> Jurgen's circuit is of interest, yes. However, I have a general rule:

Yes, indeed, it is interesting. A quick scan suggests to me that apart from the compressor/expander idea to reduce noise, the operating principle is exactly the same, i.e. two banks of all-pass filters to give 'signal' and 'signal with 90 degree phase shift', then multiply by the output from a quadrature oscillator. The interesting bit for me at least is that the oscillator is *not* a sin/cos one, but gives a triangle wave. I must presume that it's easy enough to control such an oscillator, but also the way its fed into the 1496's is intriguing - he seems to be relying on the inherent hyperbolic tan (tanh) relationship through this chip in order to 'convert' the triangle to a (pseudo ) sine wave of sorts which is needed to make the multiplications, and hence the subsequent cancellations of the 'unwanted' sideband, work. One consequence of this is the complete absence of coupling capacitors between osc and 1496 because it runs at such low frequencies. Thus it appears it *is* possible to use the 1496 in such a way to give (again presumption, especially without knowing how much the noise reduction may be needed) frequency shifting using low frequencies. Mmm, this is rather whetting my appetite (what a sad soul I'm turning into), and requires further investigation...

Tim

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