[mythtv-users] How much space should transcoding HDTV to mpeg4 save?

ryan patterson ryan.goat at gmail.com
Fri Jan 19 20:41:39 UTC 2007


On 1/19/07, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
>
> On 1/18/07, ryan patterson <ryan.goat at gmail.com> wrote:
> >  On 1/18/07, Steven Adeff <adeffs.mythtv at gmail.com> wrote:
> > >
> > > On 1/18/07, ryan patterson <ryan.goat at gmail.com> wrote:
> > > > I'm experimenting with the transcoding my over the air HDTV
> recordings
> > to
> > > > mpeg-4 to save hard drive space.  I enabled transcoding but didn't
> > change
> > > > any of the default bitrate values.  I did enable the "scale bitrate
> > > > according to capture resolution" option.  I am recording OTA HDTV
> with a
> > > > HDhomerun unit.  Here are my results from the first show I
> transcoded:
> > > >
> > > > Channel: CBS
> > > > Title: Racheal Ray
> > > > Original size: 6.8GB
> > > > Transcoded size: 6.3GB
> > > >
> > > > Does this look like usual space savings from transcoding?  Half a
> gig is
> > a
> > > > lot, but honestly I was expecting more.  It saved less then 7.5% of
> the
> > > > original file size.  Basically if I transcode everything I can have
> 107
> > > > hours of recordings on a given hard drive size instead of 100
> hours.  I
> > > > think it would be better to save the cost of the electricity used to
> > > > transcode and buy a larger hard drive.
> > > >
> > > > I didn't cut the commercials.  I know that would have saved more
> space,
> > but
> > > > I am not interested in archiving shows.  I just watch once and then
> > delete.
> > > > And I don't trust the commercial flagging enough to cut without
> manually
> > > > checking the cuts first.
> > > >
> > > > Am I missing something?  Do I need to adjust some bitrate settings
> to
> > get
> > > > better compression?
> > >
> > > Depends on how much your willing to loose in quality.
> > > Basically, xvid will give you no size advantage over mpeg2 for HD
> > > recordings. mpeg4 (without h264) was not designed for larger
> > > resolution higher bitrate encodes, thats where h264 comes in. Now, the
> > > best way to save space without compromising quality is use an inverse
> > > telecine filter and reencode to mpeg2 at the same Q level as the
> > > original "frame", this should give you very little loss in quality
> > > while saving about 15% of file size.
> > >
> > > One way to save space for 1080i broadcasts is to resize them to 720p,
> > > you won't notice much loss in detail because most 1080i broadcasts are
> > > noisy anyway.
> > >
> > > or use x264 with mencoder or similar. this with inverse telecine can
> > > net you perhaps 30% savings with about equal quality.
> > >
> > > so as you can see, transcoding HDTV is really only useful for
> > > archiving. with the price of 500GB drives under $150 now, your better
> > > off just buying one of those suckers.
> >
> > So how do I change to using mpeg4 with h264?  I didn't see that option
> while
> > configuring transcoding in the frontend or backend.  Is there a flag I
> > should set for the transcode command?
>
> right now its not supported.


Thanks.  I tried transcoding some more shows last night with the same
settings.  One saved a gig of space for a one hour show.  This was almost a
14% savings.  I'll continue experimenting.

I think the transcoding would be more useful if I had a lower resolution
TV.  My display's native resolution is 1080p and accepts that for input.  So
I'm not too interested in down sampling the resolution of my recordings.


--
> Steve
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-- 
_____________
Ryan Patterson
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