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

Steven Adeff adeffs.mythtv at gmail.com
Thu Jan 18 20:35:29 UTC 2007

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.
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