U.S. patent number 3,921,163 [Application Number 05/443,109] was granted by the patent office on 1975-11-18 for alpha-numerical symbol display system.
This patent grant is currently assigned to Thomson-CSF. Invention is credited to Philippe Giraud, Jacques Marien.
United States Patent |
3,921,163 |
Giraud , et al. |
November 18, 1975 |
Alpha-numerical symbol display system
Abstract
A colour CRT display system for displaying vectors according to
a random scanning, which enables uniform brilliance and consistent
positioning to be achieved whatever the length of trace and the
colour selected and whatever the vector co-ordinates provided which
may be polar or cartesian co-ordinates. A single integrator
receives a reference d.c. voltage U and is controlled by a
comparator receiving a threshold d.c. voltage so as to produce a
sawtooth voltage the duration of which is proportional to the
length of the trace. The sawtooth X and Y deflection voltages are
obtained as a result of multiplication and addition of
predetermined co-efficients in respective X and Y channels. Means
are provided to vary the speed of scan and the amplification gain
for deflection in discrete values when there is a change of colour.
Further means are provided to select the d.c. voltages and the
multiplying co-efficient from two sets of values corresponding to a
first operation mode with polar vector co-ordinates and to a second
operation mode with cartesian vector co-ordinates.
Inventors: |
Giraud; Philippe (Paris,
FR), Marien; Jacques (Paris, FR) |
Assignee: |
Thomson-CSF (Paris,
FR)
|
Family
ID: |
9115107 |
Appl.
No.: |
05/443,109 |
Filed: |
February 15, 1974 |
Foreign Application Priority Data
|
|
|
|
|
Feb 20, 1973 [FR] |
|
|
73.05938 |
|
Current U.S.
Class: |
345/22;
345/16 |
Current CPC
Class: |
G09G
1/10 (20130101); G01R 13/208 (20130101) |
Current International
Class: |
G09G
1/10 (20060101); G09G 1/06 (20060101); G01R
13/20 (20060101); G06F 003/14 () |
Field of
Search: |
;340/324A,366CA |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Curtis; Marshall M.
Attorney, Agent or Firm: Greigg; Edwin E.
Claims
What is claimed is:
1. An alpha-numerical symbol display system for displaying,
according to a random scanning, successive vectors representative
of graphic data on a color cathode ray tube with equal and uniform
brilliance and a consistent positioning of the trace irrespective
of the color scan selected and of the vector length, said system
comprising:
A. a color cathode ray tube with X and Y deflection means;
B. sawtooth generating means for providing a sawtooth signal having
a determined and different slope value for each color scan and
including an integrator circuit having a determined time constant
duration for integrating a reference d.c. voltage to produce a ramp
output signal, a comparator circuit for comparing said ramp signal
to a threshold d.c. voltage to control said integrator;
C. two multiplying circuits having each two inputs, receiving
simultaneously said sawtooth signal at a first input and separate
multiplying signals at the second input respectively;
D. two adding circuits each having two inputs, connected in series
with said multiplier circuits respectively and receiving separate
adding signals at the second input respectively, said adding
signals being representative of the initial X and Y coordinates of
the vector to display;
E. two variable gain amplifiers connected in series with said
adding circuits respectively and providing X and Y deflection
signals to said deflection means respectively;
F. generating means for generating said d.c. voltages and further
said multiplying signals and adding signals and including switching
means for selecting, for each vector, said d.c. voltages and said
multiplying signals from a first and a second sets of values
corresponding to a first and to a second operation modes
respectively, said generating means providing further positioning
coordinate signals of the vector in question and which are
polar-coordinate signals for the first operation mode and
cartesian-coordinate signals for the second operation mode, said
generating means comprising further control means for providing
control signals in the case of color scan change, on the one hand,
to said integrator circuit to change the said slope value by
varying its time constant duration and, on the other hand, to said
variable gain amplifiers to vary their respective gain in discrete
step values, the ratio between the threshold voltage and the
reference voltage being proportional to the length of the vector in
question whatever the operation mode selected; and
G. a controlled VHT generator for providing to said cathode ray
tube different VHT voltages corresponding to said color scans.
