Representing a video signal upon the picture screen of a video display device

A method and apparatus for representing the amplitude curve of a selected portion of a video signal upon the picture screen of a video display device, at least the selected portion being in digital form and not requiring conversion to analog form prior to such representation. This is achieved by using the line structure of the line raster produced on the picture screen of the video display device as a digital-analog converter. The range of possible digital values for the individual picture elements of the selected line of the video signal are allocated as reference values to respective lines of the line raster, and during each line period of the line raster the digital values of the elements are compared in succession with the appertaining reference value. When during any line period of the line raster the compared values agree the picture screen is momentarily brightened, so that each picture element of the selected line is represented on the screen with a vertical displacement determined by its digital value, so that an amplitude curve similar to an oscillogram is produced having a horizontal time axis. In a further embodiment of the invention, arranged to display the amplitude curve of selected picture elements each taken from a respective line of the video signal, the reference values are allocated to respective columns of the line raster of the display device. The result is the production of an oscillogram on the picture screen having a vertical time axis.

This invention relates to a method and apparatus for representing a 
selected portion of a video signal upon the picture screen of a video 
display device. 
In the periodical "Elektor", December 1971, pages 1234 to 1242, there is 
disclosed an arrangement for representing the voltage curve of a periodic 
signal upon the picture screen of a domestic television receiver which is 
scanned according to the line raster technique. In this arrangement, by 
the use of a comparator, a horizontal frequency sawtooth signal is 
compared with the periodic signal to be represented. Upon reaching a 
condition of voltage parity in the signal comparison, the electron beam of 
the picture tube is unblanked whereupon a corresponding picture element is 
brightened on the picture screen. However, in this method of operation the 
voltage curve of the signal to be represented is rotated through 
90.degree., i.e. its time axis extends in the vertical direction. Moreover 
it is possible only to represent low frequency signals, of a frequency 
smaller than the horizontal scanning frequency of the picture tube of the 
domestic television receiver. A further disadvantage is that only analog 
signals can be represented. Digital signals, e.g. PCM coded video signals, 
must first be converted into analog form when using the arrangement 
described above. 
It is therefore a primary object of the invention to provide a method of 
representing a selected portion of a video signal upon the picture screen 
of a video display device, which method is able to handle digital signals 
directly without conversion to analog form. 
Accordingly, the invention provides a method of representing a selected 
portion of a video signal upon the picture screen of a video display 
device, at least the selected portion of the video signal being in digital 
form, wherein at least some of the range of possible digital values of the 
individual picture elements of the selected portion of the video signal 
are allocated to respective lines and/or columns of a line raster 
reproduced upon the picture screen of the video display device, and 
wherein during the reproduction of the raster the brightness thereof is 
modulated in accordance with the picture elements of the selected portion 
of the video signal in such manner that each such element produces on the 
screen a contrasting brightness in that line or column if any having the 
same digital value allocated thereto as the digital value of the element, 
whereby each picture element of the selected portion of the video signal 
thus represented on the screen has a respective vertical or horizontal 
displacement determined by its digital value. 
It is a further object of the invention to provide an apparatus, operating 
according to the method of the invention, which is adapted to represent a 
selected digitally coded line of a video signal in a horizontal position 
upon the picture screen of a video display device. 
Thus, the invention further provides an apparatus for representing on the 
picture screen of a video display device the amplitude of a selected line 
of a video signal as a function of time, at least the selected line being 
in digital form, comprising storage means for storing the digital values 
of the individual picture elements of the selected line, means for 
supplying as reference values at least some of the range of possible 
digital values of the picture elements of the selected line of the video 
signal, comparison means for comparing, during each of a plurality of line 
periods of a line raster reproduced upon the picture screen of the video 
display device, the digital values of the individual elements of the 
selected line successively with a respective reference value, whereby each 
reference value is effectively allocated to a respective line of the line 
raster, and means for causing a brightness contrast in the respective 
raster line when the digital value of the element and the reference value 
are the same. 
