FIR filter configuration of the CMG-DM24 mk3
This information applies to most modular CMG-DM24 units and digital instruments manufactured after 2004.
The earlier model DM24 Mk2 has a different filter configuration.
Data from the modulator is decimated in stages down to 2000 Hz. This is dealt with by a single Crystal CS5376 unit.
- Coefficients for initial (Sinc 1) decimation by 8
- Coefficients for Sinc 2 stage 3 decimation by 2
- Coefficients for Sinc 2 stage 4 decimation by 2
- Coefficients for CS5376 FIR 1 decimation by 4
- Coefficients for CS5376 FIR 2 decimation by 2
After this, three further FIR filters are used to decimate the data. The DM24 allows the user to select four output taps, which the DM24 distributes among six filter stages. The first filter stage is pre-set to a factor of 2. Subsequently, different filters provide decimation stages (÷2), (÷4), (÷5) as required. Decimation factors of 8 and 10 are produced by cascading filters (÷4,÷2) and (÷5,÷2) respectively.
Other information, such as bit weights (in μV/count), normalisation factor, and poles and zeros, can be found on the digitizer's calibration sheet.
Recent DM24 firmware sets the digitiser type field and tap table lookup value in the GCF header. This allows the sequence of filters to be determined from the GCF data itself.