[mythtv-users] MPEG4 bigger than MPEG2?

[Ross Boylan]

On Sat, 2012-03-10 at 14:33 -0500, Raymond Wagner wrote:
> On 3/10/2012 13:43, Ross Boylan wrote:
> > On Fri, 2012-03-09 at 09:06 -0700, Tom Hayward wrote:
> >> On Thu, Mar 8, 2012 at 20:51, Ross Boylan<RossBoylan at stanfordalumni.org>  wrote:
> >>> Hi, everyone.  I'm a new user, and am wondering why transcoding is
> >>> making my files bigger.  More specifically, I think I have transcoded a
> >>> file from MPEG-2 to MPEG-4, and it got a little bigger (3.1 vs 2.9GB).
> >> The real question is: Why do you want to transcode?
> > To save space.
> 
> Most people around here are of the opinion that recordings should only 
> be transcoded for compatibility with other devices, not to save space.  
That's useful information.  I do tend to fill up my disks though...
> Transcoding is a very CPU intensive prospect, and while CPUs are 
> continually getting more efficient, hard drives are continually getting 
> cheaper.  Until the floods last October drove up hard drive prices, an 
> average HD recording might cost $0.20 of disk space per hour, 
> or maybe 
> $0.15 after spending a couple minutes defining a cutlist, and a couple 
> more running a lossless transcode.  Transcoding to H264 while retaining 
> quality might drop that to $0.05-$0.07, 
> but is going to run at a 
> fraction of real-time, and eat up considerable electrical power doing 
> so.  When you consider the cost of the increased power consumption as 
> compared to putting that machine in standby, or even just idling, the 
> cost benefit all but vanishes.  It's easier, and only modestly more 
> expensive, to just buy more hard drives.
Out of curiosity, where do those cost figures come from? 
One other factor: additional disks also use more power.
> 
> > When editing the transcode options (autodetect MPEG2) does selecting
> > "lossless transcoding" do the TS->PS transcode?  The description, e.g.,
> > "keep audio and video formats identical to the source", sounds as if it
> > will not.  If it does not, how do I get the desired conversion?  The
> > only video codes I see are MPEG-4 and RTJpeg.
> 
> TS and PS are two different types of MPEG2 containers.  A container is 
> just a wrapper that contains video, audio, and other sundry data 
> streams.  Think of it like a 'tar' file.  There is almost no CPU usage 
> to convert between different containers, as the streams are typically 
> just copied from one to the other.
I have trouble reconciling that statement with Tom Hayword's that
"Re-muxing from MPEG2-TS to MPEG2-PS can save up to 20%."  Can anyone
clarify?
> 
> MythTV only supports encoding to MPEG4 (ASP, not AVC/H264) and RTJPEG.  
> The "lossless transcode" is somewhat of a misnomer, as there really is 
> very little transcoding going on.  Mythtranscode clips out the cut 
> regions, and does some very minor repair as needed around the cuts.  
> Except for those handful of frames it may have to re-encode around the 
> cuts, it is just copying the data unmodified.
> 
> > What controls whether commercials are deleted from the transcoded
> > recording?
> 
> Go into the transcoder settings, and set the "Lossless" checkbox to 
> enabled.  Any MPEG2 (and only MPEG2) recordings transcoded using this 
> profile will be performed losslessly.  Most people use the "Autodetect 
> MPEG2" profile for this reason.  Commercials are not deleted from the 
> recordings, cuts are.  To produce a cutlist, you must go into edit mode 
> (e) during playback in mythfrontend, and define them.  Commercial 
> detection is not reliable enough to be blindly trusted, so 
> mythtranscoder will not use a commercial list.
Just to be sure I'm understannding: commercials are never cut unless I
intervene manually before the transcode runs, right?  And I probably
shouldn't, since I may end up deleting real program material unless I'm
careful (i.e., spend a lot of time verifying and tweaking the cuts).

That's too bad, because in the US an unbelievable amount of airtime goes
to commercials (Tom's estimate of 30% sounds about right).  Another
example of deregulation making capitalists richer and everyone else
worse off.

I can leave the commercials in and then play with a profile that
auto-skips the commmercials (as I am now) to avoid them, and then go
forward or back if the auto-skip  has been over-zealous.
> 
> > Finally, is it possible to set the options for a single recording?
> 
> When you queue a transcoding job through the the Job Options menu in 
> Watch Recordings, you are given the ability to select what transcoding 
> profile you want to use.  The profile selected in the Recording Options 
> section is the default profile that will be used for all subsequent 
> recordings made by that rule.  It is not applied retroactively to 
> previous recordings made by that rule.
Thanks for the info.
Ross