Also, it is an object of the invention to provide an apparatus, operating 
according to the method of the invention, which is adapted to represent a 
selected digitally coded vertical frequency portion of a video signal in a 
vertical position on the picture screen of a video display device. 
Accordingly, the invention further provides an apparatus for representing 
on the picture screen of a video display device the amplitudes of a 
selected plurality of digitally coded picture elements derived from 
respective lines of a video signal, the apparatus comprising selecting 
means for selecting desired picture elements from respective lines of the 
video signal, storage means for storing the digital value of each selected 
picture element, means for supplying as reference values at least some of 
the range of possible digital values of the selected picture elements, 
comparison means for comparing each stored digital value successively with 
the reference values during a respective line period of a line raster 
reproduced upon the picture screen of the video display device, the 
successive reference values being presented in substantially identical 
order and timing in each line period of the line raster whereby each 
reference value is effectively allocated to a respective column of the 
line raster, and means for causing a brightness contrast in the respective 
raster line when the stored digital value and the current reference value 
are the same.

Referring now to FIG. 1, this is a block schematic diagram of an apparatus 
for representing on a video display device the voltage amplitude curve of 
a selected line of a video signal as a function of time, the time axis 
being horizontal on the picture screen of the device. 
In the figure a colour video signal (F) BAS is delivered through a terminal 
1 to a controlled selector switch 2. In the case where the colour video 
signal is available at the terminal 1 in analog rather than digital form, 
there is interposed between the terminal 1 and the controlled selector 
switch 2 an analog-digital converter 3. The converter 3 is not required 
where the video signal is already in digital (e.g. PCM) form. In either 
case, therefore, the video signal is available in digital form (i.e. in 
the form of individual digitally coded picture elements) at the input line 
4 of the controlled selector switch 2. The controlled contact path of the 
controlled selector switch 2 is switched, by the presence of a gate pulse, 
to the position shown for the duration of one line period only during a 
frame of the video signal. The position of the line period within the 
frame is selectable. Normally, interest will lie in the voltage curve of a 
definite line, e.g. a test line in the vertical blanking gap of the video 
signal. 
The control of the contact path of the controlled selector switch 2 is 
effected in the following manner. The horizontal or line periods of the 
colour video signal available at terminal 1 are continuously counted by a 
line counter 5, and the running total registered by the counter 5 is 
compared in a comparator 7 with a selected line number, for example that 
of the desired test line, applied at terminal 6. When the digital number 
applied at terminal 6 coincides with the counted digital number at the 
output of the line counter 5, the output of the comparator 7 delivers a 
coincidence signal to a pulse former stage 8. The latter provides as 
output a gate pulse for the switch 2 having a pulse repetition frequency 
which corresponds to the frame frequency and a pulse width which is equal 
to the duration of a line period of the video signal. By this means the 
selected line is applied at the input of a shift register 9 connected to 
follow the controlled selector switch 2. By means of a clock signal 
produced in a clock generator 10, this selected line is shifted into the 
shift register 9, picture element by picture element so that at the end of 
the gate pulse a complete line in digital form is stored in the shift 
register 9. 
For the analog representation of the stored line upon the picture screen of 
a video display device 12 operating according to the same standard as and 
synchronised with the video signal applied at 1, the line structure of the 
line raster of the device 12 is used in effect as a digital-analog 
converter. For this purpose a respective one of the range of possible 
digital amplitude values of the elements of the stored video line is 
allocated as a reference value to a different line of the line raster. In 
the present case let it be assumed that each individual picture element of 
the selected video line has been digitalized with 8 bits. This corresponds 
to a range of 256 possible digital amplitude values (0-255 inclusive) for 
each element. Further let it be assumed that to the l00th line of the line 
raster of the device 12 there is allocated as reference value the digital 
value 255, to the 101st line there is allocated the digital value 254, to 
the 102nd line there is allocated the digital value 253, and so on, so 
that the digital value 0 is allocated as reference value to the 355th line 
of the line raster. This allocation is effected as follows. 
