Channel selecting apparatus for use with signal receiver and signal recorder

A television system having two independently tuneable, simultaneously operating, channels contains a switching means for simultaneously displaying video from the two channels on a cathode ray tube for permitting comparison therebetween and independent comparative tuning thereof. In one embodiment, video from one of the channels is displayed in the top half of the screen and video from the second channel is displayed in the bottom half of the screen. Synchronizing signals from only one of the channels is used throughout the display in order to avoid disruption in synchronization due to switching back and forth between channels. In another embodiment of the invention, video from one of the sources is displayed in the first half of each horizontal line and video from the second source is displayed in the second half of each horizontal line. Thus, side-by-side comparison can be made of displayed video from the two sources.

BACKGROUND OF THE INVENTION 
The present invention relates to video systems and, more particularly, to 
video systems having two independently tuneable video channels. 
In an apparatus having two independently tuneable video channels such as, 
for example a television receiver channel and a video tape recorder 
channel, it is customary to provide for displaying the video signal 
available to the video tape recorder on the television receiver in order 
to permit adjustment of the tuning of the signal to the video tape 
recorder. While the video tape recorder is recording, many equipments 
permit viewing of television signals on the television receiver from a 
different channel than the one being recorded. Since the channels are 
independently tuneable, it is possible to mistune one or both while they 
are tuned to the same television channel. In order to compare the tuning 
of the two channels, it has been necessary to switch one and then the 
other of the tuning channels to the television receiver in order to 
monitor them. This has been found to be inconvenient. 
In addition, when the input to the television receiver is switched from one 
of the tuning channels to the other, mistuning of the second channel may 
make the user believe that a malfunction exists in the video system where, 
in fact, no malfunction exists. 
OBJECTS AND SUMMARY OF THE INVENTION 
Accordingly, it is an object of the present invention to provide a video 
system which overcomes the drawbacks of the prior art. 
More particularly, it is an object of the present invention to permit 
simultaneous comparison of video tuned in two independently tuneable 
channels on a single display. 
According to an aspect of the invention, there is provided a video system 
which comprises first manually adjustable tuning means for tuning a first 
video signal having video and synchronizing components, second manually 
adjustable tuning means for tuning a second video signal having video and 
synchronizing components, display means for displaying at least one of the 
first and second video signals on a display screen, switch means for 
selectively applying one of the first video signal and the second video 
signal to a first area of the display screen in response to a control 
signal and for applying the other of the first video signal and second 
video signal to a second area of the display screen in response to the 
control signal, means for generating the control signal, and means for 
synchronizing the display means using the synchronizing component only 
from the first manually adjustable tuning means. 
The above, and other objects, features and advantages of the present 
invention will become apparent from the following description read in 
conjunction with the accompanying drawings in which like reference 
numerals designate the same elements.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring to FIG. 1, there is shown tuning circuitry 30 containing a first 
tuning channel identified by the letter A and a second tuning channel 
identified by the letter B. Tuning circuitry 30 may be any suitable signal 
tuning apparatus such as, for example, as in a television or a video tape 
recorder. For purposes of description only, and not as a limitation, it is 
assumed that the items of apparatus identified with the letter A appended 
to their reference numerals are part of a television receiver 10 and those 
which are identified with the letter B appended to their reference 
numerals are part of a video tape recorder 20. 
The tuning circuitry for television receiver 10 consists of an electronic 
tuner 33A, a video intermediate frequency amplifier 34A and a video 
detector 35A in cascade providing a signal to a terminal A in a changeover 
switch 36. These elements are of types well known in the art and will not 
be further described. It is sufficient for purposes of the present 
invention that electronic tuner 33A contains tuning elements, such as a 
conventional varactor tuning element (not shown), which is tuneable by an 
electrical signal applied thereto. 
The tuning circuitry for video tape recorder 20 consists of an electronic 
tuner 33B, a video intermediate frequency amplifier 34B and a video 
detector 35B which provides an output signal to terminal B of changeover 
switch 36 and to video tape recorder 20. The tuning circuitry for video 
tape recorder 20 may be identical to corresponding tuning circuitry for 
television receiver 10. Although changeover switch 36 is represented as a 
mechanical switch for ease of description, changeover switch 36 may be an 
electronic switch using, for example, one or more transistors and/or 
diodes. 
An antenna 31, or other signal source such as a cable, provides television 
signals to a signal distributor 32. The signal distributor provides output 
signals to electronic tuners 33A and 33B. Signal distributor 32 may be a 
conventional signal splitter such as a split transmission line, a 
resistive pad, or an electronic signal distributor according to designs 
well known in the art. 
