Thursday, 21 June 2012

Getting rid of Noise

Last week I got asked to explain the difference between a balanced and unbalanced audio connection. It's surprising how few musicians understand how a balanced connection works and therefore what the pros and cons of balanced vs unbalanced connections are. Most people think it has to do with the quality of the cable, which is completely wrong.

The knowledge can save you a world of pain when it comes to resolving unwanted hum and noise in both your instrument and PA system. 

An unbalanced signal is the most simple and commonly encountered audio connection as it is found on most electronic instruments including guitars. Guitar leads are un-balanced cables and have two wires in them. The black wire is called ground and it has absolutely nothing on it. It is simply connected to the earth (yes the actual dirt) and acts as a reference point of zero volts. 

The red wire has the actual signal on it represented by a positive and negative voltage relative to the ground wire.

Think of ground as being "sea level" and the signal as being the height of a mountain we are trying to measure. We can compare the heights of any two mountains in the world because they are all measured relative to their height above the sea. 

Audio works the same way, but one big problem is that our sea level isn't exactly the same everywhere you go. In some rural areas power companies transmit power through the ground using one wire above the ground and using the ground itself as the other wire. This (along with a whole host of other reasons) causes the voltage measured between two points in the earth to be different. It can even vary from one side of a venue to the other which for a musician is a real pain in the butt.

If you have two power points in a venue and half your gear is plugged into one and the other half plugged into the other and those power points have a different earth point then you get a different ground voltage (sea level) for different equipment in the same PA system. This comes through the PA as an 'Earth Hum' and is literally the sound of the earth humming. It is like putting a stethoscope on the planet. 

To compound the problem, humans are addicted to wireless things. TV, Radio, Microwave Ovens, Satellites, and Cellphones all emit radio and electromagnetic radiation. When these waves hit metal, they generate a tiny voltage. If the metal is long and thin then it acts like an antenna and guess what we musicians like to use a lot of that is made of metal and is long and thin?  

Ok, insert your 'long and thin' jokes here, but the actual answer is, guitar leads. 

Guitar leads are basically perfect antennas. All that electromagnetic radiation that is flying around the place crashes into your guitar leads and induces a tiny voltage on both the red and the black wire which you hear as noise in your amp. The net result is that the longer and poorer quality your un-balanced cables are, the more noise they will pick up and the worse the noise will get. When you use lots of un-balanced cables together in your PA system then the noise all adds together and you end up with a bit of a mess of hums, hiss, buzzes and in the worst possible scenario - Breeze FM coming through your equipment.

So what is the alternative?
The answer is to used balanced cables wherever you can and here is why.
Balanced audio cables have 3 wires instead of two.
The shield wire is a special braided mesh that surrounds the enclosed wires, it also is connected to the ground so acts as our "Sea Level".
The red wire carries the signal, just like in our unbalanced cable.
The white wire carries an exact copy of the signal but 'flipped'. If the red wire has a positive 5 volt signal on it, then the white wire has a negative 5 volts on it (referenced to sea level). Its like digging a  hole in the ground that perfectly matches the contours of the mountain that we are measuring. 

Now, when all that electromagnetic noise hits our cable, it induces the same voltage in both the red and the white wires at the same time.

At the other end of the wire, our mixing desk cleverly flips the signal on the white wire back the other way and then adds it to the red wire. Think of it as pouring clay into the hole we have dug and then superimposing the clay mould on our original mountain.

Now the original red and white signals match each other and add together to make a stronger copy of the original signal, however, the noise on the white wire is flipped the opposite way to the noise on the red wire and perfectly cancels it out resulting in absolutely nothing but a perfect copy of the original signal.
Using this method, I have transmitted audio over several kilometres without any noise, hums or buzzes.

Its pretty clever, and also the reason why you should use balanced cables and equipment wherever possible to minimise noise.

This is why, on stage we use DI (Direct Injection) boxes on anything that has an unbalanced connection. Keyboards, Acoustic Guitars, Samplers, CD players or even iPods etc are the usual suspects. If you want the best sound possible then you need to convert an un-balanced signal into a balanced signal as quickly as possible and using the shortest possible un-balanced cables you can.
Get rid of those long guitar leads and if you use powered speakers, then connect them to your mixing desk using XLR cables not jack cables.

Whenever possible, look for equipment with balanced audio outputs as opposed to unbalanced outputs. Professional equipment usually comes with balanced connectors while cheaper equipment may not offer that option. Also note that sometimes professional equipment (most often keyboards), may offer both balanced and unbalanced connections on the same jack. This is called a TRS output and is a jack cable with 3 rings instead of 2. You can then use a cheap TRS to XLR converter to convert the jack into a mic lead and therefore avoid using DI boxes on this equipment but you may need to look at your equipment user manual to find out if it has balanced connectors.

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