I must disagree with one of Nika's points:
<BLOCKQUOTE><font size="1" face="Verdana, Arial">quote:<HR>
Originally posted by Nika:
The creation of these subtones is a characteristic of the ear, and the ear is unable to hear any subtone frequencies caused by inter modulation distortion if EITHER root component is above the audible frequencies of the ear. Thus, a 24kHz and a 25kHz tone will NOT create an aubible 1kHz tone. A 22kHz tone and a 15kHz tone will NOT create an audible 7 kHz tone.
<HR></BLOCKQUOTE>
This is simply not correct. If the creation of these subtones were a function of the ear, and are not an actual electrical component of the output signal resulting from the mixing of two tones, then the science behind that could be applied just as correctly to higher frequencies as well. If that were true then a superhetrodyne radio receiver would not work.
To simplify, a superhet combines radio waves from an antenna with a set Intermediate Frequency ocsillator and then filters out all but either the sum or the difference frequency, leaving only one of these (supposedly only produced in the human ear) subtones for further processing in the receiver.
No, these subtones, the sum and difference frequencies of the two mixed tones, are a real electrical component of the output signal. Mix two sine waves, both within range of your digital system, and also whose sum and difference frequencies lie within your digital range, and look at the result in a spectrum analyzer. You will see all 4 frequencies!
Mike