[mythtv-users] shrinkage pros and cons (was Re: MPEG4 bigger than MPEG2?)

[Ross Boylan]

On Sat, 2012-03-10 at 16:24 -0500, Raymond Wagner wrote:
> On 3/10/2012 15:39, Ross Boylan wrote:
> > 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.
> 
> Additional disks use additional power, but only when in use.  When not 
> in use, or when the system is in standby, they can be spun down for 
> nearly no consumption.
> 
> Prior to the flood, 2TB hard drives were readily available for around 
> $70.  That's $0.035/GB, or $0.0375/GB if you account for binary/decimal 
> discrepancy, putting an average 6GB hour long recording at $0.21.  
> Lossless transcoding typically drops that to around 4GB, or $0.14.  The 
> H264 format is designed double the compression efficiency of MPEG2 at 
> the same quality level, but considering x264 is one of the better 
> encoders, and hardware compressors used by broadcasters tend to be less 
> efficient, 1/3 the size of the original is not unreasonable.  That would 
> bring you down to around 1.5GB and $0.05.
When you said disk space per hour, I thought you meant  per hour the
storage was in use (i.e., .20/hr -> 4.80/day of storage).  But it was
per hour of recording.

If you are in watch and delete mode, the case against transcoding is
even stronger because you pay for the disk once but use it many times as
you delete files.  In comparison, the transcoding costs are incurred
every  time.

If you put a value on your time, the issue quickly becomes what takes
the least amount of your time.  I think that favors skipping trancoding
too (esp if it means other tasks take longer because the computer is
loaded), except that if yoou have to buy a new disk, or a new system to
hold the disk,that takes time too.
> 
> The last time I made a go at transcoding HD recordings, running iVTC to 
> convert from 1080i30 to 1080p24, and doing a single pass constant 
> quantizer compression with good (but not absurd) quality settings on a 
> 2.8GHz Core 2 Duo, an hour long (44.5min clipped) recording took some 12 
> hours to drop from 13Mbps to 5.5Mbps.  That's $0.0875 saved in storage, 
> but with 150W consumption, and $0.15/kWh for power and line charges, 
> $0.27 spent in electricity compared to letting the machine go into <5W 
> standby mode.
> 
> Now computers have improved, and a modern quad-core SB i5/i7 might only 
> take 3-4 hours, and use 100W doing so under full load, but that's still 
> half what you saved, and means your computer is tied up transcoding your 
> recordings, and not being used for other purposes.
> 
[snip]
> >>> 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).
> 
> Commercial lists and cut lists are independent, correct.  "A lot of 
> time" is not that correct.  Running mythfrontend on a desktop with a 
> keyboard, it might take me 90-120 seconds to create a cutlist for an 
> hour long show.  You can import the skiplist to use as a guide, but I've 
> not found it to make things much faster.  Using a remote instead of a 
> keyboard takes me a bit longer, just because I can't hit the buttons as 
> fast.  You can also create a cutlist as you are viewing, taking a few 
> seconds per start/stop to flip into edit mode, mark the correct frame, 
> and drop back out to playback.
Could you say a little more about the mechanics of creating the cutlist?
Particularly if doing it from scratch, I think I'd need to step through
the whole program, which seems as if it would be slow.  Although I guess
if you step a minute at a time that's only 30 steps for a half hour
show--maybe not so bad.

In my experiment I went into edit mode, loaded the commercial list (Z),
and then used page up and down  to step through the cuts and the arrow
keys to see if the area around them looked OK.  The cuts before I
tweaked actually were pretty good; the only problems I noticed were that
the cut areas could have been a few seconds bigger in some cases, and
the final credits got skipped (since the picture at the time was mostly
an ad, with the credits in a small area at the bottom, a pretty tough
case).  Of course if there were ads in the middle of an area marked as
program I would never see it with that procedure.  The slightly small
cuts are probably a good thing, since it makes it easy to tell that
nothing important has been cut while you're watching.

Ross