Monitor circuits 11 in television receiver 10 process the detected video 
signal from terminal A or terminal B of changeover switch 36 and apply the 
video signal to a cathode ray tube 12 to be displayed thereby for 
monitoring by a user. 
The video signal applied by the video detector 35B to video tape recorder 
20 may be recorded therein by operation of a record switch 21. Record 
switch 21 controls recording of the video data in video tape recorder 20 
in a conventional fashion not shown in FIG. 1 and also selectively 
provides an enable signal, namely an open switch contact when record 
switch 21 is open, and an inhibit signal, namely a closed switch contact 
connected to ground when record switch 21 is closed. 
A channel selecting circuitry 40 includes first and second channel 
selectors 37A and 37B which provide control signals, preferably voltages 
having appropriate levels, to electronic tuners 33A and 33B thereby to 
control the channels to which these electronic tuners are tuned and thus 
to control the video signals which are displayed on cathode ray tube 12 
and/or recorded in video tape recorder 20. 
The first or television channel selector 37A consists of a plurality, 
suitably 10, of selector switches S.sub.1 -S.sub.10 connected to the 
collectors of respective switch transistors TA.sub.1 -TA.sub.10. Selector 
switches S.sub.1 -S.sub.10 may conveniently be momentary contact switches 
which are closeable by the user and which return to the open position when 
released. Variable resistors VA.sub.1 -VA.sub.10 are connected from a 
voltage source 61 to the junction of the collectors of respective 
transistors TA.sub.1 -TA.sub.10 and switches S.sub.1 -S.sub.10. Blocking 
diodes are connected in parallel from the movable contacts of variable 
resistors VA.sub.1 -VA.sub.10, respectively, to a control line feeding 
control voltage to electronic tuner 33A. 
A clock generator or oscillator 41 is connected in parallel to one terminal 
of each of normally open selector switches S.sub.1 -S.sub.10 to receive 
energizing potential therefrom when one of selector switches S.sub.1 
-S.sub.10 is closed. An output of clock generator 41 is applied to an 
input of a counter 42. Counter 42 may be, for example, a binary counter 
having a plurality of outputs connected to inputs of a decoder 38A. 
Decoder 38A decodes its binary inputs and provides an output signal on one 
and only one of its output terminals DA.sub.1 -DA.sub.10 according to the 
condition of its inputs. For example, upon successive outputs of clock 
generator 41, and the resulting advancement of a count in counter 42, 
decoder 38A may provide an output only on output terminal DA.sub.1 and, on 
the next change in the content of counter 42, may provide an output only 
on output DA.sub.2. Successive inputs to counter 42 may successively 
produce outputs only on output terminals DA.sub.3, DA.sub.4 and so forth. 
After an output is generated on output terminal DA.sub.10, the next output 
may be on output DA.sub.1. Outputs DA.sub.1 -DA.sub.10 are applied through 
resistors to the bases of respective switch transistors TA.sub.1 
-TA.sub.10. When an output DA.sub.1 -DA.sub.10 is applied to a switch 
transistor TA.sub.1 -TA.sub.10, the affected switch transistor is 
saturated and thus provides a low impedance path between its emitter and 
collector. 
The outputs of counter 42 are also applied through a memory circuit 39 to 
corresponding inputs of a decoder 38B associated with the second or video 
tape recorder channel selector 37B. Decoder 38B is preferably the same as 
decoder 38A. Video tape recorder channel selector 37B employs switch 
transistors TB.sub.1 -TB.sub.10, and variable resistors VB.sub.1 
-VB.sub.10 are connected between voltage source 61 and the collectors of 
the respective switch transistors in an arrangement similar to that in 
television channel selector 37A. 
Memory circuit 39 may consist of a plurality of R-S flip-flops, one for 
each input from counter 42. The R-S flip-flops are enabled or inhibited by 
the signal from record switch 21 in video tape recorder 20. When the R-S 
flip-flops are enabled in response to opening of switch 21, their outputs 
correspond with their inputs. When the R-S flip-flops are inhibited in 
response to closing of switch 21, whatever output condition exists therein 
prior to the inhibiting is latched or stored in memory circuit 39 as long 
as record switch 21 remains closed. Thus, the outputs of memory circuit 39 
remain in the latched, or unchanging, condition in which they continue to 
apply the same input signals to decoder 38B regardless of further changes 
in the inputs to memory circuit 39. 
