[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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