Todd,
I am interested to see how it works out. I had a reverse experience some years back where I was using a directional mic to pick up some voice. Worked great in rehearsal, then a live combo showed up for the real performance and although they weren’t loud, they were loud enough to keep the voice from being heard. Luckily, I corrected the oversight in the second performance. In your case, the combo is the 15KHz tone and it is “masking” the background noise.
The reason I stressed that the 15KHz should be a low distortion sine wave is to avoid the possibility of aliasing. For those who may not be familiar with aliasing, it is an undesired effect that occurs in digital signal transmission or recording when the recorded frequency exceeds one-half of the sampling frequency.
Camcorders sample audio at 48KHz (audio CDs sample at 44.1KHz) which means that a camcorder could, theoretically record audio up to 24KHz. But, if a frequency of 24,001 Hz were to be recorded, it would not appear as 24,001, but as 1 Hz. Likewise, if you tried to record 24,100 Hz, it would appear as 100 Hz! These new frequencies are called aliases and are as strong as the original frequencies that caused them. They represent serious distortion.
You don’t hear them because every sort of digital signal processor, including camcorders, includes a sharp cut-off low-pass input filter to greatly attenuate higher frequencies. That, coupled with the fact that most mics don’t have much response at those frequencies means that you normally doen’t experience aliasing. But your case may be different.
When the speaker is silent, the AGC in the camcorder will bring the gain up to the point where your 15 KHz tone is being recorded at full volume. Any harmonics due to distortion will also be present. IF – that’s a big “if” – the camera’s anti-aliasing input filter is not good enough to greatly attenuate these harmonics, you will get aliasing.
Your best bet is to try it and see what happens as it is pretty expensive to try to measure harmonic distortion and calculate it effects. If you get funny distortion or noise look for sine wave distortion caused by your oscillator or an overdriven amp. Also, don’t forget a lowpass filter on the output to get rid of the 15KHz when you play back because young people, especially ladies, can hear 15KHz and find it irritating.
Good Luck!
