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Submitted on 1 Jan 1986
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REAL TIME DIGITAL DISPLAY WITH ADJUSTABLE PERSISTENCE TIME FOR 2-D
DETECTORS
P. Leyendecker, N. Vonthron, C. Boulin
To cite this version:
P. Leyendecker, N. Vonthron, C. Boulin. REAL TIME DIGITAL DISPLAY WITH ADJUSTABLE
PERSISTENCE TIME FOR 2-D DETECTORS. Journal de Physique Colloques, 1986, 47 (C5), pp.C5-
167-C5-173. �10.1051/jphyscol:1986522�. �jpa-00225839�
REAL TIME DIGITAL DISPLAY WITH ADJUSTABLE PERSISTENCE TIME FOR 2-D DETECTORS
P. LEYENDECKER, N. VONTHRON and C.J. BOULIN
EMBL,
Physical Instrumentation Division, ~ o s t f a c h 102209, 0-6900 Heidelberg, F. R. G.
Rbsumb - Lors de la mise en place d'un bchantillon ou d'un cristal sur une expbrience utilisant un dbtecteur bidimensionnel il est important de pouvoir suivre le clichb de diffraction. Nous pr&sentons une unit; de visualisation temps-rbel, ayant une rbsolution de
256par
256blGments, une frequence d'entrbe maximale del'ordre de IMHz, et possbdant une mbmoire d'accumulation profondede
16bits. La visualisation est faite en
256niveaux de gris sur un moniteur tblbvision standard. Un algorithme de dbcroissance est appliqub aux donnbes afin de produire une visualisation instantange fidsle.
Abstract - In the initial setting up of a crystal or a sample on a 2-D detector, it is often useful to monitor the diffraction pattern. We present a real-time display unit, having a resolution of
256by
256pixels, an input rate of up to
1MHz, and a
16bits accumulation range. The patterns are displayed on a standard TV monitor with
256gray levels.
Adigitally imple- mented adjustable persistence scheme has been designed in order to provide a true life display.
I - INTRODUCTION
Modern molecular biology uses a very large number of different tools which include protein X-ray crystallography as well as fibre diffraction and small angle scatter- ing. The recording of X-ray patterns has long been done using films. One main reason is certainly the high spatial resolution and the integrating capability.
Nevertheless, the development of linear and two-dimensional electronic detectors has nowadays reached a state where it becomes possible to envisage routine use of these devices. One main advantage of the electronic detectors over film lies in the immediate availability of the digital data thus avoiding the tedious and time consuming step of film digitization. On top of that, the combined use of these detectors and very intense synchrotron radiation sources makes time-resolved measurements possible and therefore enables one to carry out kinetic studies of macromolecular changes.
At the EMBL, we currently use an area detector on the X 33 beam line at HASYLAB in HAMBURG for time-resolved small-angle scattering studies / I / . We also started work on protein crystallography in HEIDELBERG using a conventional rotating anode X-ray generator and a detector. In both cases the amount of collected data is such that appropriate interactive image processing systems for data evaluation after the experiment had to be developed 121. Our detectors are multi-wire proportional gas filled chambers and the readout is based on the delay line method / 3 , 4 / . We are currently using fast time to digital encoders to measure the event positions. The pattern recording is done by histogramming these event positions into a large memory.
The event rates we achieve with our present readout system are of the order of
200to 300 KHz.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986522
C5-168 JOURNAL DE PHYSIQUE
During set-up of a sample or data taking, the usefulness of a real-time display facility was immediately recognized. A first, simple solution had been implemented to fulfill the immediate needs. This first display module consisted of two digital to analog convecters getting the digital coordinates of each recorded event and driving a storage oscilloscope. Persistence (storage time) and beam intensity are adjusted accordingly to the event rate of the features the user wants to follow.
Although this device has proved to be useful it has a number of limitations such as a very poor dynamic range. Therefore it could not be considered as a definitive solution. We decided to use a similar approach but designed in a digital form which should overcome most of the basic limitations.
I1 - DESIGN CONSIDERATIONS
As real-time visualisation of a pattern is recognized to be an important part in the experimental set-up for X-ray crystallography, we designed a digital system able to cope with a) the event rates which can be reached by wire chambers (up to 1 MHz) and
b)the large dynamic range one has to deal with while measuring crystal diffraction patterns (1 to some lo4).
Such
a systemshould be designed as a stand-alone device which has a connection to the digital readout of the detector (a 16 bit bus with a strobe). It should provide the user with a good quality gray scale video picture. This system should work in parallel with the data acquisition but should not interfere with it.
a) - Image resolution and definition
The MWPC's we currently use produce patterns of 256 by 256 resolution elements. The imaging device should be able to cope with images of 256 by 256 pixels. However provision was made in the design in order to accommodate 512 by 512 pixels. The gray scale definition was fixed at 256 levels (eight bits) as this is the commonly used value for monochrome pictures.
b) - Real-time imaging constraints
In crystallography, measurements are made while the sample is rotating slowly and we found it necessary to provide an exact diffraction pattern during the rotation.
