On 9/8/09, James Cameron <quozl@xxxxxxxxxxxxx> 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 _______________________________________________ Linux-audio-user mailing list Linux-audio-user@xxxxxxxxxxxxxxxxxxxx http://lists.linuxaudio.org/mailman/listinfo/linux-audio-user
