Chapter 12. Configuring DM24s and CD24s

When data from DM24 and CD24 analogue to digital converters (ADCs) are used (via the the GDI to CD1.1 converter), there are two configuration settings which should be considered: state of health (SoH) reporting and block latency. It is not absolutely necessary to reconfigure the ADC but, if not done, sub-optimal CD1.1 subframes will be generated.

12.1 SoH reporting

By default, the DM24 or CD24 will only output a textual status block every 15 minutes or so. This is not sufficient for the real-time channel status field for a CD1.1 subframe. Therefore, a new GCF block type, the Unified Status block, was developed. This block is emitted every second, and is used by the GDI subsystem on Platinum to acquire all the status data needed for the CD1.1 channel status field.

If unified status blocks are unavailable, the CD1.1 frames will still be valid, but will have several warning bits set (clock differential too large, GNSS receiver unlocked, GNSS receiver off) and there will be no way to determine the clock differential from the subframe alone.

Unified status packets are turned on via a check-box near the bottom of the digitiser configuration page of the Platinum system's web interface (select Configuration → Data handling → Data acquisition and choose the appropriate ADC). Ticking this check-box and submitting the page issues the following sequence of commands to the DM24 or CD24 (note: the CD24 does not require the uspmonitor command but it is harmless):

ok-1

uspmonitor

re-boot

Clearing this box and submitting the page issues the following commands:

ok-1

-monitor

12.2 Latency

When using sample rates of 200 sps and below, the adaptive block-based transfer mechanism used by Güralp DM24s or CD24s can conflict with the fixed-duration frames used by CD1.1.

This can be solved in one of two ways. Either the ADC can be configured to emit blocks more frequently, or the latency of the subframe generation can be increased (i.e. the CD1.1 converter will wait for a longer period of time for more blocks to arrive).

To configure a DM24 or CD24 to emit blocks more frequently, visit the relevant digitiser configuration page of the Platinum system's web interface (select Configuration → Data handling → Data acquisition and choose the appropriate ADC). Scroll down to the Compression mode section and, in the drop-down menu, select “off (8 bit 20 records max)”.

If you are using a fixed-block-size transfer protocol such as Scream, this will consume significantly more bandwidth. If this is unacceptable, increase the frame assembly time in the CD1.1 converter.

If you alter neither the ADC configuration nor the CD1.1 converter's time-out, subframes which do not have all the required samples will be deferred once the assembly time-out is completed.