[sdiy] Advices for ringmod and noise source

[Czech Martin]

> Pro analog:
> o It's analog.


The real feature is: it's unpredictable. 
The shift register noise generator is 100% determined.
If you start it, it will always do the same thing.
Applies also to most random functions in software.
Often you can set a "seed", this will perfectly determine
the following sequence. Of course, you can try to insert
analog noise into the shift register, to give more randomness.

A 32bit shift register clocked with 1MHz will take about 1 hour
to repeat.

The analog noise is not predictable in terms of most musical
applications. 

This can be important if you use the noise to trigger things
for hours, like in an installation that is continuously running.

It has also some philosophical background: trying to indroduce
randomness by using a finite state machine that is perfectly
cyclical is strange.

For other purposes it is really hard to get 
real randomness. You often end up with plumbing. I.e. massive
brass enclosures and the like, in order to supress any correlation
to outside world effects.




> Con analog:
> o High gain, high impedance required to amplify the noise.  
> This amounts to
>   hum and radio station pickup being problematic.

Microphone amplifiers have the same problems, so this is
nothing unusual and the remedy is clear.

> o Transistor or diode selection is critical, avoid popcorn noise.

This is the real con against analog. 
And 1/f noise can be a problem, too.

> Con LFSR digital:
> o The spectrum is not continuous, rather it's a series of 
> lumps, the more bits
>   the more lumps, faster clock moves the lumps together.

I think it is harmonics. After all it is a perfect cyclical
waveform. For a very short sequence (1s) 
the "fundamental" will be 1Hz, and the partials will have 1Hz distance.
I think this is impossible to hear, you hear a continuous band
instead. Of course, the repetition of 1s is audible, but that is another thing.

The white feature of the signal will only be visible after many
spectral measurments and averaging.

If one wants to avoid this, there is a trick: Create a Fourier spectral
representation (a DFT table). All amplitudes set to 1.0.
But give the phase random numbers. The inverse will be a pretty noise
signal, and the average will tend much faster to the desired whiteness.

This is a nice example that phase really counts. If you take the same
amplitude distribution, but other phase, you get an impulse, something that
will certainly not sound like noise!!

m.c.