Re: LFSR flat frequency

From: Ian Stirling (root_at_mauve.demon.co.uk)
Date: 11/23/04 

Date: 23 Nov 2004 15:17:01 GMT

Rich Grise <rich@example.net> wrote:
<snip>
> I'm wondering how you get from the LFSR output into the frequency
> domain - just take the bit stream and act like it's an analog signal?
> Maybe LPF it? What would happen if, each clock cycle, you latched the
> register value into a DAC? What kind of difference might that make? Take
> the DAC output for your signal, of course.
>
> I've "listened" to a bit stream, and it definitely isn't pink
> noise! :-) It's kinda scraxxy, and (this was only about 16 bits)
> clearly periodic. I don't remember if I tried it with a DAC,
> but if I did, it must not have sounded much different or I'd have
> remembered that I had and it did. ;-)

You need to use a longer register.

16 bits (at 44Khz) repeats after a second or so, and is audibly periodic.
Add maybe 3 or 4 more bits, and you don't hear the periodicity.
Add another 16, and it repeats about once a day.
It eases the filtering a lot if you go to a 64 bit register, clocked at a
few Mhz, and just stick a R/C filter on the output.

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