We have therefore implemented an adjustable persistence model to the histogramed data in order to get a picture which reflects as much as possible the current pattern. Previous experience with the simple analog solution showed that a half-life time constant ranging from 10 milli-seconds to 3 seconds would be sufficient for most of the situations we will have to monitor.
c)
- Thresholding capability
For some samples, diffuse diffraction can be relatively strong but of no interest to the experimenter. We added a possibility allowing the user to define a lower
threshold (in the range of 0 to 255) in order to 'clean' the picture produced on the screen. This thresholding is not applied at image registration time but at display time which will still enable weak reflexions to be seen and will also help to improve the image interpretation.
d) - Visualisation schemes
The recording of crystal diffraction patterns requires a large dynamic range and therefore an eight bit deep memory is too small for the pattern histograming. The apparent memory depth required is at least 16 bits. In order to be able to carry out the optimal video representation we decided to implement two modes: a) a square root scaling that allows the viewing of the whole memory dynamic range, b) a linear scaling for which the user can choose the 8 bit display window over the whole 16 bit range.
e) - General functions
Straight histogramming is also provided by setting the half-life time constant of
A fast memory erase is implemented as this is more or less standard on imaging devices.
f) - Graphic overlay
In addition, as we want to use this system for the protein crystallography project, we decided to implement a graphic overlay. This overlay can be used by the spot prediction program to add e.g. integration boxes over the image thus giving the user helpful information about the running of the experiment (squares, boxes, crosses ...
)All these design considerations lead to the functional block diagram of figure 1.
DISPLAY PERSISTENCE TIME
M O M MICRO
J
ATTENUATOR COMPARATOR LOOK UP IMREMEN TOR
TABLES
-
VIDEO GRAPHIC UNIT
256r 256 MEMORY
GRAPHIC OVERLAY
Fig.
1- Functional block diagram.
I11 - FUNCTIONAL DESCRIPTION
The different functional parts will be described in the following section.
a) - The memory and its controller
The main memory of the device is built around dedicated video memory chips: the TMS 4161 from Texas Instruments 151. This chip is a 64-K by one bit memory which differs from standard dynamic random access memories by the availability of an internal 256 bits wide shift register which can be clocked in and out at very high frequency
(up to 150 MHz). This serial port is clearly designed to be used as the line buffer for a raster scan video display system. The 256 bits serial register is loaded in parallel from the memory array within a single access cycle of about 400 nanoseconds.
A second random access port is available. It can be used to write and read to or from the memory (Figure 2).
The major advantage of this video memory is that only 2% of the time has to be spent for the refresh and display operations in a graphic system.
To handle the video-W's, we have implemented the dedicated TMS 9161 controller
circuit 161. This controller generates the signals needed to perform all the
operations with the VRAM's, including the transfers between the internal memory
array and shift register. Furthermore it carries out the generation of the blanking
and video synchronis.ation signals. The number of lines per frame, the display mode
C5-170 JOURNAL DE PHYSIQUE
(interlaced, non-interlaced), the timings for blanking and other synchronisation signals are all programmable (Figure 3). The working conditions are defined at initialisation time after power-up. The memory refresh and the shift register update are performed automatically during the blanking. This minimizes the external logic required around the memory.
ADDRESS CONTROL
I , I
I I
I MEMORY ARRAY I
I I
I I
CLOCK
Fig. 2 -.Block diagram of the dual ported video-RAM.
COMPOSITE VIDEO
Fig. 3 - The video system controller = functionality.
b) - Event acquisition
The X-Y coordinates sent by the detector are fed to a 16 words long input FIFO
memory working at a clock frequency of
10MHz. This effectively prevents the loss of
cycle.
Data histogramming is performed via the random port in the following sequence: a) the content of the X-Y corresponding memory address is read out, b) an increment 'b' is added to the value, c) the result is stored back into the memory location which was addressed by the event position (Figure 4 ) . The complete readmodify-write cycle takes about 750 ns.
INCREMENT
"b"
1
MEMORY ARRAYI /
II I I
I I I
Fig. 4 - Histogramming scheme using the random access port.
C)
- Persistence simulation
To simulate the persistence of the phosphor screen of a storage oscilloscope, the memory content has to be regularly updated as a function of time. The attenuation or intensity decay function is done by multiplying the memory values by a factor 'a' less than
1 .For each line of the raster scan, the data are available during the video line duration (64 micro-seconds). We have used the ability of the video.RAM shift register to be clocked in and out and read and written from or to the memory array. The updating of each pixel of a video line is done with a fast multiplier working at 8 MHz. It means that at the end of the visualisation of a line the attenuated values of the corresponding pixelsareready to be transferred back to the memory (Figure 5 ) . The attenuation function is therefore performed at 50 Hz which is an adequate rate for a smooth decay function as it is the case with a phosphor. The persistence simulation appears therefore continuous. As already mentioned, each pixel value is multiplied by a factor 'a', less than one, fifty times a second. This factor is expressed as a function of the half-life t1/2:
1
(1)
a
=- I (t1/2 in seconds)
Q L
Thus, for each value of this half-life time constant ranging from 10 milli-seconds to 3 seconds, we get a factor in the range of .25 to .995.
When an event occurs at location (X,Y), an increment 'b' is added to the previous
value of this location. If the count rate at this location is stationary and of
value N, its associated value No is incremented N/50 times after each correction for
attenuation. It is then displayed as N1 with N1 equal to the following expression:
C5-172 JOURNAL DE PHYSIQUE
N1
=(No + a)
+- N a b 50
The stationary condition (NI=No=N) gives us
: (2)b
=50
9(I-a)
MEMORY ARRAY
w
CLOCK
