[mythtv-users] MPEG4 bigger than MPEG2?

[Raymond Wagner]

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.  
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.

> 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.

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.

> 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.