or not.

http://www.avsforum.com/avs-vb/showthread.php?t=206854

Above is a link on Powerstrip settings at AVS, which may do what you are after.

[Ben De Rydt]

A HDTV can have 3 ways to get a signal into it:

a) analog component input: the signal comes in as a pure 1920x1080 or 1280x720 signal with sync pulses describing the end of a line and the end of a field/frame.
b) digital uncompressed input: DVI, HDMI or HD-SDI. Each of these accepts an 1920x1080 or 1280x720 uncompressed square pixel signal.
c) digital compressed input: firewire or cable input. A compressed video and audio signal is decoded by the TV. Currently MPEG 2 is used for broadcast, MPEG 4 will be used in the future. MPEG 2 has all necessary fields for abnormal picture sizes and aspect ratios, but only 1280x720 square pixel (720p) and 1440x1080 1.33 aspect ratio are certified for broadcast.

If you're using the analog component or digital uncompressed input it's up to the camera, htpc or media player to decode the video files to the correctly sized square pixel output.

If you're using a digital compressed input you could encode your footage to MPEG 2 with different Pixel Aspect Ratios and see how the TV reacts. 720p with 1.33 aspect ratio isn't garantueed to work however.

You'll never be able to connect a DVCProHD camera to a HDTV via firewire and expect it work. The reason it works with some TVs and HDV cams is because of the similarity between the HDV MPEG 2 signal and the broadcast MPEG 2 signal.

[Kevin Shaw]

Originally Posted by John Mitchell

What you say about the Sony and the Panny is true, but I thought that conversion is done in the camera, not the display device. Aren't they stored on tape in square pixel AR (not 1440 but 1920 x 1080i)?

The Sony HDV cameras record 1440x1080 pixels to tape with a 1.33 PAR, which is transferred as such via firewire to compatible editing programs.

Regarding changing the PAR for 720p video, you could conceivably do that in post but it would make the image look visually stretched -- so it wouldn't be much good for anything. If you want a super-widescreen look, why not just shoot 1080i and crop the top/bottom off?

[David Kennett]

I think it's standard for the header in an MPEG2 file to specify aspect ratio as a separate piece of information, completely independant of pixels.

I edit with Ulead MSP, and noticed that the output window on the PC was 4x3, even though I was editing 1280x720. It was quick, so I knew it was not re-rendering - and the output was still 1280x720. In checking the output parameters, I found that the aspect ratio was indeed 4x3, so it simply displayed it that way. Setting it to 16x9 splved the problem.