[mythtv-users] DVD to AVI vs NUV

Michael T. Dean mtdean at thirdcontact.com
Fri Aug 5 21:10:09 UTC 2005


Ryan Steffes wrote:

> It seems like every time I feel like I get a good grasp on how the 
> bitrates and resolutions and codecs play together something comes up 
> that confuses me.  Like just about everyone, I'm trying to get the 
> best quality for the least space, and I thought I was doing alright.  
> My WAF doesn't tolerate graininess very well, and blockiness is a 
> killer.  I have a PVR150 that I record at 352x480 3500/ 720x480 5000/ 
> 720x480 6000  by quality level.  The HQ ones I leave alone, but the 
> low quality ones I autotranscode to mpeg4 at 352x480 2200 with the 
> four high quality options turned on.  This seems to give good quality, 
> with a half hour show coming in at around 700MB.
>
> Last night though, I ripped a few dvd's to take with me on a trip this 
> weekend, and I realized that I was ripping a full 140 minute DVD to 
> about 1GB with perfectly good quality, at only around 1000 bps VBR.  
> If I tried to transcode my shows at that, they'd look terrible.  I'm 
> sure I'm missing a basic principle somewhere, but I can't figure out 
> what it is.  I would think changing the format would have something to 
> do with it, from mpeg2 to mpeg4, but wouldn't that apply to the dvd as 
> well?  Double pass encoding might be part of the answer as well (I 
> don't think myth can do that, can it?)
>
> Can someone explain this effect to me?

I'm recording all my shows from a satellite receiver at 720x480 
(DVD-compatible resolution) at 2200kbps VBR up to 9800kbps with 
PVR-x50's.  My recordings average about 1.15GiB/hr.  Picture quality is 
very good on my 27" TV.  A friend is using the same settings, but 
playing back using a projector (8 1/2 foot TV) and--other than the 
artifacts from his analog cable signal--it looks great.  When playing 
something I recorded on his system, the quality is about the same as a 
DVD ripped to a 700MB MPEG-4.  See

http://mythtv.info/moin.cgi/UserManual_2fTechnicalDetailsAppendix_2fRecordingParameters

I've found that any type of re-encoding of the signal will either 
significantly degrade the quality (graininess/blockiness) or require me 
to crank up the bitrate to higher than the original (which is why I 
record using DVD-compliant settings...).

Also, input signal quality does make a difference.  Analog cable signals 
often require greater bitrate than digital cable/satellite (which only 
becomes analog for the short trip to the PVR-x50, which isn't long 
enough for any noticeable degradation of signal quality).

HTH.

Mike


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