— The Pan Stage —
Let’s discuss the stereo bus. We have acknowledged some of the results that occur when two versions of the same signal are combined but how do we work with the combination of two completely different signals. If we were to mix them into a mono bus which has the capability of passing only one signal, the two will become one. There will be one signal that possesses all of the sound of the two individual signals at a volume equal to the volume of each one added together. Some frequencies may indeed be affected by phase cancellation but the more different these signals are the less likely it is to be an issue. A stereo bus is a way to combine two signals without losing all of their separateness. By allowing each to reside on their own mono track and to share polarities with one another, a dual track configuration is created that allows us to hear a left and a right and what is called a phantom center. The pan stage is the routing of signals through this configuration. By raising the volume of one, but not the other, the sound will grow louder toward one side of the panoramic field. This is usually controlled by a knob called the pan potentiometer, or, pan pot. The curve at which this knob is calibrated is known as the pan law. Some mixing softwares allow the engineer to adjust the pan law so that a specific type of gear can be emulated, but this isn’t typical. Most engineers just learn to work with the pan pots they’re given.
To understand how a pan stage works, it’s helpful to break it down into mid and side. Just like the recording technique of MS stereo capturing which we touched on earlier. The stereo bus can also be thought of as having a middle section which is the content that is shared by the left and right signals. This content will sound as if it is in the middle between the speakers since it is playing in both at equal volume. As the volume or content changes there is a gradual and consistent leaning toward the left or right side depending on how different the signals are. When they are completely different, then they are opposite both in direction and in phase. This means that the extreme left and the extreme right can be combined in a mono channel and nothing gets lost. Some engineers and many plugins take advantage of a technique called mid to side processing. This is where a left and right track are added together to put the mid on a mono track and one is subtracted from the other by adding its reverse polarity, nullifying what they have in common to put the side also on a mono track. Then the stereo signal can be processed as middle and side. The process can then be reversed to put left and right back together.
One very important quality of a mix is the stereo image. The side of the mix sounds better because it adds size and space to the listening experience. However, things can sound unclear if the mid is not properly represented. There is also an issue of mono compatibility. If there is no mid or if the mid channel is weak, the song can’t play properly through a monaural system. A good mix engineer will take time to listen in both stereo and mono.