[mythtv-users] Which NVidia card

Michael T. Dean mtdean at thirdcontact.com
Mon Jan 21 19:50:21 UTC 2008

On 01/21/2008 02:14 PM, Rich West wrote:
> Michael T. Dean wrote:
>> On 01/21/2008 09:34 AM, Steve Smith wrote:
>>> On 21/01/2008, Cal <cal at graggrag.com> wrote:
>>>> Marius Schrecker wrote:
>>>>> After a Loooong break I'm trying to breathe new life into my
>>>>> MythTV box.
>>>>> Why didn't I listen to you all before when you warned bout ATI
>>>>> cards? I'd have saved myself a lot of grief! Anyway:
>>>>> Now I'm going to get a card that does proper XvMc (Nvidia the
>>>>> only choice here??)
>>>> Forget about XvMC. It's a relic of a past era where our poor
>>>> struggling sub-2G single core cpus _needed_ some graphics
>>>> processing to be offloaded in order to survive. So, the poor little
>>>>  graphics card endured constantly cooking up a storm doing XvMC,
>>>> and without even a fan to cool its brow. Now, I believe you're
>>>> better off letting a nice little dual core do the hard work. It's
>>>> better equipped to deal with the heat output too.
>>>> Worst of all, XvMC comes with some tedious artifacts. You don't get
>>>>  anything for free. My suggestion is have the graphics card do no
>>>> more than it has to, and provide the necessary grunt via cpu
>>>> instead. I have a 6200 and a 7300 (two systems), and I really can't
>>>>  pick between them. They're both excellent as long as I don't try
>>>> to include XvMC in the mix.
>>>  That's an interesting take on the use of XvMC.
>> I completely agree with Cal.
>>>  My opinion is use a specialized tool for a specialized job.
>> But the problem is that the specialized tool was designed only for video 
>> decoding.  It was not designed to be specialized for MythTV.
> What shouldn't be forgotten is that one size (of mythtv) does not fit 
> all.  MythTV can be set up in any number of ways (combined FE/BE, 
> separate FE & BE, multiple FE's and multiple BE's, etc., etc.), and it 
> doesn't make a lot of sense to generalize.
> Off-loading tasks like XvMC to the video card is to relieve the stress 
> on the CPU and, thus, reducing the requirements.  In other words, you 
> don't *need* that dual-core processor in a measly frontend-only system 
> if all that system is doing is displaying video.
> In my setup, I don't have a need for high-powered frontend systems

And you don't have a need for color OSD and you don't have a need for 
timestretch.  (And does timestretch even work with XvMC?)  And those 
using the PVR-350's TV out have their own (similar--though probably 
larger) list of things that Myth can provide that they don't need.

Oh, and there's the whole issue that XvMC can only be as good as the 
driver developer makes it.  (I.e. when using the NVIDIA proprietary 
drivers, you're stuck with what they give you.  When using MythTV to 
decode, you can fix anything yourself (or get someone to help fix issues).

> Anyhow, this particular discussion boils down 
> to the debate between hardware performance vs software performance 

Hardware is simply permanent software.  Anything implemented in software 
can be implemented in hardware and vice versa.  It all comes down to 
which is most efficient, which is a question of what you're trying to 
optimize (capabilities, flexibility, power usage, CPU usage, 
upgradeability, ...).  And, for the vendors, there's production cost, 
implementation cost, update/upgrade cost.

> With regard to the original poster's question: Nvidia is still the 
> easiest way to go with regard to the choice of a graphics card.

I definitely agree with that--whether using XvMC or not, NVIDIA cards 
are the way to go today.  That may change in a few years as ATI 
continues to open specs for its cards (or NVIDIA may follow suit and 
open their specs once large numbers of "alternative" OS's start to jump 
ship and buy ATI cards).  But, for today, I wouldn't consider using 
anything but an NVIDIA card in my (non-EPIA/non-specialized-hardware) 
Myth boxes.

However, I'd love to see some real-world power/noise comparisons.  I 
would expect there to be little difference with today's Core 2 or Athlon 
64 X2 CPU's.  When video decoding requires nearly 100% of a core, there 
may be a difference, but as processors become more powerful, the 
benefits of "hardware acceleration" tend to disappear (just like you 
won't really see a power/noise benefit using XvMC for decoding standard 
def video on a modern processor--and may actually get a power/noise 
disadvantage using something like a PVR-350's "hardware" decoder--which 
is really still using the software provided by the firmware).  And, 
since modern CPU's tend to be focusing a lot on decreased power usage 
(compared to the Athlon XP/Pentium 4 days), the more power-efficient and 
more-powerful processor may actually hold its own to an older CPU with 
XvMC GPU assist.  (Though this is all speculation as I haven't done any 
testing myself.)

I may change my mind and go back to GPU-assisted video decoding when VA 
API is usable, but only if it's truly flexible enough that I don't lose 
capabilities by using it (and, at this point, I don't expect that to be 
the case).  However, I will /never/ use XvMC as losing capability and 
getting no benefits is a no-brainer decision (that is, no benefits /for 
me/).  However, I have a setup that allows my frontend to be as loud and 
as ugly as I want (and it's both loud and ugly) since it's nowhere near 
my TV/speakers/viewing room and it can be shut down when I'm not using 
it since backends are separate.  Note, also, that my frontend is /not/ a 
"measly" frontend.  It's significantly more powerful than either of my 
backends (which can't even decode the video they capture in real time).  
I'm a believer in the idea that the backend is where you skimp on 
hardware and the frontend is where you put the power.


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