Mastering - Stereo Image

Imaging in the Mastering Process (click on figure to enlarge)

The next mastering process in the chain of post-fader inserts in the Main Output stereo bus is "imaging".  The iZotope Ozone Imager plug-in follows the EQ plug-in, as shown in the figure above.
 
The spatial balance in the mixed recording is typically discussed in terms of “width” of the stereo image and “depth” of the acoustic listening environment.  The former is primarily set by the  Left (L) – Center (C) – Right (R)  panning of the individual tracks to the Main Output stereo bus.  The latter is primarily set by time-based effects, such as Reverb and Delay, that are added to the mix. 
 
In recorded music, there has been an informal, generally held notion that the stereo image is a re-creation of a stage, an imaginary space stretching between the two speakers and reproducing the reality of live performances.  In classical music recording, this notion has some validity, because the recording techniques used to create the recording usually include the use of a concert hall or stage, and the recording engineers make the evocation of that concert hall ambience a central part of their vision.  However, such an approach is only one of many ways to use stereo to evoke a powerful sense of space and presence.  For example, in popular music, the various recorded tracks can be panned according to a desired musical style:

LCR Panning of Musical Instruments in the Stereo Image

In addition to panning to achieve a full spatial sound, there are techniques to “stereoize” mono tracks.  Such techniques include doubling tracks (either double mic’ing or simply duplicating the track), applying EQ differently on left- and right-panned tracks, and using effects such as Reverb, Delay, Vocal Doubler, and Chorus on stereo buses.
 
The iZotope Ozone Imager on the stereo main output bus works to widen (and sometimes narrow) the stereo field in four different frequency bands by using what is called  “Mid/Side” signal processing.  This Mid/Side signal processing is discussed in detail in the next post for those interested in this more advanced technique in the recording industry. 

iZotope Ozone Stereo Imager Plug-in

The four frequency bands of Ozone’s Imager offer flexibility when widening frequency-rich mixes.  Keeping the bass fairly focused near the center may actually require applying negative gain (in dB) to the bass frequency band to narrow its stereo image.  Applying positive gain (in dB) to the higher frequency bands will widen the top-end and give the illusion of a wide mix.  But like all things in music production, stereo image widening can easily be overdone, leading to disastrous results.  
 
Just as you need a mix of dry and wet sounds to achieve mix depth and a combination of loud and soft elements for broad dynamic range, you need a balance of narrow and wide signals for a mix to appear wide.  Without any narrow signals, the listener has no point of reference for width.  In fact, if every track is stretched to its limits, your mix will sound hollow and will suffer from phasing problems that will create drop-outs in the sound.  Listener ‘fatigue’ will result from a loss in focus and clarity. 
 
Ultimately, a mix that sounds good in mono will ensure a solid translation regardless of the playback device and listening environment.  A critical listening to the mix in mono serves as a quality assurance check for the sound.  If I’ve relied too much on panning and other widening techniques to expand the stereo image of my mix, this becomes clear when I hit the mono switch – phasing problems cause the whole song to shrink and lose presence.  So, a very important caveat here:  image processing must be applied sparingly and subtly !  But when used in just the right way, the imager can help give you an amazingly vibrant, spacious sound.

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