From: "Michael R. Kesti"
Subject: Re: audio line specifications
Date: Thu, 05 Dec 2002 14:46:28 -0800
Organization: MK Associates
X-Mailer: Mozilla 4.7 [en] (Win98; I)
>Yes. The impedance of the line is NOT important.
The impedance of the line is not usually important, because audio lines
are usually shorter than the shortest wavelength in the audio band.
>No body designed audio equipment to match the impedance of the line.
Sure they did, and some still do. Early audio practice was taken from
early telephone practice where long lines are typical. Matching to the
line is what the whole 600 ohm thing is about. The characteristic
impedance of open wire lines on telepone poles with glass insulators
and about five inch spacing between conductors is about 600 ohms.
Modern practice recognizes that for short lines, we need not bother to
match the line. Rather than use power transfer with matching source and
load impedances we now use voltage transfer with relatively low source
impedances and relatively high load impedances.
The broadcast industry has been slow to adopt voltage transfer and much
of the analog audio that remains in many broadcast facilities still uses
power transfer. The routing switchers and terminal equipment sold by
Grass Valley Group, Nvision, Utah Scientific, etc., are great examples
of this. Analog audio inputs typically have a way to select whether
they are bridging (high impedance) or terminating (600, 300, or 150 ohms)
either with a switch or a resistor that may be soldered between turrets.
I don't know much about products intended to drive what little analog long
lines the telephone companies might still be using, but you can be sure
that all of it matches those lines' impedance.
Michael Kesti | "And like, one and one don't make
| two, one and one make one."
firstname.lastname@example.org | - The Who, Bargain