Newsgroups: comp.dcom.cabling
Subject: Re: Coaxial vs CAT 5
References: <[email protected]>   
Robert Redelmeier  writes:

> Al Dykes  wrote:
> > isn't anyone concerned with high-end rolloff for
> > audio on UTP due to the capacitance ?
> 
> LOL!  Do you think we have Golden Ears?  Odd "capacitance"
> that doesn't affect 100 MHz would somehow disturb 10kHz.

The capacitance does affect 100 MHz signals on the cable, 
and it will affect the 10 kHz audio signal as well. 
The capacitance is there. For audio frequency signals 
the capacitance is there to affect the high frequency 
roll-off, but also the signal source impedance affect 
how much this rolloff there is. 
For high freuqency applications the CAT5 wiring 
is always terminated to 100 ohms signal and source (=low 
impedancde) and the cablign works a a transmission line 
(it nis no longer modelled as just capacitance, but the 
combination iof cable impedance and capacitance form a system 
where the effect of just pure capacitance quite much dissapears). 

A cable in audio applications for carrying microphone and line level
signals can be modeled as a low-pas filter. A first-order high-cut (or
low-pass) filter is formed by an output's source impedance and the
capacitance of the cable. The frequency at which a filter attenuates 3
dB is called its "corner frequency". With short cables (low
capacitance) and low output impedances, the corner frequency typically
occurs well above the audio band. With longer cables and higher output
impedances,the corner frequency drops and may drop into the audio
band. The formula for the corner frequency is: 

F = 1 / (2 * PI * R * C) 

where F is the corner frequency in Hz, PI is 3.14, R is the source's
output impedance in Ohms, and C is the cable's total capacitance in
Farads. A first-order filter has a slope of 6 dB per octave. This
means that beyond the corner frequency, the response will drop 6 dB
for each doubling of frequency. Generally it doesn't seem likely that
you would get detectable loss even at 20kHz unless you have one or
more of these conditions: unusually high source impedance (many
kilo-ohms), unusually high capacitance cable or unusually long cable
length (tens of meters).

Typical transmission characteristics of CAT5 wiring:
DC Resistance: 8.99 Ohms/100metres 
Capacitance: 13.5-17 pF/feet (45-57 pf/meter) 

The capacitance of a normal shielded audio interconnection 
cables is typically 2-3 times higher than the capacitance 
of the CAT5 wiring! 
One meter of typical shielded audio interconnection cable 
(RCA cable) has typically capacitance of around 100 picofarads. 


-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/