Re: [hatari-devel] Use correct white noise

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 03/09/2012 22:56, David Savinkoff a écrit :
> Hi Nicolas,
>
> I saw your message about 20 minutes after you posted it, but
> couldn't reply because talking is more natural than typing.
> Sometimes I can have a conversation, but all that I can type
> is an ACK (acknowledge). :)

Hello

patch is now applied with the 17 stage LFSR (17,14)

> I did some research and found:
>
> (1)
> Only one reference to the pseudo-random noise in the YM2149.
> Someone associated with MAME originally claimed the LFSR is
> 17 bits with taps at bits 17 and 15... MAME was later changed
> to a LFSR with taps at bits 17 and 14. I found an open source
> VHDL project that uses taps at bits 17 and 15 (so does Hatari),
> the VHDL code should be Corrected And Optimized. See
> http://www.fpgaarcade.com/resources/ym2149.zip

I read the VHDL sources of this project too some times ago. My feeling 
is that on some some precise behaviours and very rare cases (ym2149, 
shifter emulation, wd1772 timings, ...) no one really has a real 
reference from the chips' manufacturer.

So, we often have some kind of "consensus" where major emulation 
projects (mame or fpgaarcade for example) are sharing the same methods, 
but without actually fully measuring it on real hardware.
When digging, I found that several projects often had only common 
source, so having the same methods used everywhere didn't mean it was 
the official/real one.

Nice you could measure the taps on a real sample.

BTW, in Hatari the noise generator is initialized with the value "1". Is 
that OK to get a maximum length sequence or should we use a better seed 
? (in the end, I don't think it will affect the perceived noise output 
anyway)


Nicolas