Guralp Systems CMG-DM24 - Manual (Page 107)

Operator's guide
1 Data records contain one 32-bit difference
2 Data records contain two 16-bit differences
4 Data records contain four 8-bit difference
Other values are reserved for future expansion.
Number of data records : The number of 32-bit data records in the
block. Combining this with the compression code enables you to find
out the total number of sample points in the block; dividing by the
sample rate allows the time duration of the block to be determined.
You should check for status blocks (which have Sample rate = 0)
before performing this calculation. For status blocks, the number of
characters is Number of data records × 4.
The block duration is always a whole number of seconds, and always
starts on a whole second boundary. A data block has a maximum size
of 1024 bytes. If the compression algorithm changes the compression
format, blocks may appear which are not filled to the maximum
capacity.
The rest of the block contains the data fields.
The GCF body
The body of a GCF block contains
•
the absolute value of the first sample, a 32-bit signed integer
referred to as the Forward Integrating Constant or FIC (this is
not a data record, and is not counted in the Number of data
records);
•
a series of differences between following samples—32-bit, 16-bit
or 8-bit signed integers according to the Compression code;
•
the absolute value of the last sample, a 32-bit signed integer,
referred to as the Reverse Integrating Constant or RIC. The RIC
can be used as a checksum, as it should match the last
decompressed sample value. It is not a sample in itself.
Because the value of the first sample is used as the FIC, the data record
immediately after it is always zero.
Note: When data from a 24-bit digitizer is compressed into GCF blocks
and the digitizer selects compression code 1 (32-bit data records) and
the block is being transmitted over a serial link, the digitizer may
October 2009 107