2. A display system according to claim 1, wherein said generating
means provides for the first operation mode, a predetermined
constant reference voltage, a threshold voltage proportional to the
length of the vector to be displayed, and multiplying signals
proportional to the directive cosines of the polar co-ordinates of
the vector, a first signal corresponding to the sine and a second
to the cosine, said generating means providing for the second
operation mode, a predetermined constant threshold voltage, a
reference voltage which is inversely proportional to the length of
the vector to be displayed, and multiplying signals proportional to
the components which represent the cartesian co-ordinates of the
vector, a first signal corresponding to the component for an X
scanning axis and the second to that for an Y scanning axis.
3. A display system according to claim 2, wherein the integrator
circuit comprises passive components which establish said time
constant duration, and at least one sub-circuit which combines
another passive component in series with a controlled switching
circuit, a control signal being applied to the said switching
circuit from said generating means when a change of color
occurs.
4. A display system according to claim 2, wherein said generating
means comprises a digital computer to produce digital signals
corresponding to the said vector coordinate signals and to the said
switching control signals, and further four two-input switching
circuits for selecting said reference and threshold voltages and
said multiplying signals respectively according to the operation
mode in question, a common control signal being applied
simultaneously to these four two-input switching circuits.
5. A display system according to claim 4, wherein the multiplying
signals are provided in digital form to the multiplying circuits
which are of the multiplying digital-analogue converter type.
6. A display system according to claim 5, wherein said generating
means comprises further two digital subtractor circuits to supply,
on the basis of the cartesian positional coordinates provided by
said digital computer, the representative X and Y components of the
vector in question, and a digital calculating circuit to calculate
the inverse parameter of the length of the vector from the said X
and Y vector components.
7. A display system according to claim 6, wherein the said
calculating circuit comprises three programmable bipolar read-only
memories, storing m, m and m.sup.2 words respectively and
programmed to select the stored value closest to the true value
from m.sup.2 predetermined stored values of the said parameter.
Description
BACKGROUND OF THE INVENTION
The present invention relates to a cathode ray tube display system
intended for displaying graphic or alphanumeric data according to a
random scanning. In particular, the invention relates to scanning
means which produces appropriate deflection signals.
The invention applies advantageously to data-display systems such
as electronic attitude director indicators where the data to be
displayed concerns various flight parameters and where the tube is
a colour CRT display.
This data consists of vectors, curves, symbols, or alphanumeric
characters, complex traces being assumed to be formed by linking
together vectors. The scanning of the tube is of the so-called
random type, which means that the electron beam traces out the item
of data in question directly, the different items of data being
displayed one after the other in the course of each image-scan
sequence.
In such display systems where the information density is high,
there is a large number of items of data and it becomes difficult
for the operator to watch them all at once. A substantial
improvement as regards viewing is achieved by using a colour
cathode ray tube and by dividing up the different items of data to
be displayed among the various colours available. At the present
time it is easy to obtain three separate colours with colour CRTs,
i.e. red, yellow and green. The colours are selected by feeding the
tube with a very high switchable voltage at three separate voltage
levels. The result is that the electrons forming the electron beam
are propagated axially along the tube at different speeds, these
variations in speed having a corollary effect on the speed of scan,
which is known as the trace speed. The brilliance of the parameters
displayed is thus a function of the colour selected.
The problem of brilliance also exists within the limits of a single
colour. Ideal operating conditions particularly necessitate that
the brilliance be constant along the length of each vector
displayed and that the same brilliance be achieved for the
different vectors displayed.
The spot is made to move in a line between the end points of a
vector by means of X and Y deflection signals which vary in a
saw-tooth pattern. To this end, the scanning circuit includes
integrating means. If the integration takes place with a constant
integration period the trace speed is inversely proportional to the
length of the vector and brilliance varies as a function of this
parameter.
OBJECTS AND SUMMARY OF THE INVENTION
A display system of the type with which the invention is concerned
enables the following to be achieved:
Uniform brilliance within the limits of each colour as a result of
the spot moving across the screen at a constant speed whatever the
length and orientation of the vectors. In this way there is no
difference in brilliance between the items of data displayed nor
along the traces representing them;
A brilliance which is substantially the same to the eye of the
observer no matter what the colour concerned is, this brilliance
being calculated chiefly to allow satisfactory viewing under
conditions where there are considerable variations in ambient
light;
Any vector is represented in the same way from the point of view of
its position on the screen no matter what colour is selected.
