[linux-dvb] usb 2.0 v pci

Felix Domke tmbinc at elitedvb.net
Sun Jun 5 23:44:57 CEST 2005


Dirk Koopman wrote:
> Does anybody have any metrics as to which is better:-
> 
> 1. a cheapo PCI card (say a conexant based one like the Hauppauge
> nova-T).
> 2. a USB based unit.
Depends.

(wow, that's an answer, isn't it? ;)

Wheter the data comes from USB or PCI shouldn't matter that much - both
are busmastered, so both payloads come in without much CPU-Load.

However, most PCI cards don't have hardware filtering, so the whole TS
goes over the PCI-Bus, and must be processed by the CPU. It's not too
much, however. Even with 6 of them in parallel, it's still only as much
as one uncompressd PAL signal, which is handled by analog TV cards fine.

The filtering, however, could be more CPU intensive. I don't know the
relevant codepath by hearth, but I think it's the limiting factor.


USB devices, at least the USB1.1 compatible ones, have hardware filters,
so you only get the data you need. (Note: at least USB-Nova-T 1.1 cards
require calculating a checksum over the whole "muxpack", i.e. data, so
they probably eat a "lot" of CPU time.) If you want to get the whole
multiplexes, however, you need the full bandwidth anyway (and you
wouldn't get far with USB1.1 then).

Still, having 6 adapters in one PC is at least adventurous - i wouldn't
assume that it's working unless you saw it working before :)


Felix




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