Field of the invention
The present invention relates to a non-recursive system for expanding the stereo base of acoustic stereophonic diffusion apparatus. 
As is known when the distance between the two loudspeakers of an acoustic stereophonic diffusion apparatus is relatively small (e.g. 60-70 cm.)  the stereophonic effect is almost imperceptible, or anyway inadequate, for listeners positioned some distance away from the loudspeakers.  In order to simulate the effect that would occur with speakers mutually set further apart, the processing is therefore known of the input acoustic signals of the system, to improve their stereophonic characteristics, thus performing the so-called "expansion of the stereo base". 
The conventional method for achieving said processing is to use a "recursive" technique, in which a 180.degree.  out of phase crossed crosstalk is created between the two acoustic channels (left and right), usually by subtracting the output signal of each channel from the input signal of the other channel, with an appropriate amplification or attenuation of said subtracted signals.  In practice such crosstalk is imposed only on a part of the total band of the signal, in order to avoid disadvantages related to the propagation of acoustic waves and to the physiology of listening, which are well known to those skilled in the art. 
Therefore, the crossing paths of the signals fed back to the input comprise band pass filters, and generally correction filters are included in the two direct channels as well.  Both the two direct channels and the crossed crosstalk paths can therefore be considered as filters (active or passive), and when, in the continuation of the description, amplifiers will be mentioned, in any case it will be meant that the provided amplification may also comprise a desired filtering out. 
Systems of the abovementioned kind are described, as an example, in "DIGIT 2000 - VLSI Digital TV System", ITT Semiconductors, Publication Order No.  6251-190-2E, page 6.13, August 1982, or in "The German 2-Carrier System for Terrestrial TV-Sound Transmission Systems and Integrated Circuits for `High-Quality` TV Receivers", by U. Buhse, in IEEE Transactions on Consumer Electronics, Vol.  CE-28, No.  4, page 489, November 1982. 
Such known solutions, since they are all of the recursive type, require delicate design and tuning, and may give rise to instability.  Furthermore, they require four amplifier (and/or filtering) blocs, two of which for the direct channels (i.e. for driving the downstream stages of the system), and the other two for the signals being fedback to provide the expansion.  The complexity of such blocs depends on the complexity of the filtering functions which it is desired to obtain.