Re: cine sound board

> At 02:00 PM 6/10/97 -0500, you wrote:
>
> Since the goal is not to generate Normally Distributed random numbers, but
> to recreate the sounds used in Cinematronics games, I don't think you going
> to need a 20th order filter. Cinematronics certainly didn't use one in
> Solar Quest, and that game definitely used the Shift register approach in
> generating it's noise source.

        Ahhh, but it is. Your point was that the noise from your LFSR didn't have enough of a low-frequency component. If you want your noise to have more of a low frequency content, you want the mean of your normally distributed sum to be low. There is probably going to be SOME low frequency component from your LFSR generated noise. Add some gain to the passband of your filter, and you boost the low frequency component. Does that make sense? (BTW: 20th order filters, if they are, indeed, necessary aren't THAT bad to do digitally. An analog 20th order filter, though, must be HELL. Typically, the ones on the Cineamtronics Sound boards aren't any higher than 3rd order.)

        I really don't care that the noise is normally generated, but Normal distributions are the easiest to control in terms of their means and variances. I DO care about the mean value of that noise if I want it to have a certain content.
    
> > Anyways, let me know if any of you guys have any info on that noise
> generator...
>
> I've seen the S2688 noise generator replaced with a MM5586 (I think that's
> right) which was a National Semiconductor chip. At one time (many years
> ago) I had the data sheet for one of these two parts (I can't remember
> which, maybe none of the above), and I remember it being a 17 bit shift
> register with feedback. I believe there was also a noisy semiconductor
> noise generator available at the same time, and I don't remember which parts
> are which. (Maybe the "Semiconductor" part needed +/- voltages?)
>
> Solar Quest generates its noise with discreet components using the shift
> register technique. In hardware you implement a shift register, then
> through a few XOR gates you tap into some of the registers outputs to create
> a value that is shifted back into the shift registers input bit.

        Right, in Signal Processing, this is called an LFSR (Linear Feedback Shift Register.) They're used in a lot of the stuff I work with so I'm pretty familiar with their signal processing usage.
 
        I think we're talking about the same thing here. I'm sure that there are techniques to get "Brown" noise out of an LFSR, one of which might be what I described above.
           
Joe
 
Received on Wed Jun 11 11:11:02 1997