Television system and data generator and receiver suitable therefor

Television transmission or data storage system with time-division multiplex encoding. At least two signal sources are coupled via an encoding circuit for time-division multiplex encoding to a transmission or storage channel to which a data receiver including a decoding circuit can be connected. The signal supplied comprises two or more sub-picture signals during the line periods and the signal-compression or expansion time ratios for the different sub-picture signals are different. The decoding circuit is suitable for consecutively supplying signals during a line period of the data receiver, which signals largely correspond to the different sub-picture signals.

BACKGROUND OF THE INVENTION 
The invention relates to a television transmission or data storage system 
comprising at least one data generator, at least one data receiver and a 
transmission or data storage channel arranged between the generator and 
the receiver, said data generator for supplying two essentially different 
picture signals being provided with a first signal source for supplying a 
signal comprising luminance information during a first part of a line 
period of the data generator, and with a second signal source for 
supplying a signal comprising luminance information during a second part 
of the line period of the data generator, the data generator being 
furthermore provided with an encoding circuit for a time-division 
multiplex encoding of the said signals comprising information and of 
signals comprising synchronizing and identification information, said 
encoding circuit having an output for supplying a time-division multiplex 
encoded signal for transmission via the transmission channel or storage in 
the data storage channel, the data receiver comprising a decoding circuit 
coupled to the said channel having a substantially adapted operation to 
that of said encoding circuit for supplying signals comprising luminance 
information which largely corresponds to the data produced by the signal 
sources in the data generator. The invention also relates to a data 
generator and receiver suitable for use with such a television system. 
A system of this type is known for British Patent Application No. 2,140,242 
(PHN 10,986) in the name of the Applicant. In this Application (see FIGS. 
2d and 2e) a plurality of variants of a television system is described in 
which the luminance information of two different pictures is transmitted 
or stored. The two signals comprising the picture information are 
time-compressed by a time-compression factor which is equal to 0.5 for 
both of them. For each of these signals half of the signal transmission or 
storage capacity is thus used during half a line period. The picture 
information per period in the time-division multiplex encoded signal is 
sequentially composed of the time-compressed first and the time-compressed 
second luminance information. The relevant pictures may be used for the 
purpose of three-dimensional television in which one picture is intended 
for the left eye and the other picture is intended for the right eye. In 
this case both picture signals are supplied by the data receiver after 
time decompression or expansion. The transmitted or stored picture data 
utilizing the same transmission or data storage channel may also relate to 
entirely different pictures. Due to the time compression the two picture 
signals cannot be combined and supplied as such by the data receiver, that 
is to say, the receiver is to be able to select one or the other picture 
signal with the aid of a suitable indentification information. This 
transmission or storage system results in a loss of definition in view of 
the bandwidth limitation emanating from the compression and the 
decompression. 
The invention is based on the recognition that the flexibility of the known 
television system can be even better utilized without too much loss of 
picture quality. To this end a television system according to the 
invention is characterized in that the encoding circuit is suitable for 
supplying at its output a time-division multiplex encoded signal having at 
least two sub-signals in which a first luminance information in the 
sub-signal supplied during the first part of a line period of the data 
generator corresponds to the luminance information of part of a line 
period of the first signal source and in which a second luminance 
information in the sub-signal supplied during the second part of the line 
period of the data generator corresponds to the luminance information of 
part of a line period of the second signal source, said signals being 
supplied after a change or no change in duration, the signal compression 
or expansion time ratio of the signal comprising the first luminance 
information differing from the corresponding ratio of the signal 
comprising the second luminance information, said ratio indicating the 
ratio between the durations of a luminance information before and after a 
change in duration, the decoding circuit being suitable for consecutively 
supplying during a line period of the data receiver a signal comprising 
luminance information which largely corresponds to the first luminance 
information, and a signal comprising luminance information which largely 
corresponds to the second luminance information. 
Due to the measure according to the invention both sub-pictures with 
different time ratios for signal compression or expansion are transmitted 
or stored. Upon display the horizontal resolution of one sub-picture is 
therefore higher than the horizontal resolution of the other sub-picture. 
The more transmission or storage time is available for a sub-picture, the 
higher the horizontal resolution of this sub-picture and the lower the 
horizontal resolution of the other sub-picture. Thus it is obvious that a 
better quality of the first-mentioned sub-picture is at the expense of the 
quality of the second sub-picture. Consequently a choice has to be made 
between the two sub-pictures on the side of the data generator. For the 
sub-picture which is considered to be most important a high resolution may 
be given because little signal compression takes place whereas a lower 
resolution suffices for the other sub-picture because more signal 
compression is used. The above-defined time ratio is inverse to the 
above-mentioned time-compression factor. If this ratio is equal to 1:1, 
there is no signal compression and no signal expansion, whereas a ratio of 
more than 1:1 involves a signal compression and a ratio of less than 1:1 
involves a signal decompression or expansion. 