After the storage in the shift register 9 of the selected line, the contact 
path of the controlled selector switch 2 reverts, by the absence of a 
gating pulse from 8, to the upper position so that the output and the 
input of the shift register 9 are connected together. By the application 
of a suitable clock signal the stored video line is caused to circulate at 
horizontal frequency in the shift register 9, which is thus connected as a 
ring store. The individual bits of the digital values at the output of the 
register 9 are inverted in an inverting stage 13, and a comparator 11 
compares, during each raster line of the device 12, the thus inverted 
digital amplitude values of the successive picture elements of the stored 
line with the respective line number from counter 5 less a constant (in 
the present case the digital value 100) applied to the input 15 of a 
subtraction stage 14. This is equivalent to comparing the uninverted 
digital values of the picture elements of the stored line, during each of 
the raster lines 100 to 355, with a respective one of the reference values 
255 to 0 in that order, which is the allocation assumed above. 
Each time that the digital value of a picture element is the same as that 
of the currently allocated reference value, the comparator 11 is arranged 
to release a signal which causes a momentary brightening of the picture 
screen of the device 12. Thus under these conditions, in the 100th line of 
the line raster only those picture elements will be brightened which have 
a digital value of 255, in the 101th line brightening will occur only in 
respect of the picture elements having a digital value of 254, and so on. 
The result is that the selected line is represented on the picture screen 
of the device 12 by a plurality of bright spots each corresponding to a 
picture element of the selected line and having a vertical displacement 
above raster line 355 proportional to its digital value. The effect is, 
therefore, of a substantially continuous curve similar to a conventional 
oscillogram. 
In order to displace the oscillogram in a vertical direction upon the 
picture screen, it is arranged that the digital number applied at the 
terminal 15, which number is subtracted from the line number determined by 
the line counter 5, may be varied. In the above example the number 
subtracted was assumed to be 100, but if this is changed the effect is to 
vary, by the same number of raster lines in each case, the allocation of 
reference values to the lines of the raster. Thus the vertical position of 
the oscillogram may be adjusted on the picture screen without affecting 
its overall shape. 
Where it is required the analog-digital converter 3 is preferably designed 
so that, in the case of a band width of 5 MHz of the video line to be 
represented, the analog voltage of the line is sampled at intervals of 100 
nsec. In the case of a selected line which does not change from one frame 
to the next, such as a test line, the sampling can take place in such 
manner that, using a sampling and holding circuit, in a first frame the 
first picture element of the selected line has its amplitude sampled and 
held, in the next frame the second picture element of the selected line is 
sampled and held, in the third frame the third picture element of the 
selected line is sampled and held, and so on, each held amplitude value 
being subjected to a subsequent digital quantization. Thus, the selected 
video line is completely scanned and digitally converted after 64 
.mu.s/100 ns frames, in the case of the 625 line standard. Preferably in 
such case the analog-digital converter 3 operates according to the 
so-called economical counting principle in the digital quantization of the 
sampled amplitudes, whereby in successive lines of the picture 
successively higher quantization stages are adopted for comparison with 
the sampled amplitude value. 
As a measuring technique for evaluating the oscillogram amplitudes upon the 
picture screen, certain preselected lines in the line raster of the device 
12 may be brightened as a whole for example. These lines may then serve as 
reference lines if the reference values allocated thereto are known. 
For the purpose of achieving a vectorial representation of the selected 
video lines, it is necessary in principle to employ two similar circuits 
corresponding to the above described block schematic diagram. In such a 
case, however, in respect of the x direction the range of possible digital 
values of the picture elements of a selected video line are not associated 
with predetermined lines, but with predetermined columns, of the 
television raster. A brightening then only takes place in the event of 
coincidence between two associated signals. 
Referring now to FIG. 2, this is a block schematic diagram of an apparatus 
for deriving a vertical oscillogram of the largest and the smallest 
digital values within each line period of a digital video signal. 
