Here is a technical article on the Bride of FRANCINSTIEN process. Technical information on the FRANCINSTIEN T-Sym process is given here.
Leakey (JASA entitled, Measurements on the Effects of Interchannel Intensity and Time Differences in Two Channel Sound Systems (JASA Vol 31, Number 7 July 1959.) derived mathematical models to predict the sound image position in a stereo listening arrangement. At LF the image position is given by,
At HF the following, more complicated, law obtains,
where α is the angle of the perceived image, and θ is the angle of the speaker from the median line; the latter being 30o in a standard stereo arrangement. (The exponential terms in the HF expression derive from the psycho-physics which indicate that at HF the ear is sensitive to the envelope of the signal - see Leakey)
Leakey's two models are plotted on the same axes here. The ordinate (y-axis) represents α - the horizontal, angular displacement of the virtual image from the median line, and the abscissa represents channel intensity difference in dB:
The overall point is simply made.
For a given channel intensity difference, the HF components of an instrumental or vocal contribution will subtend a greater angle at the listening position than will the LF components.
We can illustrate the effect of the Shuffler and the original FRANCINSTIEN circuit by substituting the corrected values into the same model. The results are given below.
The curves demonstrate the benefits over untreated signals; especially in the all-important central region of the stereo image. However, following in EMI's footsteps, the original FRANCINSTIEN actually over-compensated the HF image; causing it to fall inside the LF image at the extreme image positions.
Is it possible to engineer a frequency-dependent channel-intensity modification so as to bring the two models closer and effect a better match for the LF and HF image?
The answer is, yes, and the effect of reducing the difference channel gain by a new (Bride of FRANCINSTIEN) function is illustrated here:
Which illustrates that an almost perfect match is obtained.
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