[mythtv-users] USB Hub Bandwidth limitations?

[Brian Wood]

On Tuesday 07 July 2009 19:58:29 Lindsay Mathieson wrote:
> On Wed, 8 Jul 2009 11:38:33 +1000, Jean-Yves Avenard <jyavenard at gmail.com>
>
> wrote:
> > 2009/7/8 Lindsay Mathieson <lindsay.mathieson at gmail.com>:
> >> Interesting - is there anyway to tell if a device is self powered?
> >
> > Most DVB-T tuners aren't.
> >
> > They draw the power from the USB host.
> >
> > running 4 DVT-T USB adapters on a single USB port is certainly going
> > to draw to much power.
> >
> > have to use a powered USB hub.
>
> Ah, thought as much :) thanks. I had this vague though that a tuner might
> be able to run of the rf signal but that didn't seem likely :)

Some devices like amplifiers and satellite LNBs can be powered by AC or DC 
sent over the coax. You need a seperate power inserter (which might be inside 
the satellite receiver), and of course you need to block the power from going 
where it shouldn't (like back into a CATV system). This is not powering 
anything by the RF, but the path is the same.

I've never heard of a tuner using ths sort of power though.

Certainly there is not enough energy in the RF signal to power anything, 
unless you're closely coupled to the transmitting antenna from a few inches 
away.

The old "crystal sets" were RF powered if you want to call it that, but they 
certainly couldn't provide any useful amplification or complex processing, 
just rectify the RF and feed the result into a set of (high impedance) 
headphones.

-- 
Brian Wood
beww at beww.org