[mythtv-users] Where to put the power..

Rod Smith mythtv at rodsbooks.com
Sun Mar 11 18:10:55 UTC 2007

On Sunday 11 March 2007 01:28, Rich West wrote:
> Simple question (or so I think)..
> Is it better to have a dramatically better processor in your FEs or in
> the BE?  And is the answer different if you are doing SD vs HD?

As with so many things, the answer to both questions is "it depends." You 
might want to think of it in terms of the CPU power required to do certain 
processes. As a very rough guideline, using Intel CPU speeds:

- SD encoding with hardware encoding card: ~50MHz
- Digital TV (HD) capture: ~100MHz
- SD playback with XvMC: ~700MHz
- SD playback without XvMC: ~1000MHz
- SD encoding with frame grabber card: ~1000MHz
- SD transcoding in 1:1 time (1 hour to do 1 hour): ~1500MHz
- HD playback with XvMC: ~2800MHz
- HD playback without XvMC: ~3500MHz
- HD transcoding in 1:1 time: ~4000MHz (??)

Some of these estimates are VERY rough; they depend on factors such as the 
recording options used, driver efficiency, etc. Still, I believe they're 
reasonable first-pass numbers, with the possible exception of the HD 
transcoding value (I've done very little of that). Also, transcoding can be 
done on systems with any CPU speed; it just takes longer on slower CPUs.

For your frontend, locate the stream type you'll be playing back and get a CPU 
in that range or faster. (XvMC is a feature supported by some video cards and 
drivers, such as nVidia-based cards using nVidia's binary drivers.) For the 
backend, locate the type of encoding you'll be doing, multiply by the number 
of tuners, and get a system with at least that much power. If you expect to 
do any transcoding at all, add in enough CPU power to do that in a time you 
consider acceptable, or take the larger of the recording and transcoding 
values if you can live with transcoding only when recording isn't occurring. 
For a combined frontend/backend system, add all these values together. Note 
that transcoding is usually required for burning DVDs, unless you use a 
hardware MPEG-2 encoding card and record in the right format and bitrate to 
begin with.

Depending on your choices, you might need more power in either the frontend or 
the backend. For instance, if you want three tuners, you'll need more backend 
power than frontend power if you use software-encoding (frame-grabber) 
tuners, but more frontend power than backend power if you use 
hardware-encoding cards and if you don't need extremely zippy transcoding 

Rod Smith

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