Wiring / sound interference

[Minwah]

I have made custom cables for my cabinet so I can switch video/audio/controls etc with one (printer) switchbox.  The leads are something like the attached pic (excuse the bad sketch).

Problem I have just noticed is that when the metal outer case of the vga and audio jack connectors touch each other it causes some interference - noise can be heard through the speakers (just like if there were a bad connection).  If I hold both connectors and touch them together the problem doesn't happen, presumably as I am creating an earth through myself.

So why is this happening?  Perhaps it is because I do not have the outer shield of the cable connected to anything as it is just foil, no wire as such.  However the video / audio have their own ground's wired in so I wouldn' t think this would be a problem.

Any ideas how I can get around this?

[jcrouse]

Not sure if it's pertinent here but in my cabinets I always use a ground loop isolator to remove any unnecessary noise from the audio circuit. In a couple of cabinets I would get feedback from the power supply coming through the speakers. the ground loop isolator completely removes it. It is about a US $15 item at Radio Shack and should probably be part of all cabinet builds.

http://www.radioshack.com/product.asp?catalog%5Fname=CTLG&product%5Fid=270-054

Also, you may get more feedback (no pun intended ) in the Audio forum.

John

[Mr. Bubblehead]

The audio cable needs to be shielded from the others, it is most likely picking up line noise from the voltage in the keyboard and video cables.  Do you have any interference in the video signal? The VGA cable should be shielded as well.

An unrelated issue that would also cause noise would be if you are using the same DC transformer to power both the speakers and the lighting for the cabinet.

[Abomination]

Yeah, you will probably need shielded cables...  Make sure no wires carrying power run parallel with your speaker cables, if they must cross make them cross at a 90 degree angle.  90% of the time the way you run the wires causes that whine.

[Minwah]

Thanks for the replies.

The problem isn't really constant noise or hum which is what these replies imply (although there is some hum, I'm not bothered about it as it is un-noticable when a system is powered up). 

The problem is when the VGA & audio plug metal cases rub together it creates a 'scratchy' noise like a loose connection.  Since the metal VGA plug case is not connected to anything at all I don't see what is causing the problem.  I could just tape up the metal parts of the connectors and therefore the metal cases would never touch together, but I was hoping to find the cause of the problem if possible...

So just to clarify the problem isn't constant noise, but momentary noise when the vga/audio plug bodies touch each other.

[Minwah]

Something interesting I just noticed...

For my Dreamcast cable the audio is affected as described.  But for my Xbox lead the video is affected but not the audio.

[SeanCyrusTowel]

I don't want to sound ignorant after I post this so here is a disclaimer...

[Minwah]

My guess is that when your printer cable touches the metal it is basically doing the same thing and creating a connection between your audio feed and your video feed.

I think you're right.  I am going to ground the outer part of the vga connectors (I think this is the problem) and change all plugs to plastic ones just in case.  I need to add a connector to the leads anyway so this is good timing - I can test it all out before putting my cab fully back together.

Cheers

[Rocky]

Yeah, you will probably need shielded cables...

[Minwah]

I don't think that is accurate. In almost every arcade game, the speaker wires are bundled with the marquee light power wires.

Well I think the theory is correct (I have heard this many times before).  But in old arcade games (& in mine) the noise generated is probably small enough not to notice, especially when the game is making a noise.

In situations where audio quality is important this is definately good advice tho.  I must admit I used to have my surround speaker cables right next to my sub power cable in some wall trunking.  I can't say it caused any noticable noise...

[BobA]

In arcade cabinets the wires to the speakers carry a signal that is already amplified and therefore is much less affected by noise.  Shielding is very important on low level audio signals that has not been amplified.

BobA