The display system further enables data received in digital form to
be used with either cartesian or polar co-ordinates as desired. It
also allows the duration of image scan to be optimised by
considerably reducing the switching dead-times between successive
items of data, and the scanning speed to be changed quickly when
changing colours.
According to the invention there is provided a cathode ray tube
display system for displaying according to a random scanning
successive vectors representative of graphic data, comprising a
cathode ray tube with X and Y deflection means; a scanning circuit
for providing X and Y deflection signals to said deflexion means
respectively and comprising an integrator circuit having a
determined time constant duration and receiving a reference d.c.
voltage for producing a sawtooth signal, a comparator circuit
receiving said sawtooth signal and a threshold d.c. voltage for
controlling said integrator, two multiplying circuits receiving
simultaneously said sawtooth signal and being each connected in
series with an adding circuit, said adding circuits providing said
deflection signals respectively; and generating means for
generating said d.c. voltages and further two multipling signals
and two adding signals for said multiplying and adding circuits
respectively, said generating means comprising switching means for
selecting, for each vector, said d.c. voltages and said multiplying
signals from a first and a second sets of values corresponding to a
first and a second operation modes respectively, the ratio between
the threshold and reference voltages being proportional to the
length of the vector whatever the operation mode selected.
The invention will now be further described with reference to the
accompanying drawings in which similar components are given similar
reference numerals and which show:
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1, the representative components of a vector and the
corresponding deflection signals;
FIG. 2, a simplified diagram of a scanning circuit used in the
invention;
FIG. 3, a diagram of part of FIG. 2, showing the special circuits
employed to modify the circuit of FIG. 2 when using a polychromatic
tube;
FIG. 4, an exemplary embodiment of the display system according to
the invention using a scanning circuit as in FIGS. 2 and 3;
FIG. 5, a diagram of a multiplying converter circuit;
FIG. 6, a diagram of an adding and variable-gain amplifying
circuit;
FIG. 7, a diagram of one possible embodiment of the calculating
circuit in FIG. 4, and
FIGS. 8 and 9, a diagram of the screen and a curve of variation
intended to show the methods employed in the calculating circuit in
FIG. 7.
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 shows the representative components of a vector and the
corresponding deflection signals.
A vector PQ is defined by two components C1 and C2 which correspond
to the differences between the co-ordinates of the end points. In a
system where cartesian axis OX and OY correspond to the horizontal
and vertical directions of scanning respectively, the co-ordinates
of P are XP, YP and those of Q are XQ, YQ while the respresentative
components are C1 = XQ - and C2 = YQ - YP. Using polar
co-ordinates, C1 = L cos R and C2 = L sin R, L being the length of
the vector PQ and R the angle formed between direction OX and this
vector (the angle in the figure being positive).
The corresponding deflection voltages UX(t) and UY(t), which are
applied respectively to the X and Y deflector elements of the tube,
are shown respectively at the bottom and at the left of FIG. 1. The
initial values UXP and UYP enable the beam to be first positioned
instantaneously at the beginning P of the trace, O being taken as
the point of rest when there are no deflection voltages. The time
at which trace PQ begins is to and the time at which it ends to +
T. The trace from P to Q is brought about by sawtooth voltages ABC
for X and DEF for Y, their duration being T. Their amplitude varies
in linear fashion from zero at time to to respective values UXQ-UXP
proportional to C1, and UYQ-UYP proportional to C2. The
proportional factor may be the same or there may be two different
factors K1 and K2, depending on whether the X and Y sensitivities
are to be the same or different. When the voltages return to zero
at time to + T this brings the beam back to the origin at O.
The case being initially considered is assumed to be that of
one-colour operation, i.e. either a monochrome cathode-ray tube is
being used or operation is taking place within the limits of a
single colour using a polychromatic tube.
To make the trace uniformly brilliant for the different vectors to
be displayed it is necessary for the scanning speed to be the same
along each vector. This trace speed is constant if it remains
independent of the length parameter L, i.e. if the condition (T/L)
= To = constant is satisfied, To being the time taken to trace a
unit component represented by a vector of length Lo = 1.
FIG. 2 is a simplified diagram of a scanning circuit for generating
deflection signals. It includes means for generating sawtooth
signals which operate in a known way by integrating a predetermined
DC voltage level, or reference voltage, for a predetermined period
of time. The generator means comprise a single integrator 1 which
receives a reference voltage U at time to and which supplies at its
output a sawtooth signal of the form K3 U.sup.. t, K3 being a
constant introduced by circuit 1. This signal is zero when t = to
and then increases in a linear fashion until time to + T.