The television system according to the invention is also suitable for 
transmitting or storing colored images. A television system in which the 
signal from the first signal source and the signal from the second signal 
source also comprise chrominance information components, the encoding 
circuit being also suitable for a time-division multiplex encoding of the 
said signals with chrominance information and the decoding circuit being 
also suitable for supplying signals comprising chrominance information 
which largely corresponds to the data produced by the signal sources in 
the data generator is characterized in that in the time-division multiplex 
encoded signal applied to the output of the encoding circuit a first 
chrominance information during the said first part of a line period of the 
data generator corresponds to the chrominance information of the said part 
of a line period of the first signal source and that during the said 
second part of a line period of the data generator a second chrominance 
information corresponds to the chrominance information of the said part of 
a line period of the second signal source, the signal compression time 
ratio of the signal comprising the first chrominance information differing 
from the corresponding ratio of the signal comprising the second 
chrominance information, the decoding circuit being suitable for 
consecutively supplying during a line period of the data receiver a signal 
comprising chrominance information which largely corresponds to the first 
chrominance information, and a signal comprising chrominance information 
which largely corresponds to the second chrominance information. 
A television system of this type is advantageously characterized in that 
the signal compression or expansion time ratio of the signal comprising 
the first luminace information is greater than the corresponding ratio of 
the signal comprising the second luminance information and that the signal 
compression time ratio of the signal comprising the first chrominance 
information associated with the signal comprising the first luminance 
information is greater than the corresponding ratio of the signal 
comprising the second chrominance information associated with the signal 
comprising the second luminance information. 
A difference may also be made between the resolutions of the two 
sub-pictures in the vertical direction. A television system in which each 
chrominance information comprises two information components is 
characterized in that the first chrominance information is present during 
a number of line periods per field which is smaller than the number of 
line periods in which the first luminance information is present and that 
at least one information component of the second chrominance information 
is present during the number of line periods per field in which the second 
luminance information is present. 
A data generator suitable for use in a television system according to the 
invention in which the encoding circuit includes a multiplex circuit for 
receiving the luminance information components from the signal sources and 
for generating the time-division multiplex encoded signal, said multiplex 
circuit comprising a switching stage controllable by means of a switching 
signal for selecting between the two signal sources, and a circuit for 
changing the duration for processing the luminance information is 
characterized in that the circuit for changing the duration is 
controllable by means of the switching signal for switching the signal 
compression or expansion time ratio between the first and the second part 
of a line period. 
A data receiver suitable for use in a television system according to the 
invention, in co-operation with a data generator of the above-mentioned 
type, in which the decoding circuit includes a circuit for changing the 
duration for processing the received luminance information is 
characterized in that the circuit for changing the duration is 
controllable by means of a second switching signal for switching the 
signal compression or expansion time ratio between a first part of a line 
period of the data receiver in which the signal comprising luminance 
information which largely corresponds to the first luminance information 
is supplied and a second part in which the signal comprising luminance 
information which largely corresponds to the second luminance information 
is supplied.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1a diagrammatically shows the signal structure of a television signal. 
A field period T.sub.V is plotted vertically and a line period T.sub.H is 
plotted horizontally. For, for example, the European television standard a 
field period has a duration of 20 ms while the line period has a duration 
of 64 .mu.s. For the sake of simplicity the line and field blanking 
periods are not shown in the Figures, that is to say, the periods at the 
beginning of each line or field in which no picture information is 
transmitted. The references Y.sub.1 and Y.sub.2 denote a signal region 
with luminance information in FIG. 1a, while chrominance information is 
denoted by the information components U.sub.1, U.sub.2, V.sub.1 and 
V.sub.2. In this case U.sub.1 and V.sub.1 and U.sub.2 and V.sub.2 are 
chrominance information components which are standardized in accordance 
with the television standard. For the NTSC television standard in 
which T.sub.V has a duration of 16.7 ms, while T.sub.H has a duration of 
63.5 .mu.s, the references U.sub.1 and V.sub.1 and U.sub.2 and V.sub.2 are 
the chrominance information components I and Q standardized in accordance 
with this standard. 
In the signal of FIG. 1a I.sub.1, U.sub.1 and V.sub.1 are picture data 
associated with a first sub-picture, while Y.sub.2, U.sub.2 and V.sub.2 
are picture data associated with a second sub-picture. These picture data 
are encoded in a different manner for transmission and this in such a 
manner that a different transmission time is allotted to each sub-picture. 
