XPost: rec.audio.pro, comp.dsp   
   From: aclark@danvillesignal.com   
      
   spam@spam.com (Don Pearce) wrote in   
   news:4ce935a6.840936228@news.eternal-september.org:   
      
   > On Sat, 20 Nov 2010 12:53:16 -0800 (PST), rickman    
   > wrote:   
   >   
   >>On Nov 20, 2:52 pm, s...@spam.com (Don Pearce) wrote:   
   >>> On Sat, 20 Nov 2010 11:43:03 -0800 (PST), rickman    
   >>> wrote:   
   >>>   
   >>>   
   >>>   
   >>> >On Nov 20, 12:02 pm, klu...@panix.com (Scott Dorsey) wrote:   
   >>> >> Dick Pierce  wrote:   
   >>>   
   >>> >> >Scott Dorsey wrote:   
   >>> >> >> I don't know, because I have never seen actual instantaneous RMS   
   >>> >> >> values ever displayed anywhere.   
   >>>   
   >>> >> >The term "instantaneous RMS value" is itself meaningless.   
   >>>   
   >>> >> BINGO! Mr. Pierce wins the kewpie doll!   
   >>> >> --scott   
   >>>   
   >>> >Except that it is wrong.  What is the instantaneous RMS value of   
   >>> >-1... +1.  RMS doesn't have to be an integral or a sum. b   
   >>>   
   >>> >Rick   
   >>>   
   >>> It isn't. It is the square root of the mean of the squares. In the   
   >>> case of -1, +1 it is 1.   
   >>>   
   >>> The inclusion of the term "mean" says that there must be at least two   
   >>> measurements - any fewer and you can't have a mean.   
   >>>   
   >>> d   
   >>   
   >>I don't recall any such restriction on N.  Certainly the measurement   
   >>has just as much meaning with N = 1 as any greater N.  Think of it as   
   >>a limit as N approaches 1 then.  The point is that it is as valid a   
   >>measurement for a single point as it is for many points.  It   
   >>represents the equivalent voltage that would produce the same power as   
   >>DC of the same voltage.   
   >>   
   >   
   > When you do it for a single point, the term RMS ceases to have   
   > meaning. For a single point it is just the voltage. For two points and   
   > above, RMS volts times RMS current give average power.   
   >   
   > d   
      
   A mean of 1 sample is valid. RMS of a one sample measurement is technically   
   valid as well, but perhaps not particularly useful.   
      
   OTOH, If you knew that the signal was DC, a single sample might not be   
   meaningless at all. With an AC signal, even a small number of samples may   
   not yield a particularly good rms value.   
      
   This issue doesn't begin to tradeoff exponential versus linear averaging,   
   which changes the result as well. A very fast exponential averaging time   
   approaches the 1 sample case.   
      
   If you have an issue with exponential averaging, you can send me all your   
   multimeters   
      
   Al Clark   
   www.danvillsignal.com   
      
   --- SoupGate-Win32 v1.05   
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