[mythtv-users] Objective IVTV quality measurement instructions

[Cory Papenfuss]

On Sat, 14 Aug 2004, dean vanden heuvel wrote:

> I myself have been experimenting with capture on the PVR-250.  I can't decide 
> with my eyes alone using normal content whether or not I gain anything by 
> capturing 720x480 as opposed to 640x480 or even less.  It takes too many 
> keystrokes in Myth to go from one to the other, and by the time I display the 
> alternate, I can't really say for sure if the higher resolution is better. 
> I do receive (and capture) HD content that is broadcast @ 1280x720, but 
> because I have only the S-video input for my PVR-250, I must use the S-video 
> out from my satellite receiver, and thus some of that resolution is lost.

 	I couldn't either... in fact, I couldn't see much of a difference from 
480x480 on up.  That's why I cooked up the resolution test pattern... to see if 
I could measure a difference.

>
> So, would your experience tell you that I can get replay that is *just as 
> good* if I capture at 640 (or less), or should I stay with 720?

 	I'm still interested to hear if other peoples' cards (different 
revisions, etc) can do better than mine, but yes.  As far as the static test 
pattern goes, I could measure no difference in quality between 480x480 and 
640x480 and 720x480.  A lot more goes into the motion side of the encoding, but 
if it can't capture a still image at high res, then there's no way it can do 
motion higher either.

for live TV, 
> storage does not matter, but for recorded content, using the lower resolution 
> and a lower bit rate could save space.

 	That's what I do.  I do record at 640x480 just to give a few more 
(albeit ficitious) dots to interpolate with when I downsample/transcode to 
352x480 to record onto DVD.  Also, the computer tends to be happier with a 4:3, 
square-pixeled video than a stretched one.

Alternatively, using the lower 
> resolution for capture and keeping the same bit rate, resulting in roughly 
> equivalent storage requirement, MAY result in a BETTER picture, If I 
> understand this correctly.

 	I would think so.
>
> What would your experience tend to recommend?
>
> -dvh

 	I'm weird... you probably shouldn't listen to me... :)  I'm an 
efficiency freak so the best bang-for-the-buck for me is to record at 
square-pixeled rate, and then denoise/resize/2-pass re-encode to 352x480 at 
about 2.5Mbps for DVD archival purposes.  6 1-hour shows (sans commercials) fit 
on a DVD at >VHS quality.  Remember, 352 lines in 4:3 -> 264 "lines of 
resolution" (352*3/4).  VHS craps out at 240 lines, and broadcasts' maximum is 
generally quoted at 330.  It's a good compromise.  If DVD supported the SVCD 
resolution of 480x480 I'd probably use that, but it's overkill for broadcast 
anyway.

-Cory