Beyond Stereo Shuffling

We say that the FRANCINSTIEN T-Sym is "beyond shuffling" because it incorporates findings derived from a theoretical analysis of a stereophonic sound reproduction system as a sub-optimal wavefront reconstruction system which post-dates the original research which informed the original Stereo Shuffling techniques by thirty years (Bennet et al. 1985). Bennett, Barker and Edeko's theoretical analysis of a two-loudspeaker stereo system as a flawed "audio hologram" permits us to build a theoretical model for the derivation of the correction signals which the inventors of the original Stereosonic system were denied. Put simply, Shufflers are emprical: FRANCINSTIEN T-Sym is theoretical.

The study by Bennett et al. is interesting because it is a modern and rather mathematical approach. The authors demonstrate that at low frequencies a two-channel system is capable of generating substantially plane waves in the region of the listener with a "tilt" or phase slope determined by the interchannel intensity-difference in the same way that a hologram recreates the amplitude and phase of the light transmitted to the eyes (and is thus able to generate a three-dimensional image). Such a synthetic wavefront is substantially identical to the characteristic of a real wavefront caused by a single, distant sound-source emanating from an angular location within the sound stage bounded by the loudspeakers. A typical two-louspeaker stereo system may thus be considered to be a reliable wavefront-reconstruction spatial-audio system at low-frequencies (up to about 1000Hz).

The authors then go further to analyse why, at higher frequencies, the stereophonic system can no longer maintain its low-frequency performance as a wavefront reconstruction apparatus. Using a spatial version of the Fourier transform they illustrate that, with just two loudspeakers, the wavefront is sub-sampled at higher frequencies leading to aliased, phantom sound-sources in the spatial-domain (in just the same way that temporal sub-sampling leads to aliased frequencies in the frequency-domain).

These aliased sound-sources change their position according to the frequency under consideration and, when the intensity of these sources reaches about 1/10 that of the amplitude of the signals direct from the loudspeakers, they cause the image to shift due to contribution of the phantom source.

FRANCINSTIEN T-Sym compensates for these "rogue" aliased, phantom images by complementary signal processing to the signals fed to the two stereo loudspeakers. The result is a stereo image of unprecedented clarity and "solidity".

References

J.C. Bennett, K. Barker, F.O. Edeko (1985). A new approach to the assessment of stereophonic sound system performance. JAES Vol.33, No:5, May 1985, p. 314-321.

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