L+R/2 is almost universally used. FM broadcast is trasmitted this way for
comptibility with monaural receivers. The "mono" switch on preamplifiers
does this. However, this simple, almost universally used transformation
does not provide a perfect mono version of a stereo source. The reason is
that stereo channels add by *power* acoustically, whereas the electrical sum
L+R/2 adds by *voltage*. Therefore, when recordings mixed with prominent
material in the center of the sound image, such as vocals, bass, etc. are
electrically mixed to mono (L+R/2), the sounds in the center of the stereo
will be 3dB higher in the resultant mono mix compared with those at the left
or right edge (known in the recording industry as the "3dB center buildup").
How to solve this? sqrt(L^2+R^2) is one method. This method would be
particluarly good for a DSP implementation. Another method is to rotate
the phase of the channels with an "all-pass" filter so that they are 90
degrees apart and add in quadrature. This method is better for an analog
implementation.
I'd love to see a deck that implemented the first solution!
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