"Conclusion: Digital recording of a theremin needs perfect low-pass filtering before sampling AND perfect grounding and shielding to avoid stray interference - a thing which is very difficult to realize!" - Thierry
I agree with everything you say Thierry -
But I dont think its quite as hopeless as you (unintentionally, I believe) seem to convey. "perfect low-pass filtering" and "perfect grounding and shielding" are impossible to obtain - but IMO "perfection" is not required - "Extremely good" is usually good enough.
I believe that your statement "Due to lazy output filtering in the theremin" is the primary source of the problem.
Alas, care and attention to detail is seriously lacking in most theremin designs when it comes to interfacing the output to studio equipment - they are designed to drive an analogue amplifier, with the fact that technology has changed since Lev's days, and that one should now expect that users will want to interface their theremin to digital (sampling and class D amplifiers for example) being overlooked or ignored.
If the final output stage of a theremin incorperates a good filter, and if the connection between this stage and the output socket is correctly shielded, then in my expierience almost all Nyquist related issues drop to acceptable levels.
Other things which greatly reduce the problems is high quality screened lead from the theremin, and ensuring that the primary ground connection between the theremin and equipment is not via the screen (in fact, I find that best results are obtained if the screen is disconnected from the plug at either the theremin OR the amplifier end (NOT BOTH!), and a seperate ground wire is run from the theremin to the equipment - this way there is no conduction path through the screen so no unwanted inductively induced ground signal coupling to the signal wire can occur)
With the above, if the theremin is kept a reasonable distance from the external equipment (so that radiated signals are not picked up strongly by this equipment) one can get a clean digital recording.
But it all starts at the theremin output - If the signal here is full of HF, nothing one does will have a major impact.. But even so, good screened cable, seperate ground wire (and perhaps removing screen connection to plug at amplifier OR theremin end) and distancing the theremin from the equipment, can improve things somewhat.
Regarding the earth connection -
If one does not have easy access to an earthing point on the theremin or equipment, one can take this off the plugs, and run the seperate earth wire along the lead.. I will draw a diagram and post it here ASAP.
It is actually better to run the ground wire as a seperate wire away from the screened cable, and certainly DO NOT wrap this wire 'round the screened cable.. Coupling will be greatly reduced because the screen is between the ground wire and the signal wire, but this will not completely eliminate any inductive coupling (only the fact that the distance between the signal wire and any ground current flow has been increased, will reduce inductive coupling).
Also - Good plugs with metal shells NOT plastic shells! - It really is worth spending money on the lead!