PSN-L Email List Message

Subject: Re: aliasing
From: "GMV" gmvoeth@...........
Date: Sat, 21 Feb 2009 14:32:50 -0700

It does not matter the sensor feeding the data into an A/D converter
unless there is something I have never heard of before.
it is simply a fact of science that sampled data will be troubled
by alizing if signals are present above 1/2 the samplke rate.
Im not knocking anyones creation simply making people
sensitive to a fact of science. we are amateures here
in this field of seismology and there may be people who have
never heard or thought of alizing before.
Anyone with a university degree in the sciences should already be aware of alizing.
What kind of college is mercer ??
Arts or Sciences ?
I think I have seen the volksmeter and like it.
If I were rich Id have a copy of everyones creations.
to compare and contrast.
But I am no University graduate myself.
Just ex SONAR tech with avid intertest in all physical sciences.
I also believe there should be no such thing as
intellectual rights. It impedes the placement of
truth/science over religion/arts.

----- Original Message ----- 
From: "Randall Peters" 
Sent: Saturday, February 21, 2009 10:45 AM
Subject: aliasing

No 'butt-in' offense to me by your email, GM.   (I hope it is acceptable to address you in this manner.)  We in physics also 'speak 
our peace' on subjects important to us.
      You are absolutely correct that periodic signals with a frequency higher than the Nyquist frequency (1/2 the sampling rate of 
an ADC) will show up as an alias.  The key element to whether or not you get such a high frequency signal masquerading as one of 
lower frequency-depends on the coherence time of the offending signal.  It must exist through a large enough number of cycles that 
what is admitted to the record saved is coherent enough to show up as a spectral line.
    In the case of the VolksMeter, there is no analog signal to begin with.  Stated differently, there is no ADC; rather the device 
is a 'direct' capacitance to digital converter (CDC).  Because of the award winning AD7745 chip of Analog Devices, that is the heart 
of the instrument, the VM is more than just a "State of the art digital seismometer", as described in the link on Larry's page.

It  is also the worlds' first fully digital seismograph'.  It is true that some detractors from this claim would say that there is 
no such thing as a fully-digital anything that interacts with our world which is itself analog.  But my point is the following. 
Unlike other commericial instruments that use analog electronics to monitor a capacitive sensor (and then finally digitize the 
analog signal), the VolksMeter has no analog components whatsoever, except for internal 'workings' of the chip-for which one would 
have great debate as to whether the 'pieces' are digital or analog.
There is one other significant 'fully'-part to the instrument.  The capacitive sensor used by the VolksMeter is "fully 
       There are a host of advantages associated with an architecture that is fully digital in the electronics with a fully 
symmetric (differential) sensor; I don't have time now to go into detail.
    Incidently, some of you amateurs might want to 'tackle' the home-brew construction of such an instrument yourself; maybe even 
extend what I've done to the world of vertical seismographs.  As Chris Chapman has indicated, they're much tougher because  of 
material problems (not my expertise).  If you want to build something that could make even the seismology pro's salivate over, then 
talk with Larry about the electronics package.  He will probably sell you the all the necessary things 'for a song', compared to 
what the big companies want for anything.



Public Seismic Network Mailing List (PSN-L)

[ Top ] [ Back ] [ Home Page ]