FIG. 1b shows the first sub-picture and FIG. 1c shows the second 
sub-picture. A picture of a teacher and a blackboard may serve as an 
example. Recording is effected with the aid of two cameras one of which 
records the teacher and the other is directed to the blackboard. The part 
which is intended for transmission is taken from both camera signals. FIG. 
1b shows that part of the signal from the first camera, for example, the 
first half is taken, whereas the rest is suppressed and FIG. 1c shows that 
part of the signal from the second camera is taken, for example, the 
second half, whereas the rest is suppressed. The picture to be displayed 
consists of the two parts obtained, with the transmission system being 
such that more transmission capacity is made available to the picture data 
Y.sub.2, U.sub.2 and V.sub.2 relating to the blackboard than for the 
picture data Y.sub.1, U.sub.1 and V.sub.1 relating to the teacher. 
FIG. 1a shows that chrominace information is transmitted during a number of 
lines. These lines constitute a horizontal strip which is divided into a 
number of parts. The chrominance information component U.sub.1 of the 63rd 
line of the field period of the first sub-picture is compressed to one 
sixteenth part of the active duration of the first line of the horizontal 
strip, which is approximately 3.3 .mu.s. The duration of the line blanking 
period is assumed to be approximately 12 .mu.s in this case. During the 
subsequent sixteenth part the information component V.sub.1 of the 64th 
line of the field period of the first sub-picture is transmitted in a 
time-compressed form. The subsequent sixteenth part of the first line of 
the horizontal strip comprises the component U.sub.1 of line 65 of the 
said field period, whereafter the component V.sub.1 of line 66 up to the 
subsequent sixteenth part is compressed. When half the signal from the 
first camera is transmitted, that is to say, when half the active duration 
of the line periods of the picture of FIG. 1 b is used, which is 
approximately 26 .mu.s, and when the time ratio for signal compression is 
defined as the ratio between the durations of the information before and 
after a change in time, it is evident from the foregoing that the time 
ratio is 8:1 for both component U.sub.1 and component V.sub.1. During the 
rest of the first line of the horizontal strip no picture data are 
transmitted. Similarly, the second line of the horizontal strip 
consequtively comprises the component U.sub.1 of line 67, the component 
V.sub.1 of line 68, the component U.sub.1 of line 69 and the component 
V.sub.1 of line 70 of the field period of the first sub-picture, after 
which no picture data are transmitted. The subsequent lines of the 
horizontal strip comprise similar data. 
Similarly, the twentieth line of the horizontal strip alternately comprises 
components U.sub.1 and V.sub.1 of the first sub-picture. Subsequently the 
chrominance information component U.sub.2 of the 63rd line of the field 
period of the second sub-picture is transmitted in a timecompressed form 
during the subsequent fourth part of the said line, which is approximately 
13 .mu.s. When half the active duration of the line periods of the picture 
of FIG. 1c is used, it is found that the time ratio for signal compression 
is 2:1 for this component. The same applies to the subsequent fourth part 
of the line of the horizontal strip comprising the component U.sub.2 of 
line 65 of the field period of the second sub-picture, as well as to the 
last fourth part comprising the component U.sub.2 of line 67 of the said 
field period. The subsequent lines of the horizontal strip comprise data 
which are similar to those of the twentieth line. Finally, the 62nd line 
of the horizontal strip consequtively comprises the component U.sub.1 of 
line 307, the component V.sub.1 of line 308, the component U.sub.1 of 
line 309 and the component U.sub.1 line 310 of the field period of the 
first sub-picture, and the component U.sub.2 of line 309 of the field 
period of the second sub-picture, after which no data are transmitted i.e. 
in the second half of the 62nd line. 
During a number of 248 lines transmitted after the horizontal strip, the 
luminance information Y.sub.1 of the relevant line of the first 
sub-picture, the chrominance information component V.sub.2 and the 
luminance information Y.sub.2 of the relevant line of the second 
sub-picture aree consecutively transmitted, information Y.sub.1 is 
time-compressed to the first fourth part of the active line period, 
whereafter information V.sub.2 is compressed to the subsequent fourth 
part. During the second half of the active line period information Y.sub.2 
is transmitted. Since the said three information components in FIGS. 1b 
and 1c are present during half the time, these processing steps imply that 
information components Y.sub.1 and V.sub.2 are time-compressed with a 
signal compression time ratio of 2:1, whereas information Y.sub.2 is not 
compressed which corresponds to a signal compression time ratio of 1:1. 
