Audio Splitter + 25khz adder box thingy

[Michael Auger]

Kyle,

If i understand the idea to track the fader using 25khz this box should do the trick.... I think.

Will require one of these for the phono into the coumputer

http://www.behringer.com/EN/Products/UFO202.aspx

I think it'd be slick to put a phono + line audio inside of this box so you plug USB to this box USB to Serato / Traktor box and you're good to go.

Thoughts? Questions? 

[Kyle McDonald]

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
the PA.
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.

[Michael Auger]

I got 25kHz from knowing there was some tone that I couldn't remember and just throwing 25kHz on it to show something was added.

Here is an updated version of the picture, this time with more color. ... and straighter lines.

In addition to what you have said, this is splitting the phono output from the turntable and sending that to the computer to track the vinyl position.

[Kyle McDonald]

yeah that's much more understandable. the only clarifications would be:

- i don't think of the SL output as going "through" the injector,
there's just a T junction there. but it works either way.
- the output from SL and output from the mixer are stereo, and it's
important that only one channel is tapped. this is what allows to
distinguish from left volume from right volume. if the fader was
linear we would only need one, but since the fader is not linear we
need both.

[Dan Moore]

>  b. an fpga that is doing analysis in hardware

I have a fairly nice fpga if anyone wants to borrow it to try to do
this. I hate writing VHDL (i.e. I'm terrible at it and forgot it
after I learned it) It is currently in storage and I'll be bringing it
to NYC with the rest of my gear in a couple weeks.

--
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Dan Moore

[quin kennedy]

hmmm, I could try to remember some VHDL...

--
Quin Kennedy
quink...@gmail.com
cell: (949)232-5775

[Dan Moore]

if you want it I'll let you borrow it. I think it is a spartan3

--
-
Dan Moore