[sdiy] PRN Noise Gennie Question

[James J. Clark]

>   > http://www.oldcrows.net/~patchell/synthmodulesII/200-1021.pdf
>   >
>   > In the above schematic, I use a 32 bit PRN source, and tap off 8 bits to
>   > run to a DAC...on this application, I did it this way so I could use the
>   > DAC output got generate control voltages in predictable sized steps....
>
>Jim,
>
>Um, it's no longer white noise then, right?  Running the shift
>register outputs to the DAC effectively creates a digital filter, so
>the result is filtered white noise.  In this case a single pulse
>through the shift register will show up as an exponentially decaying
>impulse response.

Or as an exponentially increasing one, depending on how you arrange the
LSB/MSB order of the DAC.

>
>I have not done the math so I don't know what the effect of this
>filtering is.  (Anybody?)

This is a good point. The output of the DAC, if run at the same rate as
the PRN source, will have a lowpass spectrum. But, if you have an 8-bit
DAC with a very long PRN source, you can get a (nearly) white signal by
updating the DAC at 1/8th the clock rate of the PRN. This is because the
input to successive DAC conversions will not contain any bits in common,
they will all be decorrelated. If you have an N-bit DAC you would update
the DAC at 1/Nth of the PRN clock rate.

Note that this type of "filter", while linear and having a lowpass
characteristic, can be un-done (deconvolved) in a perfect noise-free
fashion by applying an ADC giving the individual input bitstream again.
Most filters cannot be reliably de-convolved. What that is good for, I
can't say!

Jim