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

[Nick Rout]

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.