[mythtv-users] RFC: backend transcode on the fly

Michael T. Dean mtdean at thirdcontact.com
Tue Apr 17 18:10:34 UTC 2007


On 04/17/2007 01:56 PM, Brian Wood wrote:
> On Apr 17, 2007, at 10:13 AM, Yan Seiner wrote:
>> I've been kicking this around for a while and I figure I can air it  
>> out and see what happens.
>>
>> I have several front ends, none of which can display true HD.  At best
>> they do 1280x1024; my main LCD TV is 1280x720p.
>>
>> AFAIK decoding of the stream from HD and rescaling to the display
>> resolution is the CPU intensive part.
>>
>> Could this be offloaded to the backend?
>>
>> I'm thinking of a on-the-fly transcode that the backend would use to
>> reduce the resolution so it's suitable for the frontend.
>>
>> This would let me use some of my lower-powered FEs to display HD  
>> content
>> - albeit at a reduced resolution...
>>
>> Comments?  Pipe dream?
> Why "on the fly"? Why not just transcode it as a user job and leave  
> it that way. You could save the original if you wanted to as well,  
> with the obvious storage costs.
>
> I'm sure what you suggest could be done, given enough CPU horsepower,  
> but if your present setup can do it without any changes I'd think  
> about doing it that way.

Besides, modern general-purpose CPU's are pretty well taxed just 
/decoding/ HDTV in real time.  I doubt you could find a system that 
could decode an MPEG-2 HDTV recording /and/ still have enough spare 
cycles to re-encode to a lower-resolution MPEG-2 HDTV recording in real 
time.  And, note that with playback, we get to offload the scaling to Xv 
(which means it's basically no additional load on the CPU).  AIUI, with 
transcoding, you'd have to scale in software, so in fact, transcoding in 
real time requires decoding, scaling, and re-encoding all being 
performed on the CPU.

Then, when you think about things like 
skipping/jumping/fast-forwarding/rewinding, it turns out that Brian's 
approach is the most appropriate.

Chris Pinkham has worked on and eventually (though, as I understand, 
it's low on his priority list) will finish support for storing multiple 
copies of the same recording so you could record the same show 
twice--once in HDTV and once in SDTV--or you could record the show in 
HDTV and transcode a second copy to SDTV.  That way, even when you get 
your HDTV-capable frontend, you'd be able to get the right resolution 
for the system.

Mike


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