Sound Advice: Matchmaker

Hal Robertson
November 2006

The audio world is full of connectors: big, small, short, long. These essential audio items have been vexing video producers for years.

Why all the different connection versions and how do you get audio from one to another? Balanced or unbalanced? Line level or microphone level? Stereo or mono? And where is that adapter I put in my gear bag yesterday? Believe it or not, there is a method to the madness and a solution for virtually every connection dilemma. This month, we'll do some personality profiles and play audio matchmaker.

There's no simple way to explain the myriad of connection options in the audio world, so let's start with the connectors themselves. The biggest -- and easiest to recognize -- is the XLR connector. Sometimes called a Cannon connector, this is the robust 3-pin interface found at the end of most high-end microphones. With its easy-to-handle size and locking clip, the XLR is a solid performer used for mic and line level signals throughout the professional audio chain.

Next is the 1/4" or Phone plug,. receiving its name from the diameter of the business end and the fact that it was used on old-fashioned telephone switchboards. Phone plugs come in two varieties: unbalanced and balanced. The unbalanced variety has a signal connection at the tip while the rest of the plug is used for a ground connection. We often call the balanced version a TRS plug for its three conductors: tip, ring and sleeve.

A smaller category, called the 1/8" or mini-phone plug, is the same connection as the headphone connection on various portable media equipment and computer soundcards. You'll also find that this is often the only way to plug a microphone into most consumer camcorders. Often maligned and hardly a strong connector, the 1/8" mini plug is considered an unreliable way to route audio. So, why do they use it? It's very small, which frees up valuable real estate on miniature electronics. It's also capable of carrying four connections, which you'll see on some Canon and Sony cameras -- left and right audio plus video and a common ground. Just don't wiggle it too much!

Finally, there's the RCA connector, also called an RCA phono plug. I'll let you guess who invented it. RCA connectors have a center pin for the signal connection surrounded by a circular sleeve for ground. These are the most common connectors used to route audio and video signals into and out of consumer media devices. CD and DVD players are rife with these guys, as are home theater systems, TVs and game consoles. Unfortunately, a few accessory manufacturers use RCAs as power connectors, so look carefully before attaching your valuable video gear.

Here's where it gets complicated. It's possible to find a balanced or unbalanced XLR, 1/4" or 1/8" connection. They may be microphone or line level, stereo or mono -- depending on the devices you're hooking up. And don't forget the male and female versions of every connector, just to double the fun. Now don't panic, XLRs are rarely used for unbalanced or stereo connections. But it can happen, especially if you're dealing with some ancient gear or a tech-head who's gotten loose with a soldering iron.

We bring this up due to a serious issue when using balanced audio devices and stereo audio inputs. A stereo connector uses three points: one connection for the left channel, one for the right and one for ground. A balanced connector uses three points too: one for positive audio, one for inverted (or negative) audio and one for ground. If you accidentally connect a balanced audio source to a stereo input, your recorded audio will vanish when played on a mono speaker system. Confused? Try this visual example. Open a digital image in Photoshop or other image editing program and then make a duplicate layer. Invert the top layer and set its opacity to 50%. See anything? Didn't think so. The inverted layer completely cancels the other layer when they're blended. This is exactly what happens when you combine positive and negative copies of the same audio signal. Still confused? The basic rule: never plug a balanced source directly into a stereo input. Which brings us to our next topic...

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