The integration is stopped at time to + T by a threshold comparator
2 in which the signal K3 U.t is compared to a threshold value S,
duration T thus being equal to (S/K3U).
It was seen above that this relationship must be held proportional
to L, i.e. either (S/K3U) = LTo or (S/U) = K3To.sup.. L, where K3
To is a constant.
Different forms of embodiment are possible to fulfil these
conditions, two of them being more advantageous because one of the
signals S or U is held constant, the other signal being capable of
varying as a function of parameter L.
If threshold S is a DC voltage So of constant value, the reference
voltage U takes the form (Uo/L), with Uo being a constant. If on
the other hand voltage U is a constant DC voltage Uo, the threshold
S takes the form S = SoL (So being a constant).
Circuits 3 and 4 are multiplier circuits which receive the sawtooth
signal simultaneously through one of their inputs, and
multiplication signals MX and MY respectively separately through
second inputs. These circuits supply signals K3 U.sup.. t.sup.. MX
and K3 U.sup.. t.sup.. MY respectively. Co-efficients MX and MY are
chosen such that the final deflection amplitudes K1 C1 and K2 C2
(FIG. 1) of the sawtooth voltages are obtained at t = to + T.
For operation when S = So and ##EQU1## these signals are given by:
##EQU2## and ##EQU3## and they are proportional to the respective
components C1 and C2.
For operation when S = So.L and U = Uo, these signals are given by
##EQU4## and ##EQU5## and they are proportional to the directive
cosines of the vector. This second type of operation is equivalent
to using polar co-ordinates, the first type above being suitable
for use with cartesian co-ordinates.
The initial values for positioning at P are obtained by adding
values UXP and UYP respectively to the signals supplied by the
multiplying circuits 3 and 4 to form deflection signals UX and UY
(FIG. 1). A first adding circuit 5 is connected in series with the
multiplying circuit 3 and receives a signal SXP corresponding to
UXP through a second input at time to, while its output feeds a
signal UX to the horizontal deflectors 6 of a cathode ray tube 7.
Similarly, a second adding circuit 8 in series with multiplying
circuit 4 receives a signal SYP corresponding to UYP and feeds a
signal UY to the vertical deflectors 9.
Block 10 represents ancillary means producing the various signals
U, S, Mx, My, SXP and SYP. Where the application is to an
electronic attitude director indicator system, these means comprise
a digital computer which supplies the different items of data XP,
YP and XQ, YQ for each vector in digital form, these items of data
being transmitted to the display unit. The latter may for example
incorporate: an input buffer store having random and sequential
access which receives the data from the computer; a circuit for
addressing the buffer store so as to extract from it the successive
items of data to be displayed in the course of each image scan; a
circuit for decoding the data extracted; a digital/analogue
conversion circuit for the decoded data; and ancillary circuits
comprising synchronising circuits.
It will now be assumed that the display tube is of the colour CRT
type. Applying a very high voltage, the level of which may vary in
steps, to one electrode of the tube allows a colour to be selected.
For example, red, yellow and green may be produced independently in
a beam penetration cathode ray tube display by applying three
different levels of the very high voltage. The very high voltage
causes the speed of the electrons forming the beam to vary as they
move in a longitudinal direction along the gun axis. Thus, for the
same deflection voltage applied, the velocity of the line trace (or
trace speed), i.e., the brilliance, will vary, increasing or
decreasing, depending on the color selected.
To obtain brilliance which is uniform within each colour (i.e. the
trace speed is constant) and which is substantially the same for
each colour (there is a different trace speed which is determined
by each colour), it is necessary to give the sawtooth integrating
voltage a slope as determined by each of the colours i.e. the
co-efficient K3 must be different to suit the colour selected.
By way of illustration, if the slope of a sawtooth is of a given
value for green, the value will be approximately doubled for yellow
and multiplied by from 3 to 4 times for red.
In addition, steps should be taken to ensure that operation is the
same no matter what the colour is, i.e. a vector PQ the length of
which L is given by its components C1 and C2 must be shown on the
screen in the same way for each colour. Consequently, it is
necessary to modify the amplitude of the deflection signals UX and
UY when changing colours so that positioning is still the same.
