[sac-dev] Re: [SAC-HELP] doubles in sac

[Brian Savage]

All,

Taking a walk through the source code is interesting.

Many of the internal processing subroutines assume the data are  
floats (single precision).
The filtering portion of the code is a good example, bandpass,  
lowpass, etc...

All of the subroutines involved in reading and writing assume a  
specific size, and
many places in these routines assume a four byte floating point and  
have this
value hard coded in.  For example, the version number is at a  
specific byte location
in the sac header (76 bytes in).

I am not saying it would be impossible, but would take some work and  
time
to accommodate double precision for either the data or header or both.

Cheers,
Brian







On Nov 29, 2007, at  7:49 AM , Arthur Snoke wrote:

> I used the wrong address for the sac-help listserv.
>
> ---------- Forwarded message ----------
> Date: Wed, 28 Nov 2007 22:49:36 -0500 (EST)
> From: Arthur Snoke <snoke at vt.edu>
> To: Tim Ahern <tim at iris.washington.edu>, Brian Savage  
> <savage at uri.edu>,
>     Robert B. Herrmann <rbh at eas.slu.edu>
> Cc: sac-help-request at iris.washington.edu
> Subject: Re: [SAC-HELP] doubles in sac
>
> Tim and others,
>
> Bob touched on what I think is the biggest problem in modifying  
> sac: the header is fixed in length so one could not easily double  
> all the floats.
>
> The simplest interim solution as I see it that would allow sac to  
> continue to live is to have rdseed spew out single precision to a  
> sac format. Except for timing as in Bob's example or in very high  
> sample rates in general, where are doubles needed?
>
> Arthur
>
> On Wed, 28 Nov 2007, Robert B. Herrmann wrote:
>
>> sac-help-request at iris.washington.edu wrote:
>> Tim: I do not know if this response will get back to the mailing  
>> list - so you get a copy
>> If the data stream should be modified to have doubles for the  
>> data, we should also consider doubles for the SAC float header  
>> values - One CANNOT pick time correctly at 23:59:59 for 200 Hz  
>> data sets (I know this is extreme) is the continuous data stream  
>> starts at 00:00:00 - the seven digit accuracy of floating point is  
>> exceeded
>> One would just need to redefine a new header version - I believe  
>> tha tthe current header is at version 7
>> Given a define change, gsac would have it implemented in about one  
>> hour. Internally gsac uses doubles for the float header values.
>>> 1
>>> Date: Wed, 28 Nov 2007 11:41:39 -0800
>>> From: Tim Ahern <tim at iris.washington.edu>
>>> Subject: [SAC-HELP] Double Precision
>>> To: sac-help at iris.washington.edu
>>> Cc: Doug Neuhauser <doug at seismo.berkeley.edu>,	Robert Uhrhammer
>>> 	<bob at seismo.berkeley.edu>,	Chris Laughbon  
>>> <chris at iris.washington.edu>,
>>> 	Charley Weiland <cweiland at stanford.edu>
>>> Greetings
>>> We have a question for the SAC users group.  Recently a need to  
>>> support double precision floating point for data in SEED format  
>>> has been identified. While SEED itself can support this, the  
>>> problem comes with possible output formats the rdseed program  
>>> should support.  The most widely used format is SAC format in our  
>>> estimation.
>>> Our question is, is it possible for SAC to support double  
>>> precision floating point values without loosing precision?  Would  
>>> it be extremely difficult to add double precision support for  
>>> SAC?  Even if SAC can not support the format, would it make sense  
>>> to have a double precision version of the SAC Format, if you  
>>> understand what I mean by that.
>>> One further question is, assuming that SAC and SAC format are not  
>>> ready for double precision and it would be difficult to support  
>>> it...  what new format do you think we might consider supporting  
>>> as an output format for rdseed? Do any of you have suggestions as  
>>> to possible ways to deal with double precision floats as an  
>>> output format?
>>> Thanks for any help you can provide us.
>>> Cheers
>>> Tim
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