Arrangement for doubling the field frequency of a picture signal

In an arrangement for converting an original picture signal representing a sequence of frames, each of which is composed of two interlaced fields, into a converted picture signal which has a double field frequency with respect to the original picture signal, is for doubling the field frequency, for the purpose of noise reduction, motion compensation and line flicker reduction, a memory arrangement (1, 2) provided for doubling the field frequency, which memory arrangement precedes a motion compensation arrangement (5) whose output signal is applied to a noise reduction arrangement (6), and a line flicker reduction arrangement (7) is provided which receives the output signals from the noise reduction arrangement (6) and the motion compensation arrangement (5), while the converted picture signal is obtained from the output signal of the noise reduction arrangement (6), the line flicker reduction arrangement (7) or the motion compensation arrangement (5), dependent on the position with respect to time of a field to be generated of the converted picture signal.

BACKGROUND OF THE INVENTION 
The invention relates to an arrangement for converting an original picture 
signal representing a sequence of frames, each of which is composed of two 
interlaced fields, into a converted picture signal which has a double 
field frequency with respect to the original picture signal. 
When converting a picture signal into such a converted picture signal 
which, with respect to the original picture signal, has a double field 
frequency, there is the problem that every second field of the converted 
picture signal must be newly generated, because no corresponding field of 
the original picture signal is available with respect to time and also 
with respect to the picture information. 
In simple arrangements for doubling the field frequency, every field is 
doubled. A moving object in the fields of the converted picture signal is 
imaged twice in the same position before it jumps to the next position in 
the two subsequent fields. Since the human eye cannot follow these jumps, 
it is incident on the average speed of motion and observes a moving object 
from field to field at different positions. This leads to a double 
structure and motion blurr. 
In other arrangements for field doubling of a picture signal a motion 
compensation is therefore provided by means of which the motion between 
two fields of the original picture signal is determined so that the motion 
can be taken into account in fields of the converted picture signal to be 
generated therebetween as a function of time and a corresponding 
interpolation can be performed. However, such arrangements have the 
further problem that possibly present noise is also to be reduced and that 
the line flicker, which still occurs in spite of the doubling of the field 
frequency in picture signals generated by way of interlaced scanning, is 
to be reduced. In the state of the art arrangements are only known in 
which a motion compensation is combined either with a noise reduction or 
with a line flicker reduction. 
SUMMARY OF THE INVENTION 
It is an object of the invention to provide an arrangement in which the 
motion of the picture contents during generation of the compensated fields 
is taken into account when converting the picture signal into a converted 
picture signal at the double field frequency, and which moreover allows a 
noise reduction of the picture signal and a line flicker reduction. 
According to the invention this object is solved in that for doubling the 
field frequency a memory arrangement is provided which precedes an 
arrangement for motion compensation whose output signal is applied to an 
arrangement for noise reduction, in that an arrangement for line flicker 
reduction is provided which receives the output signals from the noise 
reduction arrangement and the motion compensation arrangement and in that 
the converted picture signal is obtained from the output signal of the 
noise reduction arrangement, the line flicker reduction arrangement or the 
motion compensation arrangement, dependent on the position with respect to 
time of a field to be generated of the converted picture signal. 
The actual the field frequency doubling is obtained by means of a memory 
arrangement. Consequently, the fields of the original picture signal are 
repeated at the double frequency so that a double field frequency is 
realised. However, this signal still has the above-mentioned errors. 
An arrangement for motion compensation is therefore provided, which 
arrangement determines motions in the original picture signal and, with 
reference to the known motions, allows a compensation of this motion in 
the new fields to be generated of the compensated signal. 
The arrangement for motion compensation precedes an arrangement for noise 
reduction which combines the data of two consecutive fields for the 
purpose of noise reduction. 
Furthermore, an arrangement for line flicker reduction is provided which 
receives the output signals from the motion compensation arrangement and 
the output signals from the noise reduction arrangement. 
