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If your computer has any sort of anti-virus software loaded, it's probably always running in the background.

If that's the case, and the program decides it needs to do some sort of system sweep (they do these things on their own schedule), it might cause the problem you're describing.  To get around this, disable the software when you're going to be performing.  Help files included with your security software can help you there.  There are also various Windows 'system maintanance' things that might try to start up and would cause the same thing.

The next thing to try when you’re having problems like this is whether the problem is coming from the AC power or if it’s related to the laptop.  To do that, just try unplugging the power supply so it will run from the battery.  When you unplug it, if it’s a power problem then you’ll hear the noise go away immediately.  My guess would be that this is the problem you’re having. 

If that’s the case there are two things you can do to fix it.  The first is to plug the laptop into the board using a direct box.  This can solve several issues that can create ‘noise’ in the line.  Here’s an article from Musician’s Friend that talks about that sort of stuff.  The next thing to try would be to use a filter to clean up the noise that can come from AC power interference.  Here’s an inexpensive device from Radio Shack that will catch that.  (They call it a ground loop isolator, but it’s actually a filter).

Emcee Pro and laptop noise can be fixed. First off, it is not a problem with the software of Emcee Pro, but has to do with one of two things:

1.  Ground loops between the laptop and the sound system

2.  Digital clocking noise of the laptop itself.

In any sound system there should only be one path for ground. For example, a microphone connected to a mixer input has only the one ground and the balanced plus and minus signals of the audio itself, running between microphone and the sound system. In this example no ground loop can exist. However, if you were to run a wire from the case of the microphone to the ground lug of a wall AC outlet, you would then have two independent ground paths back to the mixer.

The short path between down the mic cable. from the microphone and onto the mixer input, and the long path from microphone to wall socket, to breaker box, to the wall outlet the mixer is connected to, and eventually back to the mixer. The voltage potential between these two ground paths (and there will always be a voltage potential between various ground paths) becomes amplified by the mic-pre amplifier gain. The result is the 60 cycle line frequency is now quite audible in the sound system, and multiplies of that line frequency as well, such as 120 cycles, 180 cycles, 240 cycles. You might say, "I would never connect a ground from the mic case to a wall outlet". True enough. But when you connect a laptop computer, a mini-disc or whatever to a different wall outlet than that which is used for the mixer and amplifiers in your system, you have essentially done the exact same thing. For you now have two ground paths of unequal length and resistance. Thus a voltage potential between the two paths.

The proper method for connecting AC power is to use only one AC outlet for EVERYTHING you intend to connect to the Mixer. Everything connected to your sound system should derive its power from ONE source only. This includes laptops, mini-discs, guitar amplifiers, in-ear monitors, etc. Even then, you may have a ground loop condition on laptops, for virtually all laptops use a switching power supply in order to conserve battery drain. This type power supply turns off and on many hundreds of times per second. This switching power supply scheme can, and many times does produce a phantom ground that rides (in impedance) somewhat above true ground. In other words, it has an inherent manufactured ground that is at some voltage potential above true ground.  This is why a transformered direct box tends to help this situation considerably.

The secondary of the direct box transformer is automatically made an extension of the mixer ground. The audio portion of the signal from the laptop is tied to the ground of the laptop, but is not necessarily connected through the direct box and on to the mixer. Instead it is fed to the mixer via the transformer windings so that any AC or DC potential is lost in the windings of the first stage of the direct box. (This also blocks phantom power from reaching the laptop as transformers will not pass DC voltage.)

Now, it is best not to use a true direct box but instead a line to line isolation box. A true direct box is intended for a source impedance of 100k ohm or so, such as found on the output of a guitar pickup. The direct box transformer has a loss of about 50:1 or greater, so that the mixer "sees" the guitar as having a very low impedance of about 150 ohms or so......about the same as a typical microphone.

The output impedance of a laptop sound card is far lower than a guitar, many times less than 40 ohms. When this is ran through a typical direct box, the source impedance the mixer sees maybe almost a dead short, or less than 1 ohm.

A better method is to buy a stereo line to line isolation box. These look and feel just like a normal direct box, but they have a winding that is typically 1:1. In other words, the impedance of the output of the laptop is mirrored across the transformer and onto the mixer. It still has all the wonderful attributes of isolating the grounds one from another, but it does so without shunting the impedance down to nearly nothing. The result is a cleaner sounding signal with far more detail.

The ground lift switch on these devices should almost always be turned off, or in lift mode. This keeps the switching power supply ground noise from being transferred to the house sound system.

The other source of noise might just be the switching noise of the laptop getting into the audio and sent out as an audible signal. This is called crosstalk. If this is the case, it will not matter if you use a direct box or not as the noise will still be there. If this is your problem an external sound card, like the Turtle Beach USB device, will go a long way in cleaning up the audio.



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