It is evident from the foregoing that the television signal of FIG. 1a 
comprises the picture data of the two sub-pictures: the luminance 
information Y.sub.1 and the chrominance information components U.sub.1 and 
V.sub.1 of the first sub-picture are time-compressed, whereas the 
chrominance information components U.sub.2 and V.sub.2 of the second 
sub-picture are time-compressed and the luminance information Y.sub.2 is 
not time-compressed. In addition the components U.sub.1 and V.sub.1 of the 
first sub-picture and the component U.sub.2 of the second sub-picture are 
only transmitted every other line, whereas the luminance information 
components Y.sub.1 and Y.sub.2 of the two sub-pictures and the component 
V.sub.2 of the second sub-picture are transmitted every line. 
FIG. 2 shows a block diagram of a television data generator for supplying 
the signal of FIG. 1a in which the reference numeral 1 denotes the first 
camera and 2 denoted the second camera. Each camera supplies color 
information components R, G and B to matrix circuits 3 and 4, 
respectively, whose output signals are the luminance information 
components Y.sub.1 and Y.sub.2, and the chrominance information components 
U.sub.1 and V.sub.1, and U.sub.2 and V.sub.2, respectively. The relevant 
information components relate to the entire picture in accordance with 
FIGS. 1b and 1c, respectively, i.e. without suppression of a part thereof. 
The information components Y.sub.1, U.sub.1, V.sub.1, Y.sub.2, U.sub.2 and 
V.sub.2 are applied as input signals to an encoding circuit 5 an output 6 
of which is connected as an output of the data generator to a transmission 
channel 7, for example, a satellite connection, a transmission channel on 
earth or a cable connection. Channel 7 may be a data storage channel with 
data storage and display apparatus. The specific construction of channel 7 
is irrelevant to the invention. 
Encoding circuit 5 includes six low-pass filters 8, 9, 10, 11, 12 and 13 to 
which the information components Y.sub.1, U.sub.1, V.sub.1, Y.sub.2, 
U.sub.2 and V.sub.2 respectively, are applied. The cut-off frequencies of 
filters 9, 10, 12 and 13 may be lower than the cut-off frequencies of 
filters 8 and 11 in this case. The bandwidth-limited output signals of the 
filters are applied to analog-to-digital converters 14, 15, 16, 17, 18 and 
19 to which also clock pulses are applied, more specifically with a clock 
pulse frequency fc to converters 14 and 17 for converting the information 
components Y.sub.1 and Y.sub.2, with a clock pulse frequency 1/8 fc to 
converters 15 and 16 for converting the components U.sub.1 and V.sub.1 and 
with a clock pulse frequency 1/2 fc to converters 18 and 19 for converting 
the components U.sub.2 and V.sub.2. In this case fc is equal to, for 
example, 20.25 MHz. These converters are succeeded by a multiplex circuit 
in the form of a switching stage 20 receiving a switching signal S1. Under 
the control of signal S1 either the digital information Y.sub.1 of 
converter 14, or the digital information Y.sub.2 of converter 17 is 
applied to a compression circuit 21. In the former case the digital 
information U.sub.1 of converter 15 is applied to a compression circuit 
22, while the digital information V.sub.1 of converter 16 is applied to a 
compression circuit 23. In the latter case in which the digital 
information Y.sub.2 is applied to circuit 21, the digital information 
U.sub.2 of converter 18 is applied to circuit 22 under the control of 
signal S1, while the digital information V.sub.2 of converter 19 is 
applied to circuit 23. Consequently a choice is made between the 
information components originating from cameras 1 and 2 by means of signal 
S1 which has the line frequency with an edge in the center of the active 
duration of a line period. During the first half of the said active 
duration the information components Y.sub.1, U.sub.1 and V.sub.1 are 
transmitted and during the second half the information components Y.sub.2, 
U.sub.2 and V.sub.2 are transmitted. 
Compression circuits 21, 22 and 23 have a known construction. They comprise 
line memories i.e. digital memories whose storage capacity is sufficient 
to store the information of a television line and which are of the type 
having different write and read rates. Circuits 21, 22 and 23 receive 
clock pulse signals and switching signals. Clock pulse signals with the 
clock pulse frequencies 1/2 fc and fc are applied to circuit 21, as well 
as a symmetrical square-wave signal S2 of half the line frequency and two 
line frequency square-wave switching signals, namely signal S1 and a 
signal S3 which has a pulse level during the first fourth part and during 
the second half of the active duration of a line period and has a 
different pulse level during the second fourth part of the said duration. 