The output signal of the arrangement, i.e. the converted picture signal of 
the double field frequency, is obtained from the output signal of one of 
said three arrangements in dependence upon the position with respect to 
time of a field to be generated of the converted picture signal. This 
alternation between the output signals of the arrangements is advantageous 
because different errors occur, dependent on the position with respect to 
time of the fields of the converted picture signal. In some fields a 
motion compensation is required because these fields occur with respect to 
time between two fields of the original picture signal. This is not 
required for those fields which coincide with pictures of the original 
picture signal. The line flicker reduction is in its turn only required 
for those fields which as a consequence of the interlaced scanning method 
do not have the correct vertical position as compared with the fields of 
the original picture signal from which they are generated. 
The arrangement according to the invention thus offers a combination of 
motion compensation with line flicker reduction and noise reduction. 
An embodiment of the arrangement is characterized in that the original 
picture signal is written into a first field memory from which it is read 
at the double frequency, each field being consecutively read twice, and in 
that a second field memory is provided into which each field read for the 
second time from the first field memory is written after it has passed 
through the noise reduction arrangement. 
The first field memory is thus used for doubling the field frequency. Each 
field written into this memory is read twice consecutively. A second field 
memory already operates at this double field frequency at the input side, 
because each field, which was read from the first field memory for the 
second time and has passed through the noise reduction arrangement, is 
written into this second field memory. After this noise-reduced field has 
been written into the memory, it is available at the output of the second 
field memory. 
Consequently, two fields of the original picture signal, however, with a 
doubled field frequency are available at the outputs of the two field 
memories for the motion compensation arrangement. One of these fields is 
already noise-reduced, which simplifies the determination of motion by the 
motion compensation arrangement. 
A further embodiment of the invention is characterized in that the two 
field memories precede a line memory which buffers a picture line of one 
of the output signals of the two fields. For one of the fields information 
of two consecutive picture lines is thus time-parallel available, which is 
advantageous for the subsequent line flicker reduction. 
In a further embodiment of the invention the arrangement for line flicker 
reduction may advantageously be a median filter whose output supplies that 
input signal which has the middle amplitude value of the input signals. 
In accordance with a further embodiment of the invention the arrangement 
for motion compensation receives the output signals of the two field 
memories and the line memory, which motion compensation arrangement 
determines a motion vector from the two consecutive fields of the original 
picture signal read from the field memories, which motion vector indicates 
the motion between the two fields for a group of pixels of these fields. 
This motion vector may be used for motion compensation in those fields of 
the converted field signal which occur with respect to time between two 
fields of the original picture signal. 
A further embodiment of the invention is characterized in that the 
arrangement generates a sequence of four fields 
(A1.sub.100,B1.sup.-.sub.100,B1*.sub.100,B1.sup.+.sub.100) of the 
converted picture signal corresponding to two fields of a frame of the 
original picture signal, the first field (A1.sub.100) of the sequence 
being obtained from the output signal of the noise reduction arrangement, 
the second and third fields (B1.sup.-.sub.100,B1*.sub.100) of the sequence 
being obtained from the output signal of the line flicker reduction 
arrangement and the fourth field (B1.sup.+.sub.100) of the sequence being 
obtained from the motion compensation arrangement. 
As a consequence of the doubled field frequency of the converted picture 
signal, four fields of the converted picture signal must be generated in a 
time range in which two fields of the original picture signal are present. 
These two fields of the original picture signal and the four fields of the 
corresponding sequence of the converted picture signal will hereinafter be 
referred to as corresponding fields and corresponding sequence, 
respectively. 
The first field of the sequence is obtained from the output signal of the 
noise reduction arrangement. This is possible because this first field of 
the sequence has the right position with respect to time and location as 
compared with the first corresponding field of the original picture signal 
and because only a noise reduction is to be performed. 