In FIG. 2 a digital video signal is supplied to the apparatus at the point 
21. This digital video signal may be available direct or may be delivered, 
for example, from the output of an analog-digital converter 22 which 
receives from a video signal source (not shown in the drawing) an analog 
video signal at the terminal 23. Let it be assumed that the digital video 
signal is available with a word length of n parallel bits. The digital 
video signal is delivered to one input A of a comparator 24 and to the 
data input D of a D flip-flop 25. The output Q of the D flip-flop 25 is 
connected to the input B of the comparator 24. The comparator 24 is 
provided with an output A&gt;B. A comparator provided with such an output is 
available in integrated circuit form under the type number SN 7485. The 
A&gt;B output of the comparator 24 is connected to the CLOCK pulse input of 
the D flip-flop 25. An erase input marked CLEAR of the D flip-flop 25 
receives a horizontal frequency pulse signal over a terminal 26. 
The comparator 24 and the D flip-flop 25 serve for a determination of the 
highest digital value of a picture element during each line period of the 
digital video signal at 21. In the event that the comparator 24 determines 
that the digital value at the A input is greater than that at the B input, 
a clock pulse is delivered, which has the effect of transferring the 
binary information available at the date input D of the D flip-flop 25 to 
the output Q. This binary information remains at the output Q of the D 
flip-flop 25 until the next clock pulse is received. The horizontal 
frequency pulse signal delivered through the terminal 26 causes, 
independently of the clock pulse, a resetting of the D flip-flop to the 
digital value zero. At the end of each line period, by means of control 
exercised by a horizontal frequency pulse signal available at a terminal 
27, the binary information situated at the output of the D flip-flop 25 is 
temporarily stored in a following connected D flip-flop 28 for the 
duration of one line period, and is delivered to an input A of a further 
comparator 29. An input B of the comparator 29 is connected to an output 
of a binary counter 30 to the terminal 31 of which there are delivered 
clock pulses whose frequency is an integral multiple of the line frequency 
of a video display device 33. The counter 30 thus establishes at the input 
B of the comparator 29 during each line period a succession of digital 
reference values, the clock frequency being sufficiently high that these 
reference values include the range of possible digital values of the 
picture elements of the video signal at 21. Furthermore, as the counter 30 
is reset at the beginning of each line period each reference value is 
allocated to a respective column of the line raster on the device 33. 
During each line period, when the comparator 9 establishes indentity in the 
binary values of the information lying at the inputs A and B, a pulse is 
generated which, via an adding stage 32, serves as an unblanking pulse for 
the device 33. Thus, in each line period, that picture element having the 
greatest digital value causes the picture screen of the device 33 to be 
brightened in the column thereof having the same digital value allocated 
thereto. There will thus appear in the line raster upon the picture screen 
a vertical oscillogram of the maximum amplitudes of the picture elements 
in the individual lines of the video signal at 21, with the direction left 
to right corresponding to increasing amplitude. 
In a similar manner to the production of a vertical oscillogram of the 
highest digital values within each line period of the digital video 
signal, one can also produce a vertical oscillogram of the smallest 
digital values within each line period of the digital video signal. For 
this purpose, by means of an inverter 34, the applied digital video signal 
is inverted, and is delivered to the input A of a comparator 35 and to the 
data input D of a D flip-flop 36. The output Q of the D flip-flop 36 is 
connected to the input B of the comparator 35. Clocking of the D flip-flop 
36 is effected by pulses which are available at the output A&gt;B of the 
comparator 35. In synchronism with the action of the D flip-flop 25, the D 
flip-flop 36 is also cleared by means of a horizontal frequency pulse 
signal available at a terminal 37. However, previous to this, by the 
control of a horizontal frequency pulse signal available at the terminal 
38 in synchronism with that at 27, the information located at the output Q 
of the D flip-flop 36 is temporarily stored in a further D flip-flop 39, 
and is also delivered inverted from the Q output of the flip-flop 39 to 
the input A of a further comparator 40. A further input B of the 
comparator 40 is connected to the output of the binary counter 30. When 
the comparator 40 determines parity of value between the digital values of 
the amplitude minimum thus available at its input A and the reference 
value at its input B, a pulse is delivered also through the adding stage 
32, to the monitor 33 and there causes momentary brightening of the line 
raster in the manner previously described. The totality of all the 
brightened up picture elements in each line produces a vertical 
oscillogram corresponding to the smallest digital values within each line 
period of the digital video signal applied at the point 21. 
