[BC] audio cable termination

[Peter Smerdon]

Why do we terminate cables? Because broadcast borrowed the practice from 
the telephone companies, and old habits (and legacy installations) die hard.

My understanding is that properly terminating a cable requires the 
source impedance, cable characteristic impedance, and load impedance to 
be the same
- and -
You would only terminate a cable when either:
a) the cable length was such that it was acting as a transmission line - 
rule of thumb (heard from Bill Whitlock of Jensen Transformers - who's a 
bit of an evangelist on this subject) is that for a cable length to 
start looking like a transmission line it has to be at least 10% of a 
wavelength - and that is 4000 feet at 20kHz (to put Cowboy's "short" and 
"long" into perspective)
-or-
b) you are interested in maximum power transfer, not maximum voltage 
transfer.

Here I'm assuming the application is "short" cable lengths at audio 
frequencies.

In distributing audio, we are interested in voltage, not power, and if 
we were to terminate, we'd be just throwing away 6dB in level 
(headroom/noise) for no benefit.
For audio signals the modern practice of low source impedance (say 50 
ohms) and high input impedance (10K+) eliminates the need for terminations.

Ya just gotta watch out for interfaces to long lines, your telco or 
campus network - and if interfacing with a transformer designed for 600 
ohms, where failure to terminate adversely affects the transformer 
performance.

btw: the 600 ohms line impedance comes from the fact that that is the 
characteristic impedance of the typical "open wire" telephone line 
strung on poles with wooden crossbars and porcelain insulators. Audio 
cable Zo is usually somewhere between 80 and 100 ohms.
It's just as well maintaining correctly sourced and terminated cables 
was unnecessary, 'cos they were designed for the wrong impedance.

Cowboy wrote:
> On Thursday 03 January 2008 11:51 pm, Mario Hieb wrote:
>>  Terminating audio lines and other cables is a subject that interests 
>>  me and tends to come up now and again.
> 
> 
>>  So I throw the question out to all of you; why do we terminate 
>>  cables? When should we terminate cables?
> 
>  Depends on why there is a cable there at all !
> 
>  In most cases, the objective is to take a signal, or energy, and
>  simply transport it from one location to the other, unchanged.
>  In other cases, it is desirable to change the signal in some fashion.
>  In any case, the cable itself is a circuit component, with its own
>  characteristics, which may be desirable characteristics, or
>  undesirable characteristics and will have some affect on the
>  signal or energy being transported, which may or may not
>  be significant.
>  If the cable is "short" the effects may be insignificant, in which case
>  the decision to terminate will depend solely on what affect on the
>  signal is created by the termination itself.
>  If the cable is "long" and the effects significant, then the decision
>  to terminate will be dependent on whether it is desired to augment,
>  mitigate, or in some other fashion affect some control on the effect
>  of the cable on the signal, in concert with the affects of the termination.
>  Therefore, the decision to terminate, and with what, is a design
>  decision based on the function of the system as a whole.
> 

-- 
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Peter Smerdon.

Radio 3mp - Melbourne's Easy Music
sen-1116  - Melbourne's Home of Sport

Melbourne, Australia.
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