[LAU] OT: unbalanced to balanced

[James Cameron]

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/