[Elecraft] OT: "Line level"

[Jim Brown]

On Fri, 02 Jul 2004 12:47:44 -0400, Mike S wrote:

>Analog phone circuits are indeed transmission lines. 

ONLY if they are long enough between electronic output and electronic input to 
be transmission lines. Calculate the wavelength (in the cable of interest) at the 
highest frequency of interest. At 6 kHz, 1/20 wavelength in a 0.66 velocity factor 
cable (in the ballpark for most real cable used for audio) is nearly one mile; at 20 
kHz it is nearly one half mile.  At 3 kHz, the limit of baseband audio on POTS, 
1/20 wavelength is nearly two miles. A line must be 1/20 wavelength at the 
frequency of interest for transmission line effects to be just perceptible.  

If a line is less than 1/20 wavelength (or at frequencies where it is less than a 
wavelength), it can be completely characterized by a simple lumped parameter 
model. That is, all of its inductance in one series L, all of its parallel capacitance 
as one C, and all of its wire resistance as one R, and all of its leakage resistance 
as one parallel R. One of the most important limiting factors on real audio lines 
(including telephone lines) is how much parallel capacitance that the output stage 
can drive. Real lines typically have 40-60 pF/ft. It is not unusual for a line to be 
long enough for the capacitive reactance to fall well below 600 ohms. When this 
happens, there can be distortion as the output stage clips prematurely. 

And the characteristic impedance of a transmission line is defined by the 
physical construction of the wire, NOT a paper standard. The characteristic 
impedance of virtually all cable used to carry any form of audio signals is on the 
order of 50-100 ohms. Nothing you can write down as a "standard" or a practice 
will change that physical reality. See any EE text on transmission lines, the ARRL 
Handbook, or the ARRL Antenna Book.  

Modern telephone lines (from a central office to a home) are NOT 600 ohm lines 
because they don't use 600 ohm cable. So calling any audio line, including 
telephone line that isn't a pair of open wire spaced at something on the order of a 
foot between insulators a transmission line is simply wrong.  


Jim Brown
Audio Systems Group, Inc.
Chicago
http://audiosystemsgroup.com