[mythtv-users] Transcoding SD MPEG2 to H.264---recommendations?

Christopher Kerr mythtv at theseekerr.com
Sun Nov 28 22:36:51 UTC 2010


On Sat, Nov 27, 2010 at 9:39 AM,  <f-myth-users at media.mit.edu> wrote:
> I have a large number of PVR-x50 recordings that I'd like to transcode
> to H.264 to save disk space [1], for eventual playback under VDPAU on
> a GeForce 9500 GT-class device (and, eventually, maybe a GT220-class
> device if that would help [2]).
>
> Two questions:
> (a) What's the current favorite way to do this?  I'd like to run some
> tests standalone, and then I'd like to integrate this going forward
> as a user job under Myth.  Command lines and/or scripts that have
> worked for you would be an excellent starting point; the search space
> of transcoders and options for them is rather large.
>
> (b) What sort of performance, in both space and time, might I expect?
> I know there are a ton of variables here, so anecdotes of what's
> worked for you would be fine.  For a given CPU of the last year or
> so, would it be reasonable to expect that I could do this in real
> time?  1/10th realtime?  10x realtime?  Would multiple cores be
> helpful if I'm doing multiple separate files in parallel?  (I'm
> expecting that to be "yes", since I doubt I'd saturate the memory
> bus and the disk I/O rate should be pretty small, but I figured
> I'd check on whether lots of cores would speed this up.)
>
> If you say "this transcoded X in Y seconds", please give me a clue
> about your transcoding options, CPU speed, and how many cores were
> participating; I'd like a ballpark figure so I can do some hardware
> estimating.
>
> Thanks!
>
> [1] Yes, I know disk is cheap, but my use case is a bit unusual.
> I'm hoping to cut the space by a factor of 2 or even 3 if I can
> do it without noticeable sacrifices in quality.  Much of the video
> is (essentially) STB-quality; some is extracted from archival VHS
> and will (I'm guessing) want to be denoised somewhere (either where
> it would help the most, in transcoding, or perhaps under VDPAU if
> necessary).
>
> [2] If they come out with a fanless one---I last checked a while back,
> the ones with fans were too loud for my tastes, and cards without fans
> tended to be enormous and eat adjacent PCI slots; I haven't re-researched
> this in a few months.

I'm not doing this yet, but I'm planning to get such a thing going
over the next few weeks to help manage the endless episodes of "Escape
to the Country" and "Bargain Hunt" that are clogging up my harddrive.

I plan to use HandbrakeCLI with the Transcode Wrapper Stub on the wiki.

Transcode times vary pretty wildly depending on the complexity of the
job, but as a datapoint:

I'm using an i5 750 (quad core, currently overclocked to 3.8GHz per
core, stock is 2.66) with 4GB of RAM.

A high profile encode of PAL DTV material, using deinterlacing and
denoising, runs at 2-3 times realtime using all 4 cores.

If you trade off some filesize for speed, a more normal encode might
run at 4-5 times realtime (or about 1.5 times realtime if run on a
single core)

At the best end of things, the low-complexity low-resolution iPhone
preset with denoising runs at about 6-8 times realtime (ie. a 2 hour
film takes 15-20 minutes)

You should denoise before encoding - noise is complexity, and encoding
complexity bloat filesizes.

- Chris


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