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

Steven Adeff adeffs.mythtv at gmail.com
Fri Jan 19 19:54:17 UTC 2007


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.

-- 
Steve
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