[mythtv-users] IVTV Bitrate & Resolution

Michael T. Dean mtdean at thirdcontact.com
Mon Mar 26 21:38:04 UTC 2007


On 03/26/2007 02:36 PM, Cory Papenfuss wrote:
>> So, since there's no real-world difference, there's little real-world
>> reason to know the "theoretical" difference.  Therefore, it doesn't seem
>> to get publicized too much.  I guess it's just my OC side that wants it
>> done "right."  :)
>>     
>  	Where is that in the DVD-spec that 704x480 is "cut?"

<embarrassed>
In truth, I've never seen the DVD spec...
</embarrassed>

I'm basing the information off little bits of information that I found a 
couple of years ago when I was learning to master video DVD's (all at a 
hobbyist level--I have no ties to the industry, no education in the 
area, ...).  For example, DVD Demystified DVD FAQ, "[3.5] What's 
widescreen?  How do the aspect ratios work?" ( 
http://www.dvddemystified.com/dvdfaq.html#3.5 ) says (near the bottom of 
the section):

For anamorphic video, the pixels are fatter. Different pixel aspect 
ratios (none of them square) are used for each aspect ratio and 
resolution. 720-pixel and 704-pixel sizes have the same aspect ratio 
because the first includes overscan.

And, as an even-less-authoritative source, DVD-HQ's "Configuring TMPGEnc 
for high-quality, DVD-compliant MPEG-2" ( 
http://dvd-hq.info/Compression.html ) says (first occurrence of 704 in 
the page):

720x576 (for PAL) or 720x480 (for NTSC) (except as noted below)
These are the standard, uncropped DV / DVD / D1 full-resolution sizes. 
There are also cropped modes (704x576 for PAL, 704x480 for NTSC), which 
should be used if you have an analog capture card that does not include 
the overscan area.

And, as far as where the 704x480 comes from, see Jukka Aho's "A Quick 
Guide to Digital Video Resolution and Aspect Ratio Conversions" ( 
http://lipas.uwasa.fi/~f76998/video/conversion/ --which is linked from 
the DVD FAQ entry I mentioned above) "4.5.3 You keep saying the 'real' 
4:3 resolution is at about 711486 for 525/59.94 systems. OK, maybe 
there really are 9 extra pixels on the sides, but how do I cope with the 
fact my equipment only records 480 active scanlines, not 486?"  It gives 
a nice technical explanation (much better than I could), that basically 
says, the 720x480 is really the "made-up" resolution that comes from 
adding an MPEG-friendly 8 pixels x 2 sides for overscan to the 
"computer-friendly ideal" 704x480 frame which contains a "truly-ideal" 
702x480 frame.

(BTW, where can one stumble across the DVD spec?  I thought you had to 
purchase it.)

>   I'm not 
> calling you a liar... just that I'd never heard about such a thing... :)

I've been called far worse.  I don't generally mind being called a 
liar--as long as I'm not proved to be a liar.  :)  However, in the 
spirit of learning, I'm always open to being proved wrong.

And to start the, "What's wrong with my previous statement," bit, rather 
than say players are supposed to overscan differently, it's probably 
more technically correct to say they are supposed to crop 8 pixels from 
each side of a 720x480 frame and then output normally (i.e. the same as 
they output the un-cropped 704x480 "cut" frames), allowing the display 
device to overscan as appropriate for an NTSC signal.

Have you actually seen players that treat 704x480 and 720x480 
differently?  I haven't done any real testing, but haven't seen evidence 
that any players I've used do.  You mentioned you have seen a difference 
at the start of playback on some and then after much less than a second 
it stretches, but that sounds even worse than completely ignoring the 
difference.  I wonder why a player would be implemented that way (or if 
that's some other unrelated effect).

Mike


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