Slope and amplitude must be altered simultaneously. FIG. 3 shows
some of the circuits in the diagram in FIG. 2 as well as the
arrangements made to achieve a simultaneous action.
The integrator 1 comprises an integrating amplifier of the
operational type made up of: an amplifier 15, a resistive component
R1 between the source U and the input of amplifier 15, and a
capacitive component C10 connected to give negative feedback
between the output and input of amplifier 15.
The output signal K3U.sup.. t may be expressed in the form ##EQU6##
R1C10 being the time constant of integration of components R1 and
C10. This time constant may be varied by altering the value of
either of the components R1 and C10. In the present instance
resistor R1 is altered by switching when there is a change of
colours. A signal S1 termed the colour selection instruction on the
one hand, controls a switchable VHT generator 16 and on the other
hand, switches resistors R2 and R3 in parallel with R1 via their
respective switching circuits 17 and 18; for a first colour the
switches are open and the value of the integrating resistor is that
of R1; for a second colour switch 17 is closed and the resistance
is formed by R1 and R2 in parallel; finally, for a third colour,
circuits 17 and 18 are closed and the resistance is formed by R1,
R2 and R3 in parallel. The switching circuits 17 and 18 are
controlled via an intermediate circuit 19 which receives order S1
and identifies the circuit to be operated from it. Signal S1 may
thus be a digital signal and circuit 19 may be made up of simple
logic components.
The same circuit 19 is used to control the gain of two
variable-gain amplifiers simultaneously. The adding circuits 5 and
8 in FIG. 2 are replaced by circuits 20 and 21 which combine a
summing circuit follewed by a variable-gain amplifier. Circuits of
this type are known in the form of operational circuits. The gain
control signals are set beforehand to achieve the desired result,
i.e. that any particular vector should be positioned in the same
way whichever colour is selected. Where there are three colours,
three gain co-efficients are calculated in this way.
The reference voltage U is applied to the integrator via a
switching circuit 22. This circuit is instructed to close at the
time to when a vector begins to be traced and to open at the time
to + T when the vector in question finishes being traced. A control
signal S2, such as a pulse, is applied to one input of an
intermediate circuit 23 at time to, circuit 23 being for example a
bistable flip-flop one of whose outputs controls the switching
circuit 22. Output S3 of the threshold comparator 2 supplies the
second input of change-over circuit 23. A second output of the
latter controls a switching circuit 24 in parallel with the
integrating capacitor C10, switch 24 being made to open from to to
to + T by having signal S2 applied to it at to, and to close from
to + T to the time when the next vector begins by having signal S3
applied to it at time to + T. Switch 24 causes capacitor C10 to be
short-circuited and rapidly discharged at time to + T.
A preferred embodiment of a display system using a colour CRT, such
as a beam penetration cathode ray tube, is shown in the diagram in
FIG. 4. The system in question is of the electronic attitude
director indicator type for example. Measuring apparatus (not
shown) such as probes, sensors, inertia units, etc... supply
measurements corresponding to various flight parameters in the form
of electrical signals. After being digitally coded, this data is
transmitted to a digital computer and then to a display unit which
may incorporate a circuit arrangement similar to that described
above for block 10 in FIG. 2.
From the point of view of the invention, the point at which the
succession of display circuits is broken into is that at which the
digital signals for positioning the successive vectors and those
for controlling the switches in the scan circuit are delivered.
This point may be situated for example, at the output of the
decoding circuits. Block 30 symbolises all the circuits lying
upstream of this point, these circuits being arranged in accordance
with known techniques and lying outside the scope of the invention.
The co-ordinates are understood to be supplied either in cartesian
form (XP, YP - XQ, YQ) or in polar form (L, cos R, sin R) as the
case may be. It is in fact better for certain data to use polar
co-ordinates in view of the nature of the measurement signals and
so as not to overload the computer needlessly. For other data
cartesian co-ordinates are used.
The control signals comprise the signal S2 for the vector
considered to begin to be traced, the "colour instruction" signal
S1 and a co-ordinate instruction signal S4. The latter enables a
distinction to be made between whether cartesian or polar
co-ordinates are to be used for the current vector.