The second and third fields of the sequence are obtained from the output 
signal of the line flicker reduction arrangement, because the two fields 
of the original picture signal must be utilized for these two fields, at 
least one of which does not have the correct position with respect to time 
and neither has the correct vertical position due to the interlaced 
scanning method used. 
The signal for the fourth field of the sequence is obtained from the motion 
compensation arrangement, because this signal can only be obtained from 
the second corresponding field of the original picture signal due to use 
of motion compensation. 
The further sub-claims state how the arrangement advantageously generates 
the four fields for the sequence of converted picture signals from the 
corresponding two fields of the original picture signal. 
These and other aspects of the invention will be apparent from and 
elucidated with reference to the embodiments described hereinafter.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION 
FIG. 1 shows a block diagram of the arrangement according to the invention, 
which arrangement allows the field frequency of an original picture signal 
to be doubled and thus generates a converted picture signal, which 
generated picture signal is noise-reduced, and which performs, if 
necessary, a motion compensation and a line flicker reduction for the 
fields. 
The arrangement of FIG. 1 is divided into two blocks, the first block 
processing the received luminance signal component Y.sub.50 of the 
original picture signal and the second block processing the received 
chrominance signal component C.sub.50 of the original picture signal. In 
the embodiment shown in FIG. 1 the chrominance signal is doubled only with 
respect to its frequency. The special procedures of noise reduction and 
line flicker reduction are performed only for the luminance signal in the 
embodiment shown in FIG. 1. However, it is alternatively possible to take 
these measures both for the luminance signal and for the chrominance 
signal. 
In the arrangement shown in FIG. 1 the luminance signal component Y.sub.50 
of the original picture signal is applied to a first field memory 1 by 
means of which the field frequency of this signal is doubled. Each field 
of the original picture signal written into the field memory 1 is 
subsequently read twice. This reading process is performed at the double 
frequency. A simple doubling of the field frequency is thus already 
performed. However, the output signal of this field memory is only 
suitable for display if motion disturbances and line flicker are accepted. 
Furthermore, a second field memory 2 is provided whose input receives 
field signals to be described hereinafter, which signals already have the 
double field frequency. The signals of two consecutive fields of the 
original picture signal are parallel available at the outputs of the two 
field memories 1 and 2, which fields have already been doubled in field 
frequency. 
The two field memories 1 and 2 are followed by a multiplexer 3 allowing one 
of the output signals of the field memories 1 and 2 to be alternatively 
applied to a line memory 4. The output signals of the two field memories 1 
and 2 are applied to a motion compensation arrangement 5 via the 
multiplexer 3. The arrangement 5 thus receives the signals of the two 
field memories 1 and 2 and hence two consecutive fields of the original 
picture signal whose field frequencies have already been doubled. By using 
the line memory 4, the values of two pixels of the same line position of 
consecutive picture lines are simultaneously available for one of the two 
field signals. 
The motion compensation arrangement 5 determines, from the two fields apply 
thereto, a motion which is present in the picture contents between these 
two fields. Advantageously, a motion vector indicating the motion between 
the two fields for a group of pixels is obtained from this determined 
motion for a group of pixels. The motion compensation arrangement 5 can 
determine this motion both in the horizontal direction and in the vertical 
direction, i.e. in the line direction as well as in the direction 
perpendicular to the lines. However, the motion may exclusively be 
determined in the line direction, which is much easier to realise in the 
circuit construction and also yields good results. 
The arrangement shown in FIG. 1 also includes a noise reduction arrangement 
6. This arrangement 6 may operate in known manner in which it combines the 
signals of pixels of the same location in consecutive fields. These 
signals are applied from the arrangement 5 to the arrangement 6. Since the 
arrangement 5 has already determined the corresponding motion vector, the 
noise reduction in the arrangement 6 can already be performed with 
motion-compensated signals. 
