The basic approach to creating a useful and reliable stereo
effect from a
mono source that remains mono-compatible is based on using Middle and Sides (M&S) techniques. Consider the
mono source as the M signal, to which we need to add a 'fake' S signal. The resulting pair will then need to be decoded back into left and right for mixing into the stereo
track, of course. The advantage is that the S signal disappears in
mono, so the original source
track remains undamaged for the
mono listener, and hopefully usefully enhanced for the stereo listener.
At the most simplistic
level, the primary difference between the M and S signals is time. Imagine a source in front of an M&S mic: the direct signal reaches the M mic, while the same signal has to travel to and
bounce back from a wall to reach the sideways-facing S mic. Consequently, all we need to create a useful fake S signal is delay the original by a small amount: a simple
mono delay
line is all that's required.
Take the source, split it in two and route one to the centre of the stereo output, and the other to a delay
line. In practice, I like to pass it through a high-pass filter first and
roll off the
bass below about 150Hz, as I find that produces a better
effect; it can become bass-heavy on the left
channel if you don't do this, depending on the source material.
The secondary M signal is routed through a high-pass filter and into a delay
line. The minimum delay I would use is about 7ms, extending up to about 25ms, but feel free to experiment. With classical music, you could go as high as 80ms to create a kind of symphonic
hall effect. Changing the length of the delay changes the character of the stereo
effect. The output of the delay
line is the fake S signal, and this now needs to be combined with the M signal to decode back into stereo.
Split the output of the delay
line into two signals and phase-invert one of them. Route the normal output to the left
channel mix buss, and the inverted one to the right
channel mix buss. Ensure that the amount of +S is exactly the same as the amount of -S, so that when summing left and right channels in
mono you cancel the S signal completely: (S+(-S))=0.
The easiest way to do this is to route the two S signals through a stereo
fader, which also allows you to control the apparent width of the stereo
effect — but don't go mad. The result can sound impressive, but it gets tiring quickly, and the
mono and stereo mixes will sound radically different if you add too much.
Hope that helps. This technique is used very widely in broadcast post-production and in mastering when 'up-converting' archive
mono to stereo, or stereo to surround.