Re: Impedances of SP/DIF (distributrion)

["mark_harrison_uk2" <mph@xxxxxxxxxxxxxxx>]

--- In ukha_d@xxxxxxx, "Phil Harris" <phil@a...> wrote:

I think you're right on the high vs. low issue... however....

> The output impedance of the Sky+ box *SHOULD* be 75ohms otherwise
> you would have an impedance mismatch as an SP/DIF input *SHOULD*
> be 75ohms too.

That was how I understood impendance matching to work until I had a
long talk with Keith at UKHA2004.

After a bit of digging around I came up with
http://arts.ucsc.edu/EMS/Music/tech_background/TE-15/teces_15.html
which talks about the difference between output impedance and input
impedance.

To quote from that article:

"There was a time when output impedance (also called source impedance)
was carefully matched to the input impedance of a connected device,
because that is the most efficient way to transmit power. However,
since a connection will work if the input impedance is higher than the
source impedance but will not if the input impedance is too low (low
impedance requires more current, remember.) engineers tried to design
circuits with somewhat higher input impedance than actually necessary,
and lower output impedance. (A low output impedance implies that lots
of current is available.) There is a limit to how high input
impedances can go; if it is too high, unwanted low current sources
like radio stations will begin to affect the circuit. The usual ratio
of input impedance to source impedance is 10 to 1"

OK - let's ask another way.... Has anyone tried it????

M.