[mythtvnz] DVB-S cards: Twinhan VisionPlus VisionDTV Sat VP-1020A

[Steven Ellis]

Nick Rout wrote:
> David Zanetti wrote:
>   
>> On Fri, 2007-07-13 at 08:31 +1200, Nick Rout wrote:
>>
>>   
>>     
>>> 3. whether there is really any practical difference with the skystar
>>> (given the extra $40)
>>>     
>>>       
>> They're both DVB-S, so no practical difference.
>>
>> Verbose answer:
>>
>> Since they don't need to do anything other than provide a I2C interface
>> for a tuner, and dump the data on the PCI bus, they are largely the
>> same. The major differences come down to what tuner they provide, and
>> how they dump the data. (Oh, and maybe CI interfaces, but that's
>> irrelevent to us.)
>>
>> The card can just take the demodulated bitstream from a transponder and
>> provide it over PCI to the drivers. This is what SAA7146 and BT8xx cards
>> do. This means that even though the channel you're watching may be only
>> a 4-5Mbit/s bitstream, you will get the full 45Mbit/s over the PCI bus.
>> The driver will filter the bitstream down to the PIDs you want.
>>
>> Alternatively, you can inspect the bitstream before it leaves the card
>> and filter the stream down to just the PIDs you've asked for. This is
>> what the B2C2 chip on the SkyStar2's does. (It's called a 'hardware
>> section filter'.)
>>
>> Both work just fine, but in theory the cheaper chips without a hardware
>> section filter will result in more interrupts and traffic on the PCI bus
>> than neccecary. For the bitrates we're talking about, this isn't much of
>> a problem, tho.
>>
>> Some hardware section filters cannot dump more than X Mbit/s, but this
>> appears to be only true for the "full featured" DVB-S cards, anything
>> else is capable of dumping a full transponder. This is important because
>> if there ever are HD services over DVB-S (unlikely, but possible), the
>> section filter would need to cope with larger bitrates than typical SD
>> channels were at the time the hardware was designed.
>>
>> [I smell a pvr wiki item, except this is mostly covered by several
>> different wikis already :)]
>>   
>>     
> Excellent info David. I haven't seen that level of info before.
>   
Hmmm.. Uber geek alert :)

-- 
Steven Ellis - Technical Director
OpenMedia Limited
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