Re: [LAU] OT: unbalanced to balanced

On 9/8/09, James Cameron <quozl@email-addr-hidden> wrote:
> On Tue, Sep 08, 2009 at 05:16:00PM -0700, sevol wrote:
>> hmmm, wouldn't any TRS cable plugged into equipment with balanced
>> connections create balanced interconnects? If not, how would you
>> distinguish between a TRS unbalanced & TRS balanced cable from jsut
>> looking at them? I'm wondering.
>
> It is not possible to distinguish by looking at the cable, nor even by
> electrical means ... it is not up to the cable alone. Whether the
> system (the output, the cable, and the input) is balanced depends on all
> three components, not just the cable.
>
> As an example, an XLR connector and cable, common in stage microphone
> setups, can be used in an unbalanced fashion. Either through accident
> or intentionally.
>
> A TRS to TRS cable where the sleeve is anchored to the shield ... can be
> used as a balanced cable for one audio channel ... if the devices at
> either end are in agreement about it. It is *normally* used as an
> unbalanced cable for two audio channels.
>
> The usual audio cable is two cores with a shield ... and when it is
> used in a balanced configuration the shield is excluded from the audio
> signal path. A TRS cable is normally thus.
>
> A cable with a shield and one core cannot easily be used as a balanced
> line, since the shield will pick up a different amount of noise compared
> to the core.
>
> There is equipment out there that carefully isolates the sleeve of the
> socket from the power supply protective earth, and this reduces hum and
> may well be said to be "balanced".
>
> To test audio cables, I use a multimeter and a sewing needle:
>
> 1. test the multimeter by checking that it reads no connection when the
> probes aren't connected, and connection when the probes are connected,
>
> 2. check for isolation between each of the plug sections; tip, ring,
> and sleeve,
>
> 3. check for connection between each of the corresponding plug sections
> at either end of the cable, tip to tip, ring to ring, sleeve to sleeve,
>
> 4. stick the needle just under the surface of the cable, and check that
> the cable shield has a connection to the plug sleeve.
>
> (I'm talking above about audio frequencies. At radio frequencies, an
> unbalanced transmission line is something like coax, and balanced is
> ribbon.)
>
> --
> James Cameron
> http://quozl.linux.org.au/

Thanks much. I'm aware of enough to get in trouble. This is a help.

A few 2nd-hand stores here sometimes have had audio cables. Mostly I
pass, but picked up a pair of mono cables with 1/4 jacks. The metal
jacks alone are very decent & worth more than I paid. Actually, the
cables work fine.

So, if good TRS cables turn up (or my system develops cabling
problems)... your testing procedure will be useful.

-s
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Received on Wed Sep 9 08:15:01 2009