On Sun, 9 Jun 2002, Russell McMahon wrote:

> I could provide component values for say 2 and 4 poles of low pass if you
> are interested.

I am interested.  I did some searching & reading and came up with this:

----/\/\/\/\------|-----
     3.3K         |
                  = .47uF
                  |
                 GND

If I read the stuff I found on passive LPF design right, that should give
me a 102Hz cutoff, correct?  But I'm not sure what effect the bias
resistor(s) for the transistor will have.  I will gladly take advice and
suggestions on this matter.

I changed the tag to [EE] because we've drifted pretty much completely
away from the PIC aspect, but here's a quick overview of the PIC side.

The PIC (I'm using a 12CE674 for development, but will switch to 12C671
for real) looks for a low pulse on its input, which *should* be the peak
of the cycle of the string's fundamental frequency.  We start Timer 0
running and wait for the next pulse, indicating the next cycle.  If Timer
0 overflows, we increment a high-byte counter, ending up with a 16 bit
counter for the cycle time.  We accumulate 32 of these samples, then
divide by 32 for an average (crude, yes, I know - I may work on that).

Now comes the interesting part - since we know the desired freq of each
string, we look to see if it's more than halfway between the lowest (E)
and the next highest (A).  If so, look to see if we're more than halfway
between that and the next highest (D).  We keep doing that until we figure
out which string has been plucked, and use two LEDs to indicate whether
we're below or above that string's desired frequency.  There's a little
hysterisis so you can be within some range - right now I have it set up
for 6.25% (1/16 of the cycle time) for testing.  I don't know how close
"close enough" is when tuning a guitar, so we'll play with that once we
have it working; it's a simple software change.

The automagic string selection is neat, but also means means you could
tune, for example, the D string to a G (assuming you don't snap the bloody
thing in the process), so you have to be more or less in the ballpark to
begin with.

It's really a pretty neat little gadget, we're planning to connect it to
the amp input and mount the LEDs on the amp control panel.  The amp we're
building will use a National LM3875 driving an 8" full-range speaker.
We're roping Grandpa in to do the woodworking for the cabinet.  Figure if
he gets serious about it (who knows, the kid's 13) we can always build him
a tough little tube amp.

Dale

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