[mythtv] encoding vs decoding

Michael J. Hammel mythtv at graphics-muse.com
Mon Oct 13 15:33:20 EDT 2003

On Mon, 2003-10-13 at 09:29, Joseph A. Caputo wrote:
> If you have a PVR-250 (which I'd guess you don't), you can encode with a 
> Celeron-450, as it won't require much/any CPU to encode.  But then you'll 
> still be pushing that machine really hard to try and play back the recorded 
> MPEG-2 stream.

I have a Celeron 560MHz with a WinTV PVR 250 installed.  Recording seems
to work, but there are situations that cause the system to start
thrashing on the hard disk.  I'm trying to figure out what those
situations are - I currently think it may be a hard disk "percent
filled" issue and may be correctable with proper tuning of MythTV
recording options.  I only have a 30GB disk right now.  That seems to
hold about 6 1/2 shows and 3 one hour shows and leave me about 1/3 of
the disk clear for other stuff.  If I add a 2 hour movie to that, the
disk fills to >90% and that's when disk thrashing seems to start.

> Bottom line:  a Celeron 450 is grossly underpowered to serve as a combined 
> backend/frontend for Myth, unless you've got hardware that does both encoding 
> and decoding (say, a PVR-350).

In my case I have just the hw encoding - no hardware decoding.  It seems
to work fine for playback as long as I'm not doing any recording. 
Again, this may be a hard disk issue as opposed to CPU speed.

> ...which brings up a good question for someone (Isaac):  How little CPU could 
> I get away with (for recording and for playback) with various hardware 
> encoding/decoding options?  i.e., how little CPU could I get away with to run 
> mythbackend with a hardware MPEG-2 encoder, and how little could I get away 
> with to run mythfrontend w/full-screen playback using a hardware MPEG-2 
> decoder (i.e., PVR-350).  I've got a Pentium-150 lying around, and may soon 
> have an extra PII-333...

I'm actually trying to figure this out myself.  The box I have is
underpowered for recording and playback at the same time.  It is
probably fine for recording and playback that are not at the same time
(but that sort of defeats the purpose).  It also appears to be fine for
recording only if my assumption about disk space problems is correct.

So far its seems that a low end backend-only system (for recording only
- no frontend playback and assuming plenty of disk space) with a single
hardware encoding card could be pretty lightweight:  a PII 333MHz would
probably be plenty.  You might even be able to go below that though a
586/150 might be pushing it.  The only real heavy lifting on this box
would be transcoding (which you could turn off with lots of disk space
at your disposal).  Beyond that, you just have streaming or filesystem
access (via NFS/Samba) for remote playback.  

Take away the H/W encoding or add software playback to this system and
the CPU requirements start to multiply pretty fast.  Add multiple tuners
to this system and it goes up again (depending on H/W vs S/W encoding
and if transcoding is configured for each tuner).

Of course, that's just the video side of things.  Adding MP3 playback or
burning and game playing and the CPU requirements start to climb.

I was wondering if a 10/100 network is sufficient for streaming video to
more than 2 clients at a time.  I'd think you could get away with at
least 4 clients at the same time.  I'm not sure you could do well above
that, though.

Michael J. Hammel <mythtv at graphics-muse.com>

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