The output signal of the noise reduction arrangement 6 is applied to the 
input of the second field memory 2, to an input of a line flicker 
reduction arrangement 7 and to a first input of a multiplexer 8. A signal 
which is already noise-reduced is thus written into the field memory 2 at 
the input side, which signal corresponds to that field which is read from 
the first field memory 1 for the second time already. 
The line flicker reduction arrangement 7 which may be, for example a median 
filter and which selects, from the signals applied thereto, the signal 
with the middle instantaneous amplitude value, not only receives the 
output signal from the arrangement 6 but also the output signal from the 
motion compensation arrangement 5, because this output signal also 
contains the motion-compensated output signal of the line memory 4. This 
is necessary because a vertical interpolation must be performed for the 
line flicker reduction and consequently the pixels corresponding to the 
signals of two lines should be available, i.e. pixels of the same location 
in their line. 
The line flicker reduction arrangement 7 not only receives these signals of 
two successive picture lines of a field from the arrangement 6 but also 
the signal of another field. In a manner to be described hereinafter a 
median filtering of these signals is performed, which leads to a line 
flicker reduction. 
The output signal of the line flicker reduction arrangement 7 is applied to 
a second input of the multiplexer 8. A third input of the multiplexer 8 
receives the output signal from the noise reduction arrangement 5. 
At the output, the multiplexer 8 supplies the luminance signal Y.sub.100 
which represents the converted picture signal and which has a doubled 
field frequency as compared with the input signal Y.sub.50. In a manner to 
be described hereinafter, the multiplexer 8 is switched between its three 
inputs dependent on the field to be generated. 
FIG. 1 further shows a circuit block 9 in which the field frequency of the 
chrominance signal component C.sub.50 of the original picture signal is 
doubled. This can be effected in the same way as for the luminance signal 
but alternatively, the field frequency may be doubled only. At the output, 
the unit 9 supplies the chrominance signal component of the converted 
picture signal. 
FIG. 2 shows a Table indicating diagrammatically which fields are written 
into or read from the field memories 1 and 2 shown in FIG. 1. 
Two consecutive fields of the original picture signal are denoted by A1, B1 
and A2, B2, etc. in an unchanged form. Two fields having the same cipher 
form part of a frame. The two fields are generated in accordance with the 
interlaced scanning method. 
As is shown in the Table of FIG. 2, for example two fields A1 and B1 of a 
frame of the original picture signal are written into the field memory 1 
of FIG. 1, which field memory is denoted by FM1 in FIG. 2. Each of these 
two fields is subsequently read twice from the field memory 1, which 
reading is effected at the double frequency so that the field frequency of 
these pictures is already doubled. 
If a field is read from the first field memory 1 for the second time, this 
signal reaches the input of the field memory 2 denoted by FM2 in the Table 
of FIG. 2, after it has passed through the arrangement 5 and the 
arrangement 6 of FIG. 1. At the next reading step of the field memories 1 
and 2, two fields whose field frequencies have already been doubled are 
available at their outputs. As one of the fields, viz. the field written 
into the field memory 2 has already passed through the noise reduction 
arrangement, this field is already noise-reduced which is denoted by NR in 
the Table of FIG. 2. 
The result is that two fields from the original picture signal having an 
already doubled field frequency are available at the outputs of the field 
memories 1 and 2 in FIG. 1. 
It will now be explained with reference to FIGS. 3 to 6 how the four fields 
A1.sub.100,B1.sup.-.sub.100,B1*.sub.100 and B1.sup.+.sub.100 of the output 
signal Y.sub.100 as shown in the Table of FIG. 2, which are the signals of 
the multiplexer 8 as shown in FIG. 1, are obtained. These four fields are 
hereinafter assumed to be associated with a sequence. A frame of the 
original picture signal or two fields of this signal, viz. the fields A1 
and B1 correspond to this sequence. The four fields of the sequence will 
hereinafter be assumed to correspond to these two fields of the original 
picture signal. 
