[kwlug-disc] Multi-zone question for the audio gurus

[R. Brent Clements]

There is no problem running a line level feed to multiple rooms.  The
output of the soundcard can be reasonably dropped in multiple rooms.
I have run stereo line connections through a 50' XLR cable from a
small mixer output to a 2.1 Altec Lansing speaker set.  Never listened
closely to the sound, but it seemed all right.

The other option you may want to consider is traversing some of the
distances using your network.  If the MPD was running on a computer
with PulseAudio installed the sound can be available to any other
computer with PulseAudio on the network.  Just a thought......

But running 3 lead mic cabling is probably your best bet.  Running at
line level, you should have no noticeable degradation in sound quality
until at least 50'.  Any reasonable number of taps to this line should
be fine since the line input should be a reasonably high impedence.
It sounds like amplification and volume control will happen on a
per-room basis, which is fine.

Mic cables are usually used in a balanced configuration.  This means
that the wanted signal is sent down two wires at the same time, with
one of them inverted. At the receiving end one is subtracted from the
other.   Subtracting the inverted signal from the original is the same
as doubling the original signal,  Any electrical noise that is picked
up by the cable en route will be the same in both wires as they are
really close together.  So subtracting the noise signal in one wire
from virtually the same noise signal in the other effectively cancels
it out.  To do this tho you would have to run one wire for each
channel, as the three leads in the mic cable are used up 9 (the third
one is ground, and it is not supposed to be the same ground as the
outer shell, which attached to the outer braid.)  You would also need
something that outputs a balanced signal.  Pro audio mixing devices
provide that, but you are then looking at about 50 bucks per room for
the volume control at the other end (there is a very cheap mini mixer
by Behringer that I am thinking of,)  In this case tho even a couple
hundred feet is probably ok.

You can use the same mic cable and use the positive and negative wires
as a left and right channel instead, with a common ground.  This is
the basic headphone style connection.  You would simply put the
connectors you need on each end (you can tie the grounds of RCA or
Phone connectors together on the common ground.  Multiple branches
would be possible if you wanted to solder together the lengths
together or install y jacks in multiple places.

Do not just run the speaker output from an amplifier to speakers in
multiple rooms.  There are devices to do this for you, but different
speakers connected in parallel will affect eachother in terms of
effective volume, and you may overload the amp.  Each speaker has a
resistance (impedence is the term)  If you take 2 speakers that are 16
ohm each and put them on the same amp output, you have an 8 ohm load.
An amp has a maximum load, which corresponds to a lower number.  some
pro gear can handle 2 ohms, but most consumer stuff is rated around 8
ohms.  And if two speakers have different impedences then one will be
significantly louder than the other.

A lot of info (some of it superfluous).  And what you are proposing is
quite doable.  Use mic cabling with the connectors you need at each
end soldered on is the short answer. Or pulseaudio.....

Brent

On Tue, Apr 12, 2011 at 9:54 PM, Chris Irwin <chris at chrisirwin.ca> wrote:
> I know there are a few audio-knowledgeable folks on the list (I'm
> looking at you, Brent!). Does anybody know much about multi-zone audio?
>
> I've set up an mpd service, and I've got sound running out of my
> server's sound card. I'd like to take that output and stick it into
> various other rooms. Those rooms will either have receivers (my stereo's
> line-in) or powered PC speakers (via unused KVM port in my office).
>
> Ideally I'd like the audio running to all outputs all the time, so I can
> simply turn on the speakers in the office to listen to music, rather
> than flip A/B switches in the basement.
>
> Any suggestions for hardware? Parts? Schematics? I'm assuming it's more
> complex than soldering five wires together, and I may need some sort of
> amplified device.
>
> --
> Chris Irwin
> e:  chris at chrisirwin.ca
> w: http://chrisirwin.ca
>
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