Variable resistors VA.sub.1 -VA.sub.10 and VB.sub.1 -VB.sub.10 are manually 
adjustable to provide tuning signals at their wipers or movable contacts 
which can conveniently control their respective electronic tuners 33A, 33B 
to tune any selected one of the available television channels such as, for 
example, VHF television channels 2-13 and/or UHF television channels. 
Variable resistor VA.sub.1, for example, may be adjusted to provide a 
control voltage to electronic tuner 33A suitable to tune television 
channel 2 whereas variable resistor VA.sub.2 may be adjusted to provide a 
tuning voltage suitable for tuning television channel 4, etc. It is 
desirable that corresponding ones of variable resistors VB.sub.1 
-VB.sub.10 in video tape recorder channel selector 37B be adjusted to tune 
the same television channels as their counterparts in television channel 
selector 37A. For example, if variable resistor VA.sub.1 in television 
channel selector 37A is adjusted to tune television channel 2, variable 
resistor VB.sub.1 in video tape recorder channel selector 37B should also 
be adjusted to tune television channel 2. The provision for individual 
adjustment of corresponding variable resistors in the two channel 
selectors accommodates the fact that electronic tuners 33A and 33B may 
respond slightly differently to control voltages. Thus individual control 
is provided to correctly adjust both electronic tuners 33A, 33B. 
Clock generator 41 is normally quiescent since it receives its energizing 
signal from voltage source 61 only through one of variable resistors 
VA.sub.1 -VA.sub.10 and a closed one of selector switches S.sub.1 
-S.sub.10. Normally, all of selector switches S.sub.1 -S.sub.10 are in the 
opened position shown. Thus, clock generator 41 remains unenergized. As 
previously noted, decoder 38A provides an output on a single one of its 
output terminals DA.sub.1 -DA.sub.10 corresponding to a particular set of 
outputs of counter 42. For example, assuming that the outputs of counter 
42 are in the condition to produce an output on output terminal DA.sub.1 
of decoder 38A, this signal, applied to the base of switch transistor 
TA.sub.1, saturates transistor TA.sub.1 and effectively grounds the lower 
end of variable resistor VA.sub.1. The voltage thus available at the 
movable contact of variable resistor VA.sub.1 is applied to electronic 
tuner 33A which is thus controlled to tune, for example, television 
channel 2. 
The outputs of counter 42 are also applied to the inputs of memory circuit 
39. If record switch 21 is in the opened condition shown, the outputs of 
memory circuit 39 correspond to its inputs. These outputs are applied to 
the inputs of decoder 38B. Under the conditions described above, decoder 
38B provides an output signal on output DB.sub.1 which is applied to the 
base of switch transistor TB.sub.1. Transistor TB.sub.1 is saturated and 
grounds the lower end of variable resistor VB.sub.1 to provide a control 
signal from the movable contact of variable resistor VB.sub.1 to an input 
of electronic tuner 33B. Variable resistor VB.sub.1 is preferably adjusted 
to tune the same channel as tuned by variable resistor VA.sub.1. Although 
video detectors 35A and 35B both provide video signals to changeover 
switch 36, only the video signal from detector 35B is applied through 
monitor circuits 11 to cathode ray tube 12. The video signal from video 
detector 35A is not used in the condition shown. 
When it is desired to change the channel, a selector switch, for example, 
selector switch S.sub.2, is closed. This applies a voltage from voltage 
source 61 through variable resistor VA.sub.2 and the closed contacts of 
selector switch S.sub.2 to an input of clock generator 41. Clock generator 
41 begins producing an output signal which advances the count in counter 
42. As the count in counter 42 is advanced, decoder 38A de-energizes its 
output terminal DA.sub.1 and energizes its output terminal DA.sub.2. Thus, 
the tuning voltage previously available to electronic tuner 33A from the 
movable terminal of variable resistor VA.sub.1 is removed and is replaced 
by the voltage available at the movable contact of variable resistor 
VA.sub.2. In addition, since switch transistor TA.sub.2 is saturated, the 
energizing voltage previously available to clock generator 41 through 
switch S.sub.2 is shunted to ground through the collector-emitter path of 
switch transistor TA.sub.2. Consequently, clock generator 41 ceases 
producing output signals and television channel selector 37A remains in 
the condition selected by selector switch S.sub.2. Similarly, the output 
DB.sub.1 of decoder 38B is removed and a new output on output terminal 
DB.sub.2 is produced to thus provide a tuning signal from variable 
resistor VB.sub.2 to electronic tuner 33B. If selector switch S.sub.2 is 
again operated, no change takes place because switch transistor TA.sub.2, 
in connecting the lower end of variable resistor VA.sub.2 to ground, 
prevents an energizing signal being transmitted through switch S.sub.2 to 
clock generator 41. 
