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

Robert Tsai mythtv at tsaiberspace.net
Fri Jan 19 00:19:06 UTC 2007

On Thu, Jan 18, 2007 at 02:47:30PM -0500, ryan patterson 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?

I have "high", "medium", and "low" quality set to:

	high:	864x480	4400kbps
	medium:	720x480	3300kbps
	low:	640x480	2200kbps

It compresses 7-8GB/hr of OTA HDTV down to about 1-1.5GB/hr. There is
a noticeable loss in quality (slight jagged edges and stuff like
that), but it doesn't bother me that much (42" rear-projection TV).
You can of course increase the bitrate/resolution if that is too much
loss in quality for you.


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