Re: LF: Related Technical Query - Soundcard calibrator

["Stewart Nelson" <sn@scgroup.com>]

Hi Andy and all,

 

I was unaware of H&H, but my previous post gave the same answer.  It was based

on my naive analysis that white noise was spread evenly from 0 to fs/2,

and that the sin(x)/x effect should not affect low frequencies.

 

When I saw Stewart Bryant's post, it seemed that there really needed to be

a PI in there, so I shut up.  Now I'm not so sure.

 

I used Cool Edit to generate 1 second of white noise at 48 kHz.

Passed it through a script that changed positive values to 8 samples of

+1/2 full scale, and negative values to 8 samples of -1/2.  This is a

rough approximation to a shift register clocked at 6 kHz.

 

A small piece of the wave looks like this.

 

 

The RMS power reported is -6.02 dB, much as one would expect.

 

The frequency analysis plot looks like this:

 

 

Then, I did a low pass filter at 100 Hz.  The RMS power now displays as -20.95 dB.

This agrees fairly well with the naive -6.02+10*log(100/3000) = -20.79 dB,

and appears to support the "no pi" theory.

 

Unfortunately, I can't explain where the pi went :(

 

73,

 

Stewart KK7KA

 

----- Original Message -----

From: "James Moritz" <J.R.Moritz@herts.ac.uk>

To: <rsgb_lf_group@blacksheep.org>

Sent: Friday, December 12, 2003 3:17 AM

Subject: RE: LF: Related Technical Query - Soundcard calibrator

> Dear Andy, LF Group
> 
> I hate to throw a spanner in the works, but... A noise source of 
exactly
> this type is discussed in Horowitz & Hill "The Art of 
Electronics" 2nd
> ed., section 9.34. This gives the formula (in the form 
Andy has used)
> Vrms^2 = Vpk^2 * 2 * BW /Fclock, (giving 224mV rms) which 
is different
> by a factor of sqrt(pi) from the value Andy gets. I spent 
some time last
> night trying to figure out which one is correct, and came 
out with a
> reasonably convincing case for Horowitz & Hill's formula 
- if anyone is
> interested, I will put it into an e-mail compatible 
form
> 
> 
> Cheers, Jim Moritz
> 73 de 
M0BMU