Tue Jul 10 12:35:25 2001

A problem with the IDA MK7 data logger has been discovered that affects all data
recorded from the broadband (STS1 and KS54000) sensors. IDA personnel have prepared
a detailed report describing the nature of the problem and the impact that the problem
has upon the data already collected and distributed through the IRIS DMS. A preliminary
draft of that report is available on the IDA WWW page,

	http://quakeinfo.ucsd.edu/idaweb/

under What's New.  That report's conclusions can be summarized as follows:

  The report describes a problem in the IDA MK 7 data acquisition system (DAS) that affects
  all recorded and archived data originating from the broadband seismometers. The affected
  channels have SEED convention names of bh*, lh*, and vh*. Other channels follow a
  different processing path and are not affected.

  A series of tests conducted on a MK 7 DAS in the laboratory led to the following
  conclusions: 

  1. There is definitely a bug in the Digital Signal Processor (DSP) code that performs
  filtering and decimation of the digitized signal stream. The output of the defective
  code Y(t) = F(X(t)) + E(t), where F(X(t)) is the correctly filtered output of the raw
  digitized stream X(t), and E(t) is an error term. 

  2. The error term arises from an incorrect handling of round-off error in the convolution
  of the filter coefficients and the data. 

  3. The error term in the output of the first filtering/decimation stage, the bh* streams, is
  approximately equal to 1.6 x 10-3 times the lower 16 bits of the 2's complement
  representation of the data. Thus, the error is limited in amplitude (104 counts) and
  produces its largest effect when the input data values cross a N * 2exp16 boundary where
  N is an integer. 

  4. The error increases with each subsequent filtering/decimation so the vh* streams are
  most affected. 
  
  5. In the spectral domain, the greatest effect of the error appears in the long-period
  spectrum (greater than 10 seconds). 

  6. The DSP code is simple to fix. 

  7. At least part of the long-period information can be recovered by reprocessing the
  higher rate data with correct filtering/decimation routines. 

The full report contains some background information on the discovery of the problem, our
diagnosis, and the characterization of the effects of the error on the data recorded and
archived. 

IDA is currently developing a plan of action to correct the problem. The plan will involve
procedures to correct the hardware currently in the field and to reprocess data currently
in the IRIS data archive. We will keep the research community apprised of our plans to
implement these corrections. 

IDA staff have prepared a C program to simulate the behavior of the data logger. The text
of the code may be found in the back of the draft report and obtained by anonymous ftp.  The
program accepts 32 bit integers as input and applies filtering and decimation routines to
mimic the action of the MK7 for test purposes. A flag may be set to run the program in a
mode that eliminates the problem. 

We sincerely regret any inconvenience this may cause.

Peter Davis
IRIS/IDA Data Collection Center
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