Countrate data in "Restricted Imaging" format.

Niels Lund

DSRI JEM-X Technical Note #10

981130

1) Background

In the restricted imaging format the timing information for individual events is dropped, and

thus the timing information is limited to the image sampling rate which will typically be

between 8 and 30 s. The need for better timing data have been stressed on several occasions,

and it has been decided to set aside a fraction of the telemetry to provide a single count rate

channel with a TBD time resolution. This note suggests an implementation scheme which may

also be applicable to the pure spectrum format.

2) Imaging collection time in Restricted Imaging format.

Restricted Imaging (RI-) format will normally only be used when the average rate of good

X-ray events is about 100 cps or higher. For lower rates the Full Imaging format will be the

natural choice. The maximum rate of good events which can be transmitted in the current

RI-format is about 300 cps, corresponding to images containing 2400 events, transmitted in

8-packet sequences. Therefore we can conclude that the integration time for an image in the

RI-format will normally lie between 24 and 8 seconds, dependent on the instantaneous rate of

accepted events.

3) Count rate data in the RI-format

In the current EID-B the RI-format is defined as having 382 bytes of imaging data and 24 bytes

of countrate data in each data packet. We expect to use 8 such packets per image. Thus we

have 196 bytes of count rate data to cover a time interval between 8 and 24 s. The time

resolution of the count rate data should be fixed, otherwise it will not lend itself to be proces

sed into a standard output for general use. It seems that the best time resolution we can afford

within the above limitations is about 125 ms per channel (8 time channels per second). At this

resolution we would produce 196 time channels per 24 seconds - matching the number of bytes

we have available in 8 RI-packets.

4) Expected number of counts per channel

We anticipate that the maximum rate at which the CPU in the DFEE can process good events

is around 2000 cps. At this rate of events we will already have a very appreciable deadtime.

Therefore, if we settle for a time resolution of 125 ms we will expect a maximum of about

250 counts per time channel - i.e. a single byte can just store the maximum count value. We

can easily compress the data by using a difference scheme, so we can safely assume that one

byte per time channel will suffice to encode the countrate data in a lossless way.

5) Proposal for a simplified packet format

It has been realized that the proposed format containing both image and countrate data within

the same packet is cumbeRIome both to generate and to decode. Moreover the description of

possible changes to the greyfilter value is not complete in the existing format. We therefore

propose to separate the two types of data in each their own packet type (with different APIDS)

in the following way:

a) Image packet:

APID: JEM-X unit 1: 1100 1110 000 JEM-X unit2: 1101 1110 000

Word 0: bit 0-4: Sequence number of packet in image.

bit 5-15: Number of events in this packet.

(bit 0: MSB, bit 15: LSB)

Word 1,2,3: OBT of first event in image (0.125 ms precision)

Word 4: delta OBT for last event in image (128 ms prec.)

Word 5: X and Y position for first event in packet

Word 6-212: Image data (414 bytes, approx 330 events)

Word 213: CRC

b) Countrate packet:

APID: JEM-X unit 1: 1100 1110 000 JEM-X unit2: 1101 1110 000

Word 0,1,2 OBT of first channel in packet (0.125 ms precision)

Word 3: bit 0-5: Greyfilter valid for first channel (ch. 0)

bit 6-15: not used

Word 4-10: Room for new greyfilter values if relevant.

bit 0-5: New Greyfilter (only valid if bit 6-15 nonzero)

bit 6-15: channel number where new greyfilter takes effect

Word 11: Count value on channel 0

Word 12-212: Difference encoded count values for next 402 channels

Packet data corresponds to 51.584 seconds of input data.

Each channel 128 ms wide.

Word 213: CRC

Under nominal conditions we will have one science-tm-packet per second. Every 50 seconds

we will transmit a countrate packet when in RI-format. Thus only 2 % of the data is used for

this purpose. In fact, the new proposal allows to transmit about 10 % more image data than the

current baseline format.

Count rate packets will be generated in a continually as long as the RI-format is active. If the

formatting reverts to Full Imaging or changes to another format which includes fine timing

information, then the current count rate packet will be completed, after which the generation of

count rate packets will be stopped.

It should be straightforward to implement the same type of countrate packet when the

instrument is using the pure Spectral Format.