slightly ot: DAT clock deviation

[Magnus Danielson]

From: Martin Czech <czech at Micronas.Com>
Subject: Re: slightly ot: DAT clock deviation
Date: Thu, 10 Aug 2000 09:32:09 +0200 (MET DST)

> 
> :::Well, with the recording of the reference signal they are able to do the
> :::crosscorrelation. Now, the most straigthforward way to get precission is just
> :::to make a LONG recording. Yes, your pseudorandom noise will loop many times,
> :::but you don't care. I have been sitting too many nigths listening to the
> :::pulsating noise. Now, when you do such a recording, any drifts in clocks is
> :::at least fairly limited.
> 
> Only problem: computer memory for FFT.
> Or I have to look at programs for very long FFT length, like
> these earthquake people have... Surely pretty slow.

There are only two waveforms that you are required to hold in memory, that of
your pseudo-random noise generator and that of your impulse responce.
The measured values you can stream into it sample by sample.

If you are really into memory reduction you could drop the pseudo-random noise
memory block in favor of an generator function.

> :::You DO need a steep anti-aliasing filter. You should not settle with less 
> than
> :::8 poles. I would consider both Butterworth, Bessel and Gauss filters to do 
> the
> :::job.
> 
> I come to that in a follow up.
> 
> 
> 
> :::You could apply the filtering at the input, after the mic-amp and prior to 
> the
> :::S/H section.
> 
> I don't think so, because I need the reference signal also, and this is 
> square pulses.

Actually, my point is that since this is a linear system, it doesn't care.

If we have the amp responce Hamp(s), the speaker responce Hspk(s), the filters
responce Hfilt(s), the acoustical responce Hroom(s) and the mic responce
Hmic(s) then we get

Hmeasured(s) = Hfilt(s) * Hamp(s) * Hspk(s) * Hroom(s) * Hmic(s)
             = Hamp(s) * Hspk(s) * Hroom(s) * Hmic(s) * Hfilt(s)

However, what you said is that you need the reference signal also, and that
you can tap before hitting the amp or whatever, so that is a simple feat.

Cheers,
Magnus