Clock pulse signals with the clock pulse frequencies fc, 1/2 fc and 1/8 fc 
are applied to circuit 22, as well as signal S2 and two line-frequency 
switching signals, namely signal S1 and a signal S4 which has a pulse 
level during the first and the third sixteenth part and during the last 
three fourth parts of the active duration of a line period and a different 
pulse level during the rest of the period. Similarly, clock pulse signals 
with the clock pulse frequencies fc, 1/2 fc and 1/8 fc are applied to 
circuit 23, as well as signal S2 and two line-frequency switching signals, 
namely signal S1 and a signal S6 which has a pulse level during the 
second, the fourth sixteenth part and the second fourth part of the active 
duration of a line period and a different pulse level during the rest of 
the period. 
The digital information Y is written in a line memory with the aid of the 
compression circuit 21 for Y with the clock pulse frequency 1/2 fc during 
the first half of the active duration of a line period, and this 
information is written with the frequency fc during the second half. 
During the first fourth part and during the second half of the active 
duration of the subsequent line period the memory is read at the frequency 
fc. The same processing steps are effected with a delay of one line period 
in another line memory. Similarly, the digital information U is written in 
a line memory with the aid of the compression circuit 22 for U with the 
clock pulse frequency 1/8 fc during the first half of the said duration 
and this information is written at the frequency 1/2 fc during the second 
half. During the first or the third sixteenth part of the active duration 
of the subsequent line period and during the last three fourth parts of 
the active duration of the subsequent line the memory is read with the 
frequency fc. The same processing steps are effected with a delay of one 
line period in another line memory. The digital information V is written 
in a line memory with the aid of the compression circuit 23 for V with the 
clock pulse frequency 1/8 fc during the first half of the active duration 
of a line period and this information is written with the frequency 1/2 fc 
during the second half. During the second or the fourth sixteenth part of 
the active duration of the subsequent line period and during the second 
fourth part of the active duration of the subsequent line the memory is 
read with the frequency fc. The same processing steps are performed with a 
delay of one line period in another line memory. The obtained output 
signals from circuits 21, 22 and 23 are applied to three inputs 25, 26 and 
27 of a multiplex circuit 24 which applies these signals to two outputs 28 
and 29 under the control of switching signals S3, S4 and S5. Signal S5 has 
the field frequency with an edge after the 62nd line period. During the 
first fourth part and during the second half of the active duration of 
lines 63 to 310 inclusive input 25 and output 28 are interconnected and 
during the second fourth part of the said duration input 27 is connected 
to output 28. During the first or the third sixteenth part of the active 
duration of lines 63 to 310 inclusive and during the second, the third or 
the last fourth part of the active duration of the same lines, input 26 
and output 29 are interconnected and during the second or the fourth 
sixteenth part of the active duration of the same lines input 27 is 
connected to output 29. During the first 62 lines output 28 and output 29 
are not connected. The signal obtained at output 28 is applied to a memory 
30 for the Y signal and the signal obtained at output 29 is applied to a 
memory 31 for the chrominance signal. 
Memories 30 and 31 are field memories, which are digital memories whose 
storage capacity is sufficient to store the information of a television 
field. As a result a delay of one field period may be caused. Both 
memories receive the clock pulse signal at the clock pulse frequency fc. 
During a field period the digital information is written at the input of 
each memory and during the sebsequent field period the memory is read. 
This applies to a non-interlaced system. In an interlaced system the delay 
that occurs must be substantially equal to one field period and to an 
integral number of line periods, in this example 62+248=310 lines. Memory 
31 is controlled by an address generator 35 ensuring that the different 
parts of the chrominance signal of FIG. 1a are arranged in the correct 
position. The outputs of memories 30 and 31 are each connected to a 
selection contact of a switching stage 32 the master contact of which is 
connected under the control of switching signal S5 either to the output of 
memory 31, more specifically during the first 62 lines of each field, or 
to the output of memory 30, more specifically during lines 63 to 310 
inclusive. This shows that the information from cameras 1 and 2 relating 
to the first 62 lines of the field is suppressed. The master contact of 
stage 32 is connected to a digital-to-analog converter 33 to which a clock 
pulse signal with the clock pulse frequency fc is applied and whose output 
signal is present via a low-pass filter 34 at the output 6 of encoder 
circuit 5. It is assumed in this case that channel 7 is suitable for 
analog signal processing. The signal at output 6 is the signal of FIG. 1a. 
FIG. 3 shows an embodiment of a data receiver receiving the signal of 
channel 7 at an input 42 of a decoding circuit 41. For its operation it is 
essentially complementary to the encoding circuit 5 of FIG. 2, i.e. a 
luminance information Y and chrominance information components U and V 
occur at outputs 43, 44 and 45, respectively, of decoder circuit 41, which 
components correspond as much as possible with the parts of information 
components Y, U and V processed in encoding circuit 5. 
