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Multi-track mixing combines all the tracks you have into a stereo signal. Really two different signals; one intended for the left loudspeaker and the other for the right. You go from several tracks to two, but a curious thing happens when two similar signals are combined. To illustrate this important concept, let's take a look at a few simple sine waves; we'll extend the concept to more musical signals before we're done. If we combine these two sine waves, mix them together, the resulting signal is simply the same frequency sine wave doubled in amplitude.
Each of the two sine waves we're mixing is the same frequency and they're perfectly aligned, moving up and down sinusoidally together. Their interaction is perfectly additive. This is known as constructive interference. But let's change the second wave, sliding it to the right along the time axis by half a cycle. If we combine these two sine waves, the resulting signal is silence. These two waves are pushing and pulling against each other. At every instant one wave is doing the opposite of the other. They are the same frequency, but they're exactly misaligned.
Their interaction is perfectly subtractive. This synchronized form of opposition is called destructive interference. It took a shift to the right down the time axis to make this happen. So not surprisingly, this conversation is about delay. When we mix together any signal with a short delay of itself, constructive and destructive interference will occur. That short delay will align some frequencies for constructive interference and misalign others for destructive interference. Some frequencies get louder, others cancel out, leading to a frequency response curve known as comb filtering.
We see here the frequency footprint of a short delay, and it's a deep footprint, having strong spectral impact. It's called comb filtering exactly because of this image; the frequency response gets chopped up into this pattern, which looks very much like the teeth of a comb. Because of constructive and destructive interference, mixing in a short delay will alter the tone of any track into this pattern of peaks and dips. Some frequencies are enhanced, others are attenuated. In the next few videos we'll take advantage of this to create some really interesting effects, all built on the short delay.
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