A motion sensor is a sensor that detects motion - The Theremin sensor detects capacitance equating to proximity.
A Theremin is not a motion sensor - it can be converted to a motion sensor by adding a circuit to extract rate of change (delta) of the signal - Capacitive motion sensors operate like this.. slow changes generally are ignored or used as a new 'baseline reference' - but changes which meet the rate of change / amount of change criterion are output as "motion detected".
We could use the change in pitch as a motion detector, and do human interpretation of this to extract meaning.. But we will still hear a pitch from a theremin if we are perfectly still - we cam play a constant note without having to move - so motion sensing is not the primary function.
[i]
"I'd agree with Bob in that the theremin is NOT a synthesizer, in the sense that a synthesizer is supposed to "synthesize" the sounds of acoustically vibrating instruments - at least, that was what the guys at RCA were after in developing the Mark IV." - Joe
[/i]
Absolute disagreement from me on everything in the above.
Sound is sound, and synthesis is synthesis - the source of the sound to be synthesised is irrelevant in this context - who says [i] "a synthesizer is supposed to "synthesize" the sounds of acoustically vibrating instruments" [/i] ?
[b] Edit --> [/b]
Any simple sine wave audio oscillator (the most basic form of sound) which can be controlled, qualifies as a "synthesiser" - it is synthesising a sound. If one wants to exclude the simple sine-wave oscillator, and say it is not a synthesiser, you need to define the 'line' behind which things are not "synthesisers" and beyond which, they are..
On what would one base such a line? The number of parameters of sound which can be modified? the complexity of the circuit? how much it costs? whether it uses digital or analogue circuitry (LOL!) ?
Lets say one decided that modification of waveform (harmonics) was required.. The Theremin would qualify.
Lets say one decided that DYNAMIC modification of waveform (harmonics) was required.. The Theremin would qualify.
Lets say one decided that modification of amplitude was required.. The Theremin would qualify.
Lets say one decided that synthesis of "the sounds of acoustically vibrating instruments" was required .. The Theremin would qualify here just as much in some classes of instrument as many other "Real Synthesisers" (LOL).. (try synthesising a violin being played off-key and erratically hunting for the right note, and you will do a better synthesis of this using an EW standard than by using a MiniMoog! :-)
Lets say one decided that PERFECTLY ACCURATE synthesis of "the sounds of acoustically vibrating instruments" was required .. There would then be no such thing as a "synthesiser" !