RMS command: difference to older versions

[Franz]

From user's mailing list, see message ​http://list.seismic-handler.org/pipermail/users/2011-May/000092.html

There's a huge difference between recent new and old version using ShRms. The old result seems to be correct.

SeismicHandler version 5.0a 13-Jul-2006, Copyright (C) 2006
|sh> rms 1 0 3600
     rms: 9.336159e+00

SeismicHandler version 5.5 February-2011, Copyright (C) 2011
|sh> rms 1 0 3600
     rms: 2.505154e+03

[MarcusWalther]

The difference also exists between version 2008 and 2011:

SeismicHandler version 5.0e 03-May-2008, Copyright (C) 2006
|sh> cresharp
|sh> rms 1 ;;;
   rms: 8.657193e+01

SeismicHandler version 5.5 February-2011, Copyright (C) 2011

|sh> cresharp
|sh> rms 1 ;;;
   rms: 2.942311e-01

So it should be related to a chance in or after 2008...

[MarcusWalther]

This chance seems to be related to the fix in r373. Since the symbol "mean" was uninitialized, the result of RMS was unpredictable.

[MarcusWalther]

For the records another result of CRESHARP test:

The value for the 2006 version is rms: 9.304403e-03

[MarcusWalther]

Explaining the difference between 2008 and 2011:

If "main" was initialized with 0 at runtime, the summed values where not divided by sample number and square root was also not taken. If we catch up on these operations, we get in case of the CRESHARP test (1000 samples):

>>> import math
>>> "%e" % math.sqrt(8.657193e+01/1000)
'2.942311e-01'

This is the result of 2011 version. The result from 2006 cannot be explained that way.

[MarcusWalther]

Replying to MarcusWalther:

If "main" was initialized with 0 at runtime, ...

Of course I meant "mean".

[MarcusWalther]

Wikipedia RMS formula

[MarcusWalther]

Franz sent an explanation for the 2006 result. In this version the following code is executed:

>>> import math
>>> "%e" % (math.sqrt(8.657193e+01) / 1000)
'9.304404e-03'

The RMS value for a set of n values is defined as follows:

So the RMS in 2011 version is the only correct one.

[MarcusWalther]

I introduced two qualifiers to the RMS command. "SRS" computes the square root sum of the trace (the false alternative in 2008 version) and "SAM" simulates the behaviour of the 2006 version.