[BC] audio cable termination

[Peter Smerdon]

Thanks for the trip down memory lane, Robert.

Ah yes - the "Voltage Transmission for Audio" paper.
That's a classic.
It's still well worth a read.
It opened my eyes back then.

I now wonder, was Richard just documenting an evolving practice or did 
he invent the concept?

Regards,

R A Meuser wrote:
> 
> This is all covered in the original AES paper from 1980.
> 
> http://www.richardhess.com/be/aes-80.htm
> 
> 
> 
> Peter Smerdon wrote:
>> 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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