The time axis of the oscillograms represented upon the monitor 33 proceeds 
vertically with respect to the line direction from the top to the bottom 
of the picture screen. If the binary counter 30 begins to count from zero 
at the beginning of each line period, then the picture elements having the 
smallest amplitude values are indicated at the left hand side of the 
picture screen of the monitor 33 and those with the largest amplitude 
values are indicated at the right hand side. By a suitable choice of the 
frequency of the clock signal at 31 it is possible to obtain a desired 
scale in the horizontal direction of the picture screen, as the clock 
frequency determines the width of the columns having respective reference 
values allocated thereto. 
A saving in the amount of capital expenditure on circuitry can be achieved 
by eliminating the stages 35 to 39 if the greatest and smallest digital 
values are determined alternately line by line. This can be achieved by 
supplying non-inverted and inverted digital video signals alternately line 
by line to 24 and 25, and by switching alternately line by line between 
the Q and Q outputs of the D flip-flop 28. Instead of line by line, the 
alternation can be frame by frame if desired. 
For measurement purposes it is furthermore possible to superimpose 
calibration or reference lines upon the vertical oscillogram on the 
picture screen of the monitor 33. The production of calibration lines 
takes place in a calibration line generator 41. The derivation of 
calibration lines takes place in a manner equivalent to the method above 
described, except that instead of using a digital video signal a digital 
reference voltage is processed having an amplitude corresponding to the 
desired column of the line raster in which the calibration line is to 
appear. If desired, it is possible to produce the calibration lines as 
dashed lines by reducing the reference voltage to zero at regular 
intervals. Furthermore it is possible for the dashes in the calibration 
lines to be produced with reference to a predetermined time scale so that 
an optically legible time base is obtained. 
If it is desired that the vertical oscillograms represented upon the 
picture screen of the monitor 33 be observable for a long period of time, 
independently of the applied digital signal at 21, then it is possible to 
provide between the adding stage 32 and the monitor 33 a storage device 42 
for storing the pulses which bring about the raster brightening operation. 
The control of this store is effected by control signals which are derived 
from the synchronous component of the digital video signal applied at the 
point 21. 
FIG. 3 is a block diagram of an apparatus for producing a vertical 
oscillogram of the picture elements occurring in the same position in 
successive lines of an input digital video signal, i.e. elements which 
would be reproduced vertically below one another if the digital video 
signal were reproduced according to conventional practice. 
In the block schematic diagram of FIG. 3 the n-bit digital video signal 
applied at the point 21 is delivered to the data input D of a D flip-flop 
45. Upon the occurrence of a horizontal frequency scanning pulse H.sub.A 
at the terminal 46, the current digital value of the video signal is taken 
from the output Q of the D flip-flop 45 and conveyed to the data input C 
of a further D flip-flop 47. In each line of the video signal at 21 the 
pulse H.sub.A selects the element at the same position for transference to 
the flip-flop 47, whereby the digital values processed are those 
corresponding to picture elements which would be vertically aligned in 
conventional reproduction. The time of occurrence of the pulses H.sub.A 
may be adjusted so as to select a different line position of the elements 
to be evaluated. In dependence upon a horizontal frequency pulse signal 
applied at a terminal 48, the digital information situated at the input D 
of the flip-flop 47 is accepted by the output Q and remains there 
available and uninterrupted for the duration of the next line period. The 
output Q of the D flip-flop 47 is connected to the input A of a comparator 
49 and to the D input of a further D flip-flop 50. The other input B of 
the comparator 49 is connected to the output Q of a binary counter 51. 