FIG. 3 shows diagrammatically, above a broken line, two fields B0.sub.NR 
and A1 of the original picture signal read from the two field memories 1 
and 2 of FIG. 1. Below the broken line, a field A1.sub.100 is shown which 
represents the first field of a sequence of the converted picture signal. 
This signal of the field A1.sub.100 is to be generated by the arrangement 
of FIG. 1. 
To this end the output signal of the first field memory 1 is used, from 
which field memory the field A1 of the original picture signal (at the 
doubled field frequency) is read. The field B0 of the original picture 
signal was already previously written in a noise-reduced form into the 
field memory 2. At the output, this signal is now available as signal 
B0.sub.NR at the output of the second field memory simultaneously with the 
signal A1. The first field A1.sub.100 of the sequence is obtained from 
these two output signals of the field memories 1 and 2 in accordance with 
the diagrammatic representation in FIG. 3. 
This field A1.sub.100 to be generated has the correct position vertically 
and with respect to time as compared with the field A1 of the original 
picture signal. Therefore, only a noise reduction should be carried out, 
and a line flicker reduction in particular is not necessary. 
The output signals of the field memories 1 and 2 are utilized for the noise 
reduction, while it is advantageous to submit the field read from the 
field memory 2 and not having the correct position with respect to time as 
compared with the field A1.sub.100 to be generated to a motion 
compensation of its picture contents. The motion vector determined by the 
motion compensation arrangement 5 in accordance with FIG. 1 is utilized 
for this purpose. This motion vector is denoted by v.sub.x in FIG. 3. 
For a pixel marked in picture line 3 of the field A1.sub.100 in FIG. 3, the 
pixel of the same line position and the same line number of the field 
A1.sub.100, as read from the field memory 1, is utilized. Moreover, the 
pixel of the field B0.sub.NR as read from the second field memory and 
offset by the motion vector v.sub.x is used. This pixel is taken from line 
4. A noise-reduced signal is obtained from these two pixels of the two 
fields. A factor k is provided for this purpose, indicating the degree of 
noise reduction. The pixel from the field A1 is multiplied by a factor 1-k 
and the pixel from the field B0.sub.NR is multiplied by a factor k. These 
two multiplied values are added and constitute the value of the marked 
pixel of the field A1.sub.100. 
If k is chosen to be small, only a small or no noise reduction is to be 
performed and this pixel is essentially obtained from the corresponding 
pixel of the field A1. With a larger factor k, the value of the pixel is 
increasingly being taken from the field B0.sub.NR. 
The generated field A1.sub.100 thus corresponds to the field A1 of the 
original picture signal, but for the performed noise reduction. It is 
written into the second field memory 2 of FIG. 1 and is available as 
A1.sub.NR for subsequent fields to be generated. 
During the generation of the first field A1.sub.100 the multiplexer 8 is 
switched to its first input in accordance with FIG. 1, because the output 
signals for the noise reduction are used as output signals in accordance 
with the diagrammatic representation in FIG. 3 and hence as signals for 
the field A1.sub.100. 
FIG. 4 is a representation, corresponding to FIG. 3, for obtaining the 
second field B1.sup.-.sub.100 of the sequence. 
As compared with the two fields of the original picture signal, this second 
field of the sequence neither has a vertically correct position nor a 
correct position as regards time. Therefore, a motion compensation and a 
line flicker reduction are performed. 
At the instant of generating this second field, the field B1 of the 
original picture signal is read from the first field memory and the field 
A1 of the original picture signal is read in a noise-reduced form from the 
second field memory. 
In the representation in FIG. 4 a pixel of the picture line 2 is marked for 
the field B1.sup.-.sub.100. The value of this pixel is generated from 
three values by means of median filtering, which values are obtained from 
the fields A1.sub.NR and B1. 
The first of these values is obtained from the picture line 3 for that 
pixel which, after being offset by half the motion vector (v.sub.x.1/2) 
has the same position as the pixel to be generated in the field 
B1.sup.-.sub.100. The second input signal of the median filter is obtained 
from the pixel of the same line position of line 1 of the field A1.sub.NR. 
