H264 decoding speed (in Mbps, not fps)

[asarian]

Hello,

I'm trying to shop for a good HTPC system, but I can't get a clear picture on what decoding speed to expect for H264 encodings. I have a lot of re-encoded 1080p HD material, some of which will peak at bitrates between 60-80Mbps (those are the extremes, of course; but still). I found an Asus M4A89GTD PRO motherboard, with an onboard ATI Radeon 4290, which is, far as I can tell, the absolute low-end graphics card one could find. :( But it supports a hexacore Phenom II @ 3.2 Ghz (which is comparable to an i7 920, it seems).

So, my question is, would such a board + Phenon II be able to decode H264 at those bitrates? Using, say, XMBC with DXVA 2.0 support. I also simply do not fully understand the DXVA API; I mean, if the GPU lacks the raw decoding power, will the CPU come to help it out? Or is it, the monent you use hardware acceleration, all strictly GPU-only?

Thanks.

[nurbs]

The decoding isn't done by the GPU itself. They usually have an ASIC for that. There have been benchmarks with DGIndexNV that showed that a cheap GT 210 outperforms a GTX 295 for decoding simply because it has a newer Purevideo hardware. So just because that card is low end doesn't mean it will be slow in DXVA. The 42xx cards use UVD 2. Thats two revision behind the newest. UVD 2.2 had a redesign for better performance and UVD 3 added ASP and MVC decoding.
If the DXVA can't handle it the CPU can't help out, but with that CPU you should be able to decode it in software if you have a multithreaded decoder. As a general rule though it's good to stick to level 4.1 if you want to have maximum compatibility.

[mariush]

Wait until November - December after the high end 6xxx cards are launched, for the low end htpc cards in the series... right now the cards in the 6xxx series are mid-level, for gaming. You'll be able to get a 70$ video card of the latest generation and combine it with a cheap dual core AMD processor and a cheap motherboard and you'll be set.

[asarian]

The decoding isn't done by the GPU itself. They usually have an ASIC for that. There have been benchmarks with DGIndexNV that showed that a cheap GT 210 outperforms a GTX 295 for decoding simply because it has a newer Purevideo hardware. So just because that card is low end doesn't mean it will be slow in DXVA. The 42xx cards use UVD 2. Thats two revision behind the newest. UVD 2.2 had a redesign for better performance and UVD 3 added ASP and MVC decoding.
If the DXVA can't handle it the CPU can't help out, but with that CPU you should be able to decode it in software if you have a multithreaded decoder. As a general rule though it's good to stick to level 4.1 if you want to have maximum compatibility.

Thanks for your reply.

Okay, it looks like the 4290 may have a problem. It *is* guaranteed for smooth 1080p HD playback, btw; but how much more bitrate it can handle beyond the 4.1 @ High profile is unclear, of course.

I guess I'll have to find some CPU-only decoder tests.

EDIT: Well, if I read the wiki chart right, level 4.1 should, per the specs, be able to support a max bitrate of 62,500Kbps. Somehow I doubt the 4290 will handle that. :P

[Blue_MiSfit]

@asarian:
You're correct. HP@L4.1 does imply a max bitrate of 62.5mbps.

As high as this may sound, I'd be surprised if any decoder marked as compatible with HP@L4.1 couldn't handle this! Consider that your encodes have only peak bitrates near this level. I think you'll be okay. If your peaks are above 62.5, all bets are off, but I still think you'll be okay as long as your ref count and other things that can sometimes foul up hardware decoders are kept in check.

Derek

[nurbs]

It's easy to test, it's just that no one has bothered. I'll give it a try when I find time.

[asarian]

It's easy to test, it's just that no one has bothered. I'll give it a try when I find time.

I would very much appreciate that, nurbs! Short of bying a board and just crossing my fingers, there's no way of telling for sure upfront.