[mythtv-users] DVD to AVI vs NUV

[Joe Votour]

Your transcodes will only look as good as the source
data you're transcoding.  Garbage in = Garbage out.

DVDs are very high bitrate (I've heard of them being
up to 9Mb/s), but the important thing is that the
source input is usually very clean, is 720x480 (or
720x576 for PAL), and most importantly, digital (which
means no resampling to analog).

The streams that you record on your TV though
(assuming an analog connection, since that's what the
PVR-x50 cards take) aren't so great.  First, they've
likely been downsampled in resolution (they were
mucked with to get the VBI and Closed Captioning
information in there).  Then, there's the potential
for signal/data loss as the signal comes down the wire
(and splitters cause signal loss too).  So, by the
time the signal comes to your TV, it might be crap,
just that you might not notice it so much on the TV.

Then, the PVR-x50 card will take that analog stream,
and convert it into a digital MPEG-2 stream.  The
720x480 resolution that you record at isn't
necessarily true 720x480 - sure the output file is
that size, but the input data (the signal on the TV)
might not be.

Probably not the best explanation, but I hope that it
helps you out.

-- Joe

--- Ryan Steffes <rbsteffes at gmail.com> 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?
> 
> Thanks,
> -- Ryan
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>
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> 


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