This binary counter 51 counts the pulses of a clock signal applied at a 
terminal 52, the clock pulses having a frequency which is an integral 
multiple of the line frequency. The digital value available at the output 
of the binary counter 51 increases from 0 to 2.sup.n-1 during each line 
period and thus allocates to vertical columns of the line raster of a 
monitor 53 respective reference values in the same way as in FIG. 2. 
During each line period the comparator 49 delivers a logical 1 at an 
output A=B when the binary counter 51 achieves a value equal to that of 
the scanned digital picture signal at the A input of 49. In the simplest 
case this logical signal can be indicated directly upon the monitor 53 by 
brightness control of the picture screen in the manner previously 
described. Because each individual line of the digital video signal is 
evaluated in such manner, there will thus be produced on the line raster 
of the monitor 53 a punctiform vertical oscillogram. 
If the digital values of adjacent lines differ from each other materially, 
then the brightened picture elements will appear upon the picture screen 
of the monitor 53 in a scattered formation, which will lack the desired 
substantially continuous line formation of the previously described 
oscillograms. The D flip-flop 50 serves to connect by horizontal lines the 
individual points of the vertical oscillogram, this flip-flop cooperating 
with a comparator 54 and a logic network comprising two AND gates 55 and 
56 and an OR gate 57. Upon the appearance of a pulse from the line 
frequency signal applied to the terminal 48, the information situated at 
the data input of the D flip-flop 50 is accepted at the output Q and 
delivered to an input A of the comparator 54. Thus during the subsequent 
line period the digital value applied to the A input of the comparator 54 
is derived from the previous line to that applied at the A input of the 
comparator 49. The other input B of the comparator 54 is connected to the 
output of the binary counter 51. The comparators 49, 54 are provided each 
with three outputs A&gt;B, A=B and A&lt;B. Such comparators are available in 
integrated form under the type numbers SN 7485. In the voltage-time 
diagrams of FIG. 3a indicates the binary signal at the output A&gt;B, b the 
binary signal at the output A=B, and c the binary signal at the output A&lt;B 
of the comparator 49, in each case for a digital value at input A lying 
adjacent the left hand dashed line and with B increasing from left to 
right. For the comparator 34 d indicates the binary signal at the output 
A&gt;B, e the signal at the output A=B, and f the signal at the output A&lt;B, 
in this case for a digital value at input A lying adjacent the right hand 
dashed line. By the logic interaction of the logic network 55, 56 and 57 
the result is achieved that the output signal of 57 assumes a logical 1 
value if in the actual line in question A=B, or in the previous line A=B, 
or when the count at B is situated between the two A values. 
In addition to generating the digital reference values for the comparators 
49 and 54, the binary counter 51 can also serve the function of producing 
calibration lines which appear upon the line raster in the form of 
vertical lines at, for example, the amplitude levels 0%, 25%, 50%, 75% and 
100%. 
An AND gate 58 connected to follow the OR gate 57 serves for blanking out 
the retained signal by means of a blanking signal available at a terminal 
39. 
FIG. 5 illustrates an apparatus similar in principle to that of FIG. 1, but 
wherein there are simultaneously reproduced in a superimposed arrangement 
upon the picture screen of the video display device the horizontal 
oscillograms of a plurality of selected line periods of a digital video 
signal. 
In FIG. 5 a digital video signal is available at the point 61. This digital 
video signal may be produced, for example, by an analog-digital conversion 
process in an analog-digital converter 62, to a terminal 63 of which an 
analog video signal is delivered from a video signal source (not shown). 
