[linux-dvb] DVB API update

Rudy Zijlstra rudy at grumpydevil.homelinux.org
Tue Sep 18 14:05:29 CEST 2007


Manu Abraham wrote:
> Janne Grunau wrote:
>   
>> On Tuesday 18 September 2007 01:41:04 Manu Abraham wrote:
>>     
>>> Johannes Stezenbach wrote:
>>>       
>>>> On Tue, Sep 18, 2007 at 02:50:09AM +0400, Manu Abraham wrote:
>>>>         
>>>>> Johannes Stezenbach wrote:
>>>>>           
>>>>>> The recording filters are exactly the piece from V4 which has the
>>>>>> "mmap DMA buffers" zero copy API. But to be honest, I don't think
>>>>>> it's important on a PC which can copy > 1GByte/s in RAM. More
>>>>>> interesting would be the ability to have multiple independant
>>>>>> filtered TS outputs instead of just one dvr device.
>>>>>>             
>>>>> Currently have you tried playing back a High Bit rate H.264 stream
>>>>> default of a DVB-S2 stream ? I guess not.
>>>>>
>>>>> If you have had, you will see my reasons why i am trying to
>>>>> optimize the overheads.
>>>>> BTW: it is not RAM that matters here, but CPU horsepower
>>>>>           
>>>> A demux doesn't decode, and what matters is memory bandwidth.
>>>>         
>>> Try running a software decoder alongwith and tell me that that
>>> decoding doesn't need CPU
>>>       
>> right the software decoder needs cpu power.
>>
>>     
>>> and then the options what you can look at 
>>> is cutting whatever overheads it is.
>>>       
>> Wrong, you would start optimizing parts with take significant time. I 
>> can record 20mbps streams on a machine capable of decoding H264 1080p 
>> video with more than 99 percent idle. So even if you can optimize 
>> capturing the stream to taking zero cpu cycles (and you can't) you will  
>> see at most 1% increase in decoding speed.
>>     
>
>
> The case of a 20Mbps stream getting recorded is not a great thing. when
> you have a TS with symbol rate 27.5Msps, (capturing the complete TS) the
> normal TS itself is about 27Mbps (in a very crude rounded off case)
>
> So, the situation that you have isn't larger than a situation having a
> normal single DVB-S card.
>
>   

A single DVB-C card can go up to about 55Mbps when using QAM256...

And more and more operators are now using QAM256 on cable.

Cheers,

Rudy



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