[mythtv-users] OT: Hardware decoders (was Re: completely new and have questions before I buy hardware)

[Michael T. Dean]

On 09/08/2008 10:16 AM, Marc Randolph wrote:
> On Fri, Sep 5, 2008 at 7:01 PM, Michael T. Dean wrote:
>   
>> [...]
>> (So, what about HDTV decoders?  Well, they're completely unnecessary for
>> MPEG-2-encoded HDTV, but could be useful today for MPEG-4-AVC-encoded
>> HDTV (H.264) /if/ there were any that worked in Linux.  However, by the
>> time there is hardware that works in Linux for decoding H.264 HDTV,
>> there won't be a need for the decoders...  And, since decoders only
>> limit what Myth can do and don't provide any benefits over simply
>> selecting an appropriate general-purpose CPU...)
>>     
> Drifting off topic...
> Understand and agree for the most part.  But hardware decoding for
> H.264 or maybe VC1 could be useful for:
>
> * Those on a super tight buget who want to go with lowest cost CPU

Assuming that the cost of "budget" CPU + dedicated hardware decoder is 
less than the cost of "appropriate" CPU that can handle software 
decode.  This could be the case, but I think when that hardware decoder 
is available and supported in Linux and MythTV, it won't be the case.  
(After all, CPU's are pretty cheap today--just ask AMD.)

> * Those trying to minimize every last Watt (with HW decode, you can
> even underclock, rather than having to run ~2.75 GHz)
> * Those trying to eliminate fans required to run at ~2.75 GHz (either
> for space or noise reasons)
Though, for the last two, since most all "hardware" decoding is/will be 
implementations of software decode on hardware that's not the CPU, it's 
usually not the panacea most expect.

True, Intel's next-generation audio implementation, to replace Intel 
HDA, which replaced AC'97, is the first Intel audio spec that's not a 
CPU-based sound implementation but uses a discrete DSP.  Intel finally 
realized that the inefficiency of the design (which can actually 
increase power consumption by 4-6W when playing back a movie with Dolby 
True HD audio, though primarily because of interrupts) is causing 
problems due to the large number of mobile/laptop users***.  So, designs 
/could/ be made to be more efficient on dedicated hardware.  Key words, 
though, being, "could be."  Don't just assume that because your CPU 
usage is low, you're saving power/heat/money.

Chances are, a "hardware" video decode implementation would save a few 
watts on interrupt processing.  It can save power elsewhere, but it can 
also be inefficient, especially depending on which "hardware" is doing 
the decoding (i.e. is it an ASIC or simply using software in 
programmable shaders on a power-hungry GPU).  However, until I see 
actual power usage numbers showing on-CPU/off-CPU decoding usage, I'm 
not going to assume power savings are worth the limitations.  True, by 
spreading the heat generation out over multiple chip packages/heat 
sinks, you may be able to have fewer/quieter fans, but those who build 
small/quiet MythTV frontends are already making compromises and 
accepting limitations.  (I'm a fan of big, ugly, and in a different room 
when it comes to Myth frontends.)

IMHO, assuming that hardware decode lowers power usage/heat generation 
is akin to assuming a low-power Intel CPU will make for the most 
efficient system, while completely ignoring the effects of the chipset 
and integrated peripherals (where, when considered, the higher-power AMD 
CPU may actually use less power overall).

But, whatever.  If people really want to live with (and are willing to 
not complain about) the limitations they get with "hardware" decode, 
more power to them.  I won't be using it myself.

The main point of my post, though, was that the PVR-350's hardware SDTV 
decoder is a waste, and I firmly stand by that assertion.

Mike

***And, since Intel has pretty much locked up the Intel motherboard 
chipset business, they can still make some money by including a Realtek 
DSP in their chipset packages with appropriate markup.  And because 
Microsoft Vista has already taken up the job previously held by on-CPU 
audio processing--finding a way to make users need ludicrously fast 
CPU's for other than high-end computing--Intel can still continue to 
sell high-end CPU's to low-end users while making gamers/laptop users happy.