[BC] audio cable termination

[Ron Youvan]

   Terminating any cable also lowers the overall impedances and reduces the crosstalk
from other nearby circuits and RF in general.
   600 Ohms was also the impedance of the twisted AWG 19 cotton covered twisted pairs
in the original lead covered telephone cables.  I was told it exceeds 900 Ohms by a
Verizon engineer with the current AWG 24 (or smaller) with polyethylene insulation.
-- 
   Ron


> 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.

>> 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.
-- 
    Ron  KA4INM - My wife never had any problem spending money
                  her problem was just with me spending money!