Simple project: audio cable tester (assistance needed)

[Pixie]

Hi,

I want to build this fairly simple audio cable tester, but my electronics skills are fairly limited, so I'd like input on the following idea.

I often find myself disassembling the connectors to determine wether a given cable is balanced or unbalanced, checking to see if it is shielded or not shielded etc.. This is both annoying and grinding the connector parts unnecessarily.

The idea is that the "box" has two XLR (male/female) and two female jack slots. One side is XLR (f)/Jack and the other is XLR (m)/Jack. This should suffice.

When a cable is connected btwn the two sides, some leds may light up:

1. General Failure (one or more of the signal wires are broken, or there's a shortcut not supposed to be there).
2. Wrong wiring (are hot, cold or shield switched)

3. Balanced/Unbalanced cable (determine if shield and "cold" is seperated).

4. Instrument/Speaker cable (somehow detect if cable is shielded).


I've tried to find a diagram for such a device, without luck. I guess the task of determining if a cable is shielded or not is the hardest part.

All comments are welcome.


Best regards, and TIA

Michael

 

[stevez]

I sounds like you need to break this apart - first to understand what tests you want to run in order to verify that the cable functions or to identify what it is. Second you'd figure out how to run those tests. It does seem like a good job for a PIC.

Something I've been pondering is a tester for my coaxial cables. Mine might be much simpler. In an article about testing contacts and other things an author suggested using higher amounts of current (the amount is relative to what you are doing) that might be in a normal continuity checker. Certainly you wouldn't want to burn out the wires. The whole point was that extremely low current flow might not expose a fault.

What I might end up doing is flowing 100 ma or maybe more, thru a circuit with a speaker in it - so I'd hear the noise if a connection were loose. I don't know how much I can flow thru a speaker but you should understand the point. Could do the same with a light bulb I suppose. I'd connect the cables then wiggle them, pull on them, etc - so as to reveal loose or poor connections. All this test would do is verify the performance at DC but most of the failures I experience would be revealed this way.

Measuring the impedance of the cables might reveal something about the condition. One friend of mine does this with a TDR, time domain reflectometer. Google for an explanation. There are several descriptions of home brew TDRs and they don't look all that difficult to build. You basically put a pot at one end - feed pulses from the TDR and adjust the pot for no returns of the pulse. That means the resistance of the pot equals the impedance of the cable so you measure the pot. The impedance might tell you what kind of cable it is.

Another thing you might measure is the insulation resistance between the lines or the sheild. I don't know if the megohm reading on a DVM is all that good - maybe brew your own with slightly higher voltages - possibly some AC.

Just some thoughts on what might be measured.