JEM-X data can be transmitted in several different telemetry formats which vary in their information content for position, energy or time and the required bandwidth per event. In addition, a ``grey filter'' mechanism exists eliminating a fraction of the incoming events in a randomized way. The possible transmission settings range from to of the incoming events. These mechanisms allow the instrument to cope with sources of very different brightness despite its limited telemetry allocation.
For formats with poor time resolution (REST, SPEC) countrate data packets are also transmitted to provide some data for timing analysis. However countrate data is not an independent data format.
For a given observation a primary and a secondary telemetry format are defined and they can be identical. If the observed data rate is too high to be transmitted completely, first the grey filtering will be increased to reduce the number of processed events. Should this not be sufficient the instrument will autonomously switch to the secondary telemetry format, continuing to adapt the grey filter as necessary. For decreasing input rates the instrument will reduce the filtering and possibly switch back to the primary format. All these changes are driven by the filling status of an on-board buffer, the mechanism includes a certain hysteresis in order to avoid rapid switching between formats.
The characteristics of primary and secondary telemetry formats are
listed in Table ![[*]](crossref.png)
. The default primary format is Full Imaging
and the default secondary format is Restricted Imaging. Note that in
the Spectral Timing format the actual spectral resolution will be
slightly lower than that of the Full Imaging mode due to spatial gain
variations in the detector. It is recommended, however, that the full imaging format is used both as primary
and secondary format.
| Detector Image | Timing | Spectral | Events | |||
| Format Name | Resolution | Resolution | Resolution | per | ||
| (pixels) | (channels) | packet | ||||
| Full Imaging | (FULL) | 256 x 256 | 1/8192s = 122 s | 256 | 105 | |
| Restricted Imaging | (REST) | 256 x 256 | 32 s | 8 | 320 | |
| Countrate | ( ) | None | 1/8 s = 125 ms | 1 | n/a | |
| Spectral Timing | (SPTI) | None | 1/8192s = 122 s | 256 | 210 | |
| Timing | (TIME) | None | 1/8192s = 122 s | None | 550 | |
| Spectrum | (SPEC) | None | 1/8s = 125 ms | 64 | n/a |