The digital video signal may have a word length of n bits and be available 
in parallel form. Such a video signal is delivered to the input of a store 
64, the storage capacity of which is sufficient for storing the digital 
values of a complete line period. The control of the store 4 in respect of 
the reading in and reading out operations is effected by two signals, 
which are derived in a control signal generator 65. Thus a certain line of 
the video signal at 61 may be selected and the digital values of the 
individual elements thereof read into and stored in the store 64. The 
digital information circulates in the store 10 at line frequency. During 
the subsequent frame period the digital values successively available at 
the output of the store 64 as the information circulates are delivered to 
an input A of a comparator 66. To another input B of the comparator 66 is 
applied the digital value counted by a line counter 67, to which there is 
delivered for counting from a terminal 68 a horizontal frequency pulse 
signal functioning as a clock signal. The line counter 67 is reset in each 
vertical period and thus establishes the allocation of reference values to 
the raster lines of the display device 72 in a similar way to FIG. 1. Each 
time the information applied to the comparator 66 from the output of the 
line counter 67 is equal to the digital information from the output of the 
store 64, a recognition signal is produced at the output A=B of the 
comparator 66, which signal represents an element of the desired 
oscillogram and is applied through an OR gate 69 to an input of a further 
store 70. A control signal for writing-in the recognition signal available 
at the input of the store 70 is generated in the control signal generator 
65. The output of the store 70 is connected to its input through the OR 
gate 69. By this means it is possible for the information which has been 
written in to the store 70 by the recognition signal to circulate in a 
manner similar to that in a ring store. In the present example the storage 
capacity of the store 70 is so designed that it is possible to store a 
complete frame of information. By the provision of a further store 71 
(indicated in dashed lines) the signal available at the output of the 
store 70 is delivered to the monitor 72, and in the manner previously 
described causes a momentary brightening of the line raster upon the 
picture screen. By means of a calibration line generator 73 it is possible 
to superimpose calibration lines upon the oscillograms reproduced upon the 
picture screen of the monitor 72. For this purpose pulse signals are 
delivered from the line counter to the calibrating line generator 73 for 
the purpose of synchronization. The signals received at the output of the 
calibrating line generator 73 can be added, through the OR gate 69, to the 
information circulating in the ring mode in the store 70. 
During the successive comparison of the individual digital values 
circulating in the store 64 with the respective reference values 
established by the counter 67, all the recognition pulses (oscillogram 
elements) from the comparator 66 are written into the store 70 and 
circulate therein at the frame frequency. 
The same procedure is now followed for one or more succeeding lines of the 
video signal at 61, and in each case the newly arriving oscillogram 
elements, if they differ from those which have already been inscribed, are 
additionally recorded in the store 70. This procedure continues until as 
many as desired or all of the lines of the video signal have been 
evaluated and the respective newly arriving oscillogram elements have been 
inscribed in the store 70. Thus the content of the store 70 represents a 
sum total of the oscillograms of (in the extreme case) all the lines 
comprising a frame and can be taken into the following store 71. The 
content of this store is read out at frame frequency and the signal thus 
produced is delivered to the monitor 72. The store 71 may be omitted if 
the structure of the sum oscillogram, and its clearance upon switching off 
the monitor 72, is found not to introduce disturbance. 
The installation of the store 71 is of advantage, however, in that by 
taking over the information contained in the store 70, the store 70 can be 
cleared and a new sum oscillogram can be established. The establishment of 
a sum oscillogram may be accelerated by evaluating, not each line, but 
each nth line of the video signal. A further possibility of increasing the 
speed of oscillogram production when superinscribing horizontal 
oscillograms may be achieved by provision of a plurality of stores 64 and 
comparators 66. By such means it is possible to obtain parallel processing 
of all the lines in a frame. 
The operation of the apparatus is not restricted to what has been described 
above. By variations in the control of the stores it would, for example, 
be possible to represent only one selected line of the digital video 
signal as a horizontal oscillogram, or to select a plurality but less than 
all of lines of the digital video signal for superimposed representation. 
This possibility is indicated in the drawing by the two additional lines 
shown at the left hand side underneath the block symbol of the control 
signal generator 65.