I encode data in audio by energising 5
frequencies between 17.5kHz
and 19.5kHz
:
I play the WAV on a sports stadium (20k seating, so BIG) PA rig, placing a microphone somewhere in the spectator seating area.
This is the data (I did it at vol=0.6
and vol=1
):
As can be seen, there is some kind of 'ringing' occurring. Now the frequency channels are using are all integer multiples of a fundamental frequency (~100Hz
), and the ghost frequencies appear at exact harmonics. If I crank up the volume I also get energy midway between harmonics.
There is one further effect: Ghosting at 1/2 f
. This is most visible if I analyse a sweep (vol=0.8
:
As can be seen, much of the energy at frequency f
has been redirected to 1/2 f
.
Can anyone explain these unwanted artefacts?
PS Often I observe a perfectly valid SE question attracts several instant down/close-votes before making it back to the surface. It always makes me scratch my head a little. It seems rather feeble to downvote without supplying a comment to summarise the decision logic. e.g. Is it OT / malformed / inarticulate / lazy / etc, and how so?
PPS Someone has now suggested _intermodulation_ in a comment. Now this is valuable information. The question together with this keyword, once catalogued by Google, adds to the effectiveness of the Internet as a resource for Information. Yet the question is teetering on 4 (un--publicly-justified) close-votes. I do not think this is right. :/