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

[David Zanetti]

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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 :)]

--=20
David Zanetti <dave2@wetstring.net>
http://hairy.geek.nz/

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