In decoding circuit 41 input 42 is connected to the input of a low-pass 
filter 46. The output of filter 46 is connected through an 
analog-to-digital converter 47, to which a clock pulse signal is applied 
with the frequency fc, to a chrominance memory 48 controlled by an address 
generator 49, to a first selection contact 51 of a switching stage 50 and 
to a first decompression circuit 53. An output of memory 48 is connected 
to a second selection contact 52 of stage 50 and to a second decompression 
circuit 54. The master contact of stage 50 is connected to a third 
decompression circuit 55. Decompression circuits 53, 54 and 55 are 
designed in known manner. They comprise line memories which are of the 
type having different write and read rates and they receive clock pulse 
signals and switching signals. 
Clock pulse signals with the clock pulse frequencies 1/2 fc and fc are 
applied to circuit 53, as well as a line-frequency square-wave switching 
signal S8 which has an edge in the center of the active duration of a line 
period, a line frequency switching signal S9 having a pulse level during 
the second fourth part of the active duration of a line period and having 
a different pulse level during the rest of the period, a symmetrical 
square-wave signal S10 of half the line frequency and a field-frequency 
switching signal S11 having an edge after the 62nd line of each field. The 
incoming digital information at the clock pulse frequency fc is entered 
into a line memory with the aid of the decompression circuit S3 for Y 
during the first fourth part and during the second half of the active 
duration of lines 63 to 310 inclusive. During the first half of the active 
duration of the subsequent line period the memory is read at the frequency 
1/2 fc and during the second half it is read at the frequency fc. The same 
processing steps are performed in another line memory with a delay of one 
line period. Under these circumstances a signal is present at the output 
of circuit 53, which signal is time-decompressed or expanded with a time 
ratio for signal expansion of 1:2 during the said first half and which has 
undergone no decompression during the said second half, which is a time 
ratio of 1:1. During the first 62 lines of the field circuit 53 does not 
have an output signal. It is evident from the foregoing that the output 
signal of circuit 53 which is present corresponds to the luminance 
information Y.sub.1 of the first sub-picture during the first half of the 
line and to the luminance information Y.sub.2 of the second sub-picture 
during the second half. The output of circuit 53 is connected to the 
output 43 of decoding circuit 41 via a digital-to-analog converter 56 and 
a low-pass filter 59. Converter 56 receives switching signal S8 with which 
switching between the clock pulse signals with the frequencies 1/2 fc and 
fc is effected, the first-mentioned being applied during the first half of 
each line and the second being applied during the second half of each 
line. 
Memory 48 receives switching signal S10 and the clock pulse signal with the 
frequency fc for writing during the first 62 lines of each field. Memory 
48 is read at the same clock pulse frequency with the aid of address 
generator 49, and this in such a manner that the correct chrominance 
information is passed on so that the output signal of memory 48 does not 
contain any information during the first 62 line periods, after which it 
contains the following information components for each line: the component 
U.sub.1 of the same line during the first sixteenth part of the active 
duration of the line, the component V.sub.1 of the subsequent line during 
the second sixteenth part and the component U.sub.2 of the same line 
during the second fourth part. The compression of the said components has 
not been changed. Clock pulse signals with the clock pulse frequencies 1/8 
fc, 1/2 fc and fc are also applied to circuit 54 which receives this 
output signal, as well as switching signals S8 and S10 and a 
line-frequency switching signal S12 having a pulse level during the first 
sixteenth part and during the second fourth part of the active duration of 
a line period and having a different pulse level during the rest of the 
period. A line memory is written at the clock pulse frequency fc with the 
aid of the decompression circuit 54 for U during the said intervals and 
the memory is read at the frequency 1/8 fc during the first half of the 
active duration of the subsequent line period and at the frequency of 1/2 
fc during the second half. The same processing steps are performed with a 
delay of one line period in another line memory. Under these circumstances 
a signal is present at the output of circuit 54, which signal is 
time-decompressed during the said first half with a time ratio for signal 
expansion of 1:8 and which is time-decompressed during the said second 
half with a time ratio for signal expansion of 1:2. In the first case this 
signal corresponds to the chrominance information component U.sub.1 of the 
first sub-picture and in the second case it corresponds to the chrominance 
information component U.sub.2 of the second sub-picture. The output of 
circuit 54 is connected to the output 44 of decoding circuit 41 via a 
digital-to-analog converter 57 which receives a clock pulse signal which 
is switched with the aid of signal S8 between the frequencies 1/8 fc and 
1/2 fc and a low-pass filter 60. 