Two digital subtracting circuits 31 and 32 supply components C1 and
C2 respectively on the basis of signals XP, XQ and YP, YQ. A
digital circuit forms a signal the value of which is equal to or
proportional to ##EQU7## from C1 and C2. A switching circuit 34
receives signal 1/L at one input, and a digital signal U.sub.o at a
second input, and is controlled by signal S4. The output of this
circuit transmits 1/L or U.sub.o, depending on whether the
co-ordinate instruction S4 is equivalent to the use of cartesian or
polar co-ordinates. Value U.sub.o may be produced constantly by a
store circuit or a register 35. A digital/analogue conversion
circuit 36 receives the output of switch 34 and supplies the
appropriate reference voltage U to integrator 1 via switching
circuit 22. Circuits 1 to 4 and 19 to 22 are equivalent to those in
FIG. 3.
A change of threshold in response to the co-ordinate instruction is
accomplished by a switching circuit 37 controlled by the signal S4,
which gives the threshold S appropriate to the type of co-ordinates
being used. With polar co-ordinates, circuits 30 supply a digital
value L which, having been converted into analogue form in block
36, is applied to a first input of switching circuit 37. A second
input of this circuit receives from a circuit at 35, a voltage So
which may be produced in the same way as Uo.
Two other switching circuits 38 and 39 controlled by S4 are used to
select the MX and MY signals appropriate to the type of operation
in question. Circuit 38 supplies MX to multiplying circuit 3,
having received value C1 at one input and value cos R at a second.
Similarly, circuit 39 receives value C2 and sin R and supplies MY
to multiplying circuit 4.
The signals SYP and SXP applied to the combined summing circuits
and variable-gain amplifiers 20 and 21 result from the conversion
of digital signals XP and YP in 36. The appropriate connections are
not shown to simplify the figure.
The signals C1, C2 or cos R, sin R applied to the switching
circuits 38 and 39 may also be in analogue form after being
converted in 36, although the control S4 may still be in digital
form. The signals are preferably applied in digital form and MX and
MY are then digital signals likewise and the circuits 3 and 4 used
are of the multiplying digital/analogue converter type. The D/A
converting circuit 36 may be simplified in this way.
FIG. 5 shows, as a reminder, a diagram of a multiplying D/A
converter circuit which may be used to form circuit 3 for example.
The signal MX considered is one of four digits so as not to
overburden the Figure. Each of the connections which transmits a
digit in a given position in MX controls a respective switching
circuit (41, 42, 43 or 44). A local reference voltage U1 is
transmitted by each of the circuits 41 to 44 if the appropriate
digit is a "1". For a "0" value the appropriate switch is in the
open state, a resistor network 45 and an operational amplifier 46
complete the circuit. By choosing the resistors in the network in a
given way, the output signal is made equal to NU1, N representing a
number corresponding to the digital signal MX. In the example
shown, the binary value 0011 of MX is equal to a number N of 3 and
the value of the output is 3U1. An extra connection allows
information on the sign of MX to be transmitted by, for example,
letting digit 0 be equivalent to + and digit 1 to -. The sign is
given by the the digital information C1 or cos R discussed above
and depends on whether XQ is less than XP or whether angle R is
negative. An additional switching circuit 47 is controlled by the
sign data and switches through value + U1 or - U1 as the case may
be, the output signal becoming + U1 . N or - U1 . N
respectively.
FIG. 6 shows a diagram of a summing and variable-gain amplifying
circuit such as circuit 20 (FIG. 3 or 4) which receives signal SXP
and the output of the associated multiplying circuit 3. It
incorporates an operational amplifier 50 which is connected in a
known way as an adder by applying input signals to it through two
resistive components 51 and 52 which form an adder 53. The
change-over between discrete predetermined levels of gain which
occurs when changing colours is accomplished by altering the value
of the resistor connected to give negative feedback between the
output and input of the amplifier. In the same way as was described
for changing slope with reference to FIG. 3, the appropriate
circuit combines three resistive components 54, 55, 56 and two
switching circuits 57, 58 controlled from circuit 19 by signal S1,
assuming that the type of display envisaged uses three different
colours.
