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

[Phil Bertram]

On Saturday 14 July 2007 11:40 pm, Steven Ellis wrote:
> 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.

Nick,

I guess you are going to be the 'test geek' with the Twinhan :)

Phil
> >>
> >> 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 :)