[sdiy] Truly white noise

[Scott Gravenhorst]

Robotboy8 at aol.com wrote:
>
>Content-Transfer-Encoding: 7bit
>
>In light of recent threads I have a few general observations about white 
>noise.  They might be wrong - in which case I'm sure I'll be corrected - and a few 
>of these points are probably common knowledge, but I feel that some on here 
>need reminding.
>
>*Most people only need something that sounds like white noise.  For instance, 
>a detuned radio.  No, it isn't white, but it sounds good enough to mix with a 
>VCO before filtering.

Which is why I mentioned an LFSR with 32 or more bits, clocked nice and fast.  It
sounds like white noise (to me).  I will say that I was not looking at it from a
random voltage standpoint of nonrepeatability, which I think Magnus pointed out. 
However, if the noise source is clocked independantly and simply generates a voltage
that is sampled and held with another independant clock, I think the distinction is
minor.  I think it becomes problematic if you wanted to clock the shift register
once per sample.  Then the sequence of voltages becomes quite predictable and
repeatable.

For a noise sound, I think there is little perceived difference between a good
analog noise source and a good digital noise source.

>*White noise is only necessary (truly white, I mean) for testing, scopes, 
>etc.  Most other applications - well, see above.

As far as I know, yes, this is true, pure white noise probably isn't any more useful
than the "white" noise we make one way or the other for music.  And it goes through
an un-white-ning filter anyway most of the time.

>*Obtaining truly white noise is difficult because most components 
>(amplifying, etc) will color the spectrum.

Yes.

>*To get truly white noise, why don't we just build massive amounts of 
>sinewave oscillators, record them all tuned hundredths of a cent off and phased 
>slightly off from one another, then layer that recording over them all tuned an 
>increment higher until we have literally every frequency we can generate?

This is very much like the spectral composition of an LFSR source, where more bits
is like having more oscillators.  It's not truly white because there aren't an
infinite number of sinewave oscillators, so you've got a spectrum of lumps with
spaces between them.

>*Would this truly white noise not be loopable at whatever smallest number 
>ratio all oscillators have in common?

Hmm.  My guess is yes.

>*Truly white noise would have to be scalable - containing all frequencies in 
>the same amplitude, such that if you pitchshifted it up some it would still be 
>white, because all frequencies a human can hear are still present (those that 
>were subaudible or above audible before are now shifted into audible range as 
>others are shifted out).

Interesting, I think this is true.

>Am I right in these?  Just trying to place myself and help anyone else, not 
>to mention spark some interesting conversation.
>
>       -eric
>

---------------------------------------------------------
- Where merit is not rewarded, excellence fades.
- Hydrogen is pointless without solar.
- What good are laws that only lawyers understand?
- The media's credibility should always be questioned.
- The only good terrorist is a dead terrorist.
- Governments do nothing well, save collect taxes.

-- Scott Gravenhorst | LegoManiac / Lego Trains / RIS 1.5
-- Linux Rex         | RedWebMail by RedStarWare
-- FatMan: home1.gte.net/res0658s/FatMan/
-- NonFatMan: home1.gte.net/res0658s/electronics/
-- Autodidactic Master of Arcane and Hidden Knowledge.