FIG. 7 shows a simplified diagram of an embodiment of the
calculating circuit 33 in FIG. 4. The digital components C1 and C2
at the outputs of the subtracting circuits 31 and 32 include an
item of sign information. The so-called twos complement binary code
is usually used with systems of this type which include a computer,
and digital circuits 61 and 62 enables the absolute values
.vertline.C1.vertline. and .vertline.C2.vertline. to be obtained
which are used for circuit 33. Circuits 61 and 62 may consist of a
switching or multiplexer circuit, a complementing circuit, and an
adding circuit. The calculating circuit is arranged to calculate
the value 1/L to an approximation such that consecutive variations
in brilliance and position are small, of no consequence and
imperceptible to the observer. The calculating circuit 33 may be
produced in a compact and relatively simple form using integrated
circuits. The amount of change represented by each component C1, C2
is calculated as a function of the appropriate dimension of the
screen along X and Y, and of the positioning of the point of origin
0 on the screen. If point 0 is considered to be at the centre of a
screen as shown in FIG. 8, the maximum values C1M and C2M of the
components are equivalent to half the appropriate dimensions of the
screen, and their sign may be either negative or positive. Each of
the distances 0 to C1M and 0 to C2M is divided into a small number
m of areas so as to give only m values of C1 and m values of C2, m
being 16 for example, m is selected to such a way as to give rise
to only a limited amount of error which is compatible with the
operating criteria mentioned above.
In FIG. 9 is shown a function 1/.sqroot.X.sup.2 + A which is
equivalent to 1/L where C2 = A is a constant. A given change DL in
Y corresponds to a small change in X as the origin is approached
and a greater and greater one as it recedes. The m values selected
for C1 (and for C2) are therefore distributed over the envisaged
range C1M (and C2M) in a non-uniform manner. The distribution is
such that the ratio between two successive selected values of the
function is substantially constant, i.e. ##EQU8## The result is
m.sup.2 predetermined values of 1/L i.e. 256 when m = 16.
For each value C1 and C2 which occurs, the means employed allow
firstly the nearest predetermined values C1 and C2 to be selected
and then the corresponding value of 1/L. For the X channel which
receives .vertline.C1.vertline., the circuits used comprise a
circuit 63 for addressing a store 65 and, for the Y channel which
receives .vertline.C2.vertline., similar circuits 64 and 66. The
function of the addressing circuit 63 (or 64) is to identify the
area to which the input signal corresponds and to select the
appropriate nearest value of .vertline.C1.vertline. or
.vertline.C2.vertline. in the store. To this end each of the stores
65 (or 66) permanently contains m binary bits corresponding to
these areas which constitute the m separate values of the component
C1 (or C2) selected for the calculation. If C1S and C2S are the
values extracted from the stores, when m = 16 these values may be
four-digit binary words. They are then used in the same way to
address, via cricuits 67 and 68, a third store 69 in which are held
the m.sup.2 predermined values of 1/L given by the m values for X
and the m values for Y. Circuits 63 to 69 are produced using known
techniques. The combination 67, 68, 69 may for example be formed as
integrated circuits of the programmable bipolar read only memory
type. Each of the groupings 63, 65 and 64, 66 may also be produced
from a programmable bipolar read only memory by dividing the
digital input connections into two sets and programming the store
accordingly; for example, if .vertline.C1.vertline. is an eight bit
number, two set of four bits may be selected to address the store
after decoding and to extract the desired value from the m stored
values.
Similarly, the various digital circuits used in the system
described which are not dealt with in detail are produced in a
known way using basic logic circuits generally formed by integrated
circuits. By way of example, a switching circuit such as circuit 23
in FIG. 2 may be a simple fieldeffect transistor. Furthermore, no
details are given of the form of the various signals S1, S2 and S4
intended for the various switching operations and which may be
produced in different ways. These signals are generally made up of
one or more pulses depending on the type of control to be
exerted.
The scan circuit and the system described with reference to FIGS. 2
to 7 may be modified in a number of ways which provide the
specified characteristics and these modifications are understood to
fall within the scope of the invention. The facility of using polar
or cartesian co-ordinates as desired and the fact of employing
predominantly digital processing make possible a compact and
lightweight embodiment which is of advantage when used to form a
piece of airborne equipment.
Regarding the scanning circuit of the system, reference may be made
to U.S. publications "Electrical Design News" Vol. 16, Aug. 1,
1971, No. 15, page 49, "High Speed Multiplying DAC Simplifies CRT
Displays" and, "Electronic Equipment News" Vol. 14, Feb. 1973, No.
10, page 61 "Multiplying D/A converters simplify CRT displays", and
to U.S. Pat. No. 3,325,802.
Of course the invention is not limited to the embodiment described
and shown which has been given solely by way of example.
* * * * *