The CCD camera employs a 12-bit ADC but data are transferred to the DP (telemetry) as 8-bit numbers. For purposes of gain calibration and helioseismology it is important to maintain the full 12-bit accuracy. This is accomplished by commanding compressed data and the low-order 8 bits for the same region, and then restoring the full 12 bits (see the routine RESTORE_LOW8 described in the Reference Guide).
The compression of ADC numbers uses the following algorithm. Taking DN,
or Data Number, to be the CCD digital camera output, and 
the 8-bit
compressed value, and 
the 12-bit decompressed value,
64 
The SXT compression algorithm is not lossless, and so the decompressed
values, , contain a small error due to decompression: 
. The algorithm has been selected so as to make the
decompression error, 
, comparable to the statistical
uncertainty. The decompression error is given by:
which are optionally returned by the SXT_DECOMP routine.