720 v. 704 pixels - please get it right!

8 replies [Last post]
RichardJ
Offline
Joined: May 7 1999

In the Feb edition review of the Hitachi MV270 camcorder, Peter Wells wrote

"... records at the resolution of 704x576 pixels - differing from the 720x576 ratio of DV - due to the camcorder's use of square pixels."

Aaarrgghhh Peter!

The 704/720 pixel-width distinction seems to be one of the most misunderstood issues in video formats, yet is extremely simple.

The difference is essentially nil! The pixels are exactly the same shape in both formats, the 720 variant has 8 extra pixels each side which do not form part of the visible frame - it's just spare data. Conversion between the two is (or should be) a matter of cropping or padding, NOT resampling. Using 704 simply saves space by avoiding storing pixels that will never be seen.

704 pixels are NOT square, in fact in the world of Digital video, TV, and DVD, pixels are NEVER square. Video engineers have no trouble dealing with this concept, but software engineers seem to have congenital difficulty getting their heads round the idea!

I'm not an expert on all the software out there, but I would like to know how many programs actually convert between 704 & 720 correctly (i.e. pad/crop rather than scaling the entire image), as well as convert say a square-pixel format to DV by scaling to 704 then padding. In what I've seen, programmers just can't grasp the idea that the 720 pixels cover an area wider than the real image in any other format.

There's a detailed article on my Web site at http://www.activeservice.co.uk/video/pixels/page1.htm

CV's always been good at getting the facts right. Bob, Peter, et al, please can we have this one right too?

Cheers

------------------
Richard Jones, http://www.activeservice.co.uk
Home of the MediaStudio Pro 6 Tutorial, and free 6.5 Supplement

[This message has been edited by bcrabtree (edited 02 January 2003).]

Richard Jones, http://www.activeservice.co.uk
Home of the MediaStudio Pro Tutorial - Edition 3 for MSP 7

Alan Roberts at work
Offline
Joined: May 6 1999

Well spotted, this is one matter that I repeatedly come back to on this board. About every 5 weeks it all starts again. Perhaps we should have a new forum, one with tutorials and explanations, not for chat like the FAQ tends to be.

By the way, my test cards stuff deals with all this and much more, and, at least today, it's at
http://www.computervideo.net/testcard/Videotestcards.zip

Philip Woods
Offline
Joined: Feb 7 2002

Sorry Gents,Both Misfired!
http://www.activeservice.co.uk/video/pixels/page1.htm http://www.computervideo.net/testcard/Videotestcards.zip

[This message has been edited by bcrabtree (edited 02 January 2003).]

RichardJ
Offline
Joined: May 7 1999

Well Alan's link works for me (it goes straight to a zip file, which should download), all that's wrong with my link is that the "." at the end of the sentence has been included in the link (smart Web board, huh? ).

Try http://www.activeservice.co.uk/video/pixels/page1.htm
it should work this time!

------------------
Richard Jones, http://www.activeservice.co.uk
Home of the MediaStudio Pro 6 Tutorial, and free 6.5 Supplement

Richard Jones, http://www.activeservice.co.uk
Home of the MediaStudio Pro Tutorial - Edition 3 for MSP 7

Philip Woods
Offline
Joined: Feb 7 2002

RichardJ,Yes yours now works, Thanks!
Alan's, Still a blank frame, No info!
Regards,
Philip

johnpr98
Offline
Joined: Aug 20 1999

quote:Alan's, Still a blank frame, No info!

Right Click on the link, 'Save Target As'

Regards

John
http://www.johnpr98.com

johnpr98
 
If you have any Forum Suggestions please post them here

Alan Roberts at work
Offline
Joined: May 6 1999

To be fair, my link is a temporary one that Bob installed just so that I could see if we're talking about the correct files. He's planning to get it linked properly very soon.

There's a couple of bigger zipped files with the test cards in them, I'm assuming no-one wants access to them until the links are set up right. If that's wrong, let me know.

Philip Woods
Offline
Joined: Feb 7 2002

johnpr98

Alan's, Still a blank frame? Nope, Plenty!
Didnt really notice the zip part and I groaned as I read your remark!!!

Thanks for your help!

Regards,
Philip75

pcwells
Offline
Joined: Jun 10 1999

Hi Richard,

Yes, it seems that I did indeed misunderstand the difference between different PAL video resolutions.

Thanks for pointing that out. It's much appreciated.

Oh, and for future reference, you can find my email address in my profile. While I don't have a problem with admitting my mistakes, I'd rather not be informed of them with a megaphone.

Thanks again,

Pete