Friday, 19 October 2012

On limiting and stereo depth

The main complaint that people have about heavy limiting in the mastering stage is that it reduces dynamic range. This a perfectly well founded complaint but this heavy limiting also has a detrimental effect on other factors in the mix. One of the more overlooked side effects is the way that it damages stereo imaging.

The main way that we perceive a stereo signal is differences in the left and right channels (or speakers). If a sound is identical in both channels and our listening setup is relatively sane, we hear that sound from a, ghost, central position (mono). As if there was only a single speaker centrally playing the sound. If we add changes to the two sources individually and with different values (time shifts, pitch shifts, different eq etc) we hear the signal as stereo. A well crafted mix will have many of these differences and will make use of all of the stereo field to put the sound across.

Now if we consider what a limiter does, it essentially chops off peaks in the signal when it reaches a certain threshold of volume. So lets say that the left side has a signal which fluxuates around -6db and the right side a signal which is simply just a single sine wave hit at -3db. For the period that they are both over a threshold of -7db, both signals will simply be a flat-line at -7db. Not only have we lost dynamic information from either side but we'v lost that important difference between the two, and as we discussed earlier, this is what triggers our perception of a stereo signal. This is why simply slapping a limiter over a mix at the mastering stage can often lead to a more mono sounding mix.

As a side note, this also applies when using a brick wall limiter, and even aggressive compression, on stereo sources within the mix as well as at the mastering stage.

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