The value of this pixel is also multiplied by a factor k. Moreover, that 
pixel of the picture line 2 of the field B1 which, after use of half the 
negative motion vector (-v.sub.x.1/2) has the same picture line position 
as the pixel to be generated of the field B1.sup.-.sub.100 is multiplied 
by a factor 1-k. These two values are added and the sum constitutes the 
third input signal for the median filtering. Due to the median filtering, 
the input signal having the middle instantaneous amplitude value is 
selected from these three input signals. This signal is constituted by the 
value of the marked pixel of the second field B1.sup.-.sub.100 of the 
sequence. 
As already shown in the representation according to FIG. 4, a motion 
compensation for all signals is required for this field. Moreover, a line 
flicker reduction is to be performed. Consequently, the multiplexer 8 is 
switched to its second input for generating the value of the field 
B1.sup.-.sub.100 in accordance with the representation in FIG. 1, which 
input receives the output signal from the line flicker reduction 
arrangement 7. 
FIG. 5 is a representation corresponding to FIGS. 3 and 4, but in the 
representation according to FIG. 5 the third field B1*.sub.100 of the 
sequence is to be generated. 
The two corresponding fields A1 and B1 of the original picture signal are 
used again for generating this field. The field B1 is read from the field 
memory 1 of FIG. 1. The field A1, which is already noise-reduced, is read 
from the field memory 2 of FIG. 1. 
A median filtering is performed again, because the output field B1 has the 
incorrect vertical position. The output field A1.sub.NR additionally has 
the incorrect position with respect to time so that also a motion 
compensation has to be performed for this field. 
A median filtering of three input signals is carried out for generating one 
of the pixels marked in FIG. 5, of the picture line 3 of the field 
B1*.sub.100. 
The first of these input signals represents the value of the pixel of the 
picture line 2 of the field B1, which has the same picture line position 
in its picture line as the pixel to be generated in its picture line. 
Moreover, from the field A1, as read from the second field memory, that 
pixel is used which after correction by the motion vector v.sub.x has the 
same line position as the pixel to be generated. This motion-compensated 
pixel represents the second input signal of the median filter. The third 
input signal is formed by the sum of the value of the same line position 
of the pixel of the picture line 4 of the field B1, multiplied by a factor 
1-k, and the value of the second input signal of the median filter, 
multiplied by a factor k. This sum represents the third input signal of 
the median filter and is simultaneously written as input signal into the 
second field memory from which it can be read again for fields to be 
subsequently generated. 
The multiplexer 8 of the block diagram in FIG. 1 is switched to its second 
input for generating the third field B1*.sub.100 of the sequence, because 
a line flicker reduction as well as a motion compensation have to be 
performed. 
In FIG. 6, corresponding to the representations in FIGS. 3 to 5, the values 
of the fourth field B1.sup.+.sub.100 of the sequence are to be obtained. 
Since the field B1 used for this purpose (in a noise-reduced form) of the 
original picture signal has the correct vertical position, i.e. the same 
position as the field B1.sup.+.sub.100, a line flicker reduction is not 
necessary in this case. The field B1.sub.NR has, however, the incorrect 
position with respect to time so that a motion compensation is necessary. 
Consequently, for a pixel as marked by way of example in FIG. 6 in picture 
line 2 in a given position, that pixel of the field B1.sub.NR as read from 
the field memory 2 is used which has the same line position as the pixel 
to be generated in its picture line after correction by half the motion 
vector (v.sub.x.1/2). 
Since only a motion compensation (in addition to noise reduction) is 
necessary in this case, the multiplexer 8 of FIG. 1 is switched to its 
third input. 
The way of generating a sequence of four fields in accordance with FIGS. 3 
to 6 is continuously repeated, with four corresponding fields of the 
converted picture signal being obtained for two output fields of the 
original picture signal.