Miller, Steve wrote:
>
> Howdy,
>
> Has anyone tried to make a PIC produce white noise in the audio range?  Just
> like Dilbert, they have "Upgraded" us from offices to cubicles.  The noise
 level
> makes thinking difficult.  I want to make a system to produce modulated white
> noise as a masking signal.  I have used audio tapes of the ocean that work ok
 as
> a masking signal, but rewinding the tapes gets to be a hassle when you have to
> do it all day long.   I have a circuit to do this via analog means, but it
> requires a ton of resistors and caps.  Therefore I would like to use a PIC to
> generate all of the signals and then just have a resistive summer and an
 output
> amp to drive a set of headphones.   Any thoughts on how to make the PIC
 generate
> white noise, or "pink" noise?  I can write all of the loops etc, I just do not
> know how to randomize it.  Thanks.
>
        An easy way to make white noise is to use a "psuedo-noise sequence,"
also called a PN sequence or maximal length sequence. This is done by
XORing taps on a shift register and feeding this output back to the
input. You must choose the taps correctly if you want a true PN sequence
and you must not allow the all 0 state or it will get stuck there. I
suggest look at a table in a communication book. The frequency spectrum
will consist of spectral lines (discrete frequencies) spaced at an
interval of fc/(2^N - 1) with nulls at intervals of fc, where fc is the
shift register clock frequency and N is the register length. You want fc
higher than (at least 2 times) the highest audio frequency you want to
cover but you also want N big enough so that the spectral line
frequencies are not audible. A true PN sequence will go through all
possible states except the all 0 state once and only once and then
repeat. These sequences are commonly used for spread-spectrum
communications systems because their autocorrelation properties allow
for easy synchronization. If you want pink noise--noise that decreases
as the frequency rises--just filter the output with an RC filter with a
low cutoff frequency. You will have to amplify to get the signal level
back to where it was without the filter.
        Having said all this it's probably easier not to use a PIC but just use
some shift registers and XOR gates or a PAL.

Dan Mulally