Switching stage 50 is controlled by switching signals S9 and S11. During 
the second fourth part of the active duration of lines 63 to 310 inclusive 
the master contact is connected to contact 51 so that the digital 
information component V.sub.2 is passed on to circuit 55. During the rest 
of the period of the said lines the master contact of stage 50 is 
connected to contact 52 so that the digital information components 
U.sub.1, V.sub.1 and U.sub.2 originating from memory 48 are passed on to 
circuit 55. Clock pulse signals with the clock pulse frequencies 1/8 fc, 
1/2 fc and fc are also applied to circuit 55, as well as switching signals 
S8 and S10 and a line-frequency switching signal S13 having a pulse level 
during the second sixteenth part and during the second fourth part of the 
active duration of a line period and having a different pulse level during 
the rest of the period. With the aid of the decompression circuit 55 for V 
writing is effected in a line memory at the clock pulse frequency fc 
during the said intervals. During the first half of the active duration of 
the subsequent line period the memory is read at the frequency 1/8 fc and 
during the second half it is read at the frequency 1/2 fc. The same 
processing steps are performed in another line memory with a delay of one 
line period. Under these circumstances a signal is present at the output 
of circuit 55, which signal is time-decompressed during the said first 
half with a time ratio for signal expansion of 1:8 and which is 
time-decompressed during the said second half with a time ratio for signal 
expansion of 1:2. In the first case this signal corresponds to the 
chrominance information component V.sub.1 of the first sub-picture and in 
the second case it corresponds to the chrominance information component 
V.sub.2 of the second sub-picture. The output of circuit 55 is connected 
to the output 45 of decoding circuit 41 via a digital-to-analog converter 
58 which receives a clock pulse signal which is switched with the aid of 
signal S8 between the frequencies 1/8 fc and 1/2 fc and a low-pass filter 
61. 
It will be noted that the circuits described with reference to FIGS. 2 and 
3 may include other parts which are not important to the invention and 
which have therefore been omitted for the sake of simplicity. Delay 
elements for compensating delays caused by different elements, 
specifically filters are such parts. A refinement can be introduced in 
known manner in the decoding circuit according to FIG. 3, which consists 
in that the information component U.sub.1 and/or U.sub.2 in the data 
receiver is repeated after one line period for display instead of the 
non-transmitted corresponding information of the subsequent line. As a 
rule the repeated information deviates only very little from the 
non-transmitted information so that an improvement of the vertical 
resolution is obtained. A further improvement is obtained in that the 
information which replaces the non-transmitted information is not the 
information of the previous line, but is the result of the interpolation 
between this information and the information of the subsequent line, for 
example, for the component U.sub.1 of line 64 it is half the sum of the 
component U.sub.1 of lines 63 and 65. 
FIG. 1d shows the picture produced upon display with the aid of the data 
receiver of FIG. 3. The picture consists of two sub-pictures with each 
sub-picture covering half the width of the total picture in this example. 
The sub-picture with the information components Y.sub.1, U.sub.1 and 
V.sub.1 is displayed on the left, which is the teacher in the example, and 
the sub-picture with the information components Y.sub.2, U.sub.2 and 
V.sub.2 is displayed on the right, which is the blackboard in this 
example. A number of lines without picture data is present at the top of 
the picture. If desired these lines may be displayed in black because the 
data receiver includes a circuit for suppressing the display during these 
line periods. It is evident from the foregoing that the sub-pictures are 
not transmitted and displayed in the same manner. Signal compression time 
ratios of 2:1 for the luminance information Y.sub.1 and of 8:1 for the two 
chrominance information components U.sub.1 and V.sub.1 apply to the first 
sub-picture in FIG. 2. If the bandwidth of the transmission channel, for 
example, a satellite channel is 8.4 MHz, the described transmission 
results in a reduction of the bandwidth by a factor of 2 for the luminance 
information and by a factor of 8 for the two chrominance information 
components, i.e. 4.2 MHz and 1.05 MHz, respectively. The corresponding 
figures are 2.5 and 0.625 MHz for a cable system having a bandwidth of, 
for example, 5 MHz. Signal compression time ratios of 1:1 for the 
luminance information Y.sub.2 and of 2:1 for the two chrominance 
information components U.sub.2 and V.sub.2 apply to the second sub-picture 
of FIG. 2. In the said satellite channel transmission a bandwidth of 8.4 
MHz is obtained for Y.sub.2 and of 4.2 MHz for U.sub.2 and V.sub.2 and in 
the said cable system the bandwidth is 5 MHz for Y.sub.2 and 2.5 MHz for 
U.sub.2 and V.sub.2. These figures show that the second sub-picture is 
displayed with a higher resolution, both for luminance and for chrominance 
than the first sub-picture, the chrominance resolution being lower than 
the luminance resolution for both sub-pictures. A selection as to which 
sub-picture will be displayed with more resolution can be made in the data 
generator. 
