Brian Gaff wrote:

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I know this to be true but have often wondered about the science behind it.
In them old analogue days I built a Mullard circuit for an image width
control. Basically it was a device for adding the sound from the opposite
channel to the other from fully in phase, ie Mono, to completely out of
phase, but in the process, you could often find a position where the stereo
was wider or the sound was more spacious. Of course it did reduce the
central image a bit.. However I built such a control into Goldwwave using
the channel mixing adjustments and yes it works the same way.
The question is, why?



** Most stereo recordings rely on amplitude differences to create apparent positions for sounds when listening to a stereo pair of speakers. Same amplitude and phase in each channel creates an image in the middle of the pair, for an ideally situated listener. A dB or two difference in amplitude moves the image over to the stronger speaker. A 10dB or more difference moves it all the way.

A "width control" used at max ( ie expand) setting sums a reverse phase version of one channel with the other so cancelling any central image on the recording. Sounds that appear left and right of centre do not have equal amplitude and undergo far less attenuation. Sounds that appear purely in one channel suffer no attenuation.

The reverberation on a stereo recording has little correlation in the two channels so is unaffected by the expand setting of a width control.

The net result is as you described, the central image is diminished while far left and right sounds PLUS any reverberation remain the same.


..... Phil