the one you link to has a 48 kHz samplerate,so while the 25 kHz is
below the nyquist rate it would still suffer from phasing problems.
for mathematical reasons, you want something only as high as the
samplerate / 4. so in the case of 96 kHz you want a 24 kHz sine (or
square wave). i'm going to refer to it as a 24 kHz wave from now on.
i'm not sure why we were ever calling it a 25 kHz wave.
i'm actually a little confused by your drawing so i'm just going to
explain with english, for a single turntable:
1 the input and output (post crossfader) of the mixer are split. the
input is probably coming from SL, and the output is probably going to
2 the input split has a 24 kHz signal injected ("mixed") via the
split. the signal has a low volume so it does not distort the audio
going to the PA
3 the output split (or "tap") is going back to something that can
analyze the amplitude of the 25kHz signal. this could be:
a. a computer that is doing analysis in software
b. an fpga that is doing analysis in hardware
c. an analog circuit that is amplifying then bandpassing the signal
at 24 kHz (or just high passing at 20kHz) and an arduino that is
reading that value
for two turntables instead of one, it would make sense to simply
inject the left channel on channel 1, and the right channel on channel
2, then tap both the left and right outputs separately.