The considerations concerning resolution relate to the horizontal 
resolution. In the example described the vertical resolution for the 
luminance information components Y.sub.1 and Y.sub.2 of the two 
sub-pictures is the same. For the chrominance information components 
U.sub.1 and V.sub.1 of the first sub-picture a reduction by a factor of 2 
is effected, while the same reduction is effected for only the component 
U.sub.2 of the second sub-picture and not for the component V.sub.2. The 
structure of the signal of FIG. 1a may have many variants while 
maintaining the measure according to the invention. For example, the lines 
whose picture data are not displayed may be divided into a part at the 
beginning and a part at the end of the field so that a dark horizontal 
strip is visible upon display both at the top and at the bottom of the 
picture, with the height of each strip being half the height of the 
horizontal strip in FIG. 1d. The chrominance information components 
transmitted during the said lines may also occur in a different sequence. 
It will be noted that the television system described can also be used as 
a monochrome television system for at least one sub-picture, for example, 
for displaying text and/or graphic characters with no chrominance 
information being transmitted. If no sub-picture which is to be displayed 
comprises chrominance information, the picture data of all lines of the 
field are transmitted and displayed. In addition the transmission time for 
information Y.sub.2 may be extended in this case relative to FIG. 1a. This 
implies that the useful duration of the picture in the horizontal 
direction in FIG. 1c is longer than the corresponding duration in FIG. 1b 
and/or that the part of period T.sub.H in FIG. 1a in which Y.sub.2 is 
transmitted is longer than the part in which Y.sub.1 is transmitted. In 
this case, which is for that matter also possible in a television system 
in which chrominance information is also transmitted and displayed, a time 
expansion for information Y.sub.2 may take place. As a result a still 
wider band and consequently a still better horizontal resolution is 
obtained for this information than has been described in the foregoing. A 
compression for this information is then to be effected in the data 
receiver. It will also be noted that the part in FIGS. 1b and c intended 
for transmission may be arbitrarily located relative to the entire 
picture, for example, in its center. It is also possible to transmit and 
display more than two, for example, three sub-pictures which can be 
encoded in different manners for transmission and to which sub-pictures 
different transmission times can be allotted so that the picture displayed 
reveals differences in horizontal resolution between the sub-pictures. The 
limits between the different signal regions allotted to the information 
components Y.sub.1, U.sub.1, V.sub.1, Y.sub.2, U.sub.2, V.sub.2 . . . in 
FIG. 1a or in similar figures can be varied in known manner so that the 
different time ratios for signal compression or expansion may be variable. 
FIG. 4 shows a variant of the signal according to FIG. 1a in which the 
lines of the horizontal strip formed by the first 62 lines at the top are 
entirely utilized for transmitting information. In FIG. 4 fractions denote 
parts of the active duration of the line periods. Nothing has changed with 
respect to FIG. 1a for the information components Y.sub.1, U.sub.1 and 
V.sub.1 of the first sub-picture, which implies signal compression time 
ratios of 2:1, 8:1 and 8:1, respectively. The time after the first fourth 
part of the said active duration is not divided by three but by four, 
which results in the fraction 3/16. This shows that both the components 
U.sub.2 and the component V.sub.2 of the chrominance information of the 
second sub-picture undergo a time compression with a signal compression 
time ratio of 1/2:3/16=2.67:1, i.e. more than was the case in FIG. 1a. On 
the other hand the luminance information Y.sub.2 of the second sub-picture 
is transmitted during 9/16 of the active duration of lines 63 to 310 
inclusive. If half the time of the camera is still used for this 
sub-picture, as was the case in FIG. 1d, this implies a time expansion 
with a signal expansion time ratio of 1/2:9/16=1:1.13. As compared with 
the system of FIG. 1, this system thus yields an improvement of the 
horizontal resolution for Y.sub.2. With reference to the foregoing 
description it will be evident to those skilled in the art in which manner 
the data generator of FIG. 2 and the data receiver of FIG. 3 must be 
modified for the signal of FIG. 4 or of a variant thereof which is not 
shown. In such a variant the components of the first 62 lines may be 
positioned, for example, in such a manner that a component U.sub.2 occurs 
during 3/16 of the said duration every time after a component U.sub.1 
during one sixteenth part of the active duration of a line period. 
Particularly circuit 21 performs a time expansion for the system of FIG. 4 
during the time when information Y.sub.2 is processed, while circuit 53 
performs a time compression.