If a different selector switch, for example, switch S.sub.1, is operated, 
the output DA.sub.2 is removed and outputs DA.sub.3, DA.sub.4 -DA.sub.10 
are produced in sequence until the output DA.sub.1 is again produced. When 
an output on output terminal DA.sub.1 again appears, outputs from clock 
generator 41 and counting in counter 42 are stopped. Thus, in the manner 
previously described, control voltages from variable resistors VA.sub.1 
and VB.sub.1 are applied to their respective electronic tuners 33A and 
33B. 
It would be clear to one skilled in the art that the outputs DA.sub.1 
-DA.sub.10 of decoder 38A may be applied directly to memory circuit 39 
instead of the outputs of counter 42. In this case, decoder 38B can be 
omitted and the outputs of memory circuit 39 can be connected directly to 
the base resistors of switch transistors TB.sub.1 -TB.sub.10. 
It will be noted in the preceding that the signal being displayed on 
cathode ray tube 12 is the signal from video detector 35B that is 
available to video tape recorder 20. Thus, the viewer is able to adjust 
the appropriate variable resistor VB.sub.1 -VB.sub.10 for the received 
channel to ensure that, when recording is begun, proper tuning is 
completed. 
When record switch 21 is closed, recording of the previously selected 
channel begins in video tape recorder 20. In addition, the latch or 
inhibit signal from record switch 21 applied to memory circuit 39 inhibits 
all R-S flip-flops therein and prevents further changes in the outputs of 
memory circuit 39 applied to decoder 38B thus latching or storing the 
values then existing at the input of memory circuit 39. At the same time, 
the latch signal from record switch 21 changes over changeover switch 36 
from terminal B to terminal A. In this condition, the output of video 
detector 35A is applied through monitor circuits 11 to cathode ray tube 
12. As previously explained, corresponding variable resistors VA.sub.1 
-VA.sub.10 and VB.sub.1 -VB.sub.10 are adjusted to tune the same 
television channels. Consequently, when record switch 21 is closed, the 
signal being tuned in electronic tuner 33A should be the same as that 
being tuned in electronic tuner 33B. Consequently, if both channels are 
accurately adjusted when record switch 21 is closed, the viewer should 
notice no difference in the displayed picture. If necessary, the viewer 
can adjust the appropriate one of variable resistors VA.sub.1 -VA.sub.10 
to correctly tune the television channel. 
While recording is being performed in video tape recorder 20, the viewer 
may operate one of selector switches S.sub.1 -S.sub.10 to change the 
channel being displayed on cathode ray tube 12. For example, if selector 
switch S.sub.10 is operated, energizing voltage from voltage source 61 is 
applied through variable resistor VA.sub.10 and the closed contacts of 
switch S.sub.10 to clock generator 41. Counter 42 and decoder 38A function 
as previously described to commutate the outputs DA.sub.1 -DA.sub.10 until 
an output signal is provided at output DA.sub.10. In the manner previously 
described, clock generator 41, counter 42 and decoder 38A are stopped at 
that point and a control voltage is applied from the movable contact or 
wiper of variable resistor VA.sub.10 to electronic tuner 33A. No change 
takes place in decoder 38B due to the inhibited condition of memory 
circuit 39. Thus, one television channel can be recorded in video tape 
recorder 20 while the viewer tunes other television channels using 
selector switches S.sub.1 -S.sub.10. 
If the viewer opens record switch 21, memory circuit 39 becomes enabled and 
thus changes its output signals to correspond to its inputs. At the same 
time, changeover switch 36 changes over from its terminal A to terminal B 
thus disconnecting video detector 35A from monitor circuits 11, and again 
applying the video signal from video detector 35B to monitor circuits 11. 
However, since memory circuit 39 now permits its outputs to correspond to 
the actual count supplied by counter 42, video tape recorder channel 
selector 37B selects the same television channel selected by television 
channel selector 37A, and the viewer should notice little or no change on 
cathode ray tube 12 as a result of the changeover. 
It will be noted that, as the user switches into and out of the recording 
mode, the signal being displayed on cathode ray tube 12 changes from the 
signal tuned by electronic tuner 33B under control of one of variable 
resistors VB.sub.1 -VB.sub.10 to the signal tuned by electronic tuner 33A 
under the control of variable resistor VA.sub.1 -VA.sub.10. In order to 
properly adjust both variable resistors associated with a given channel, 
it is necessary for the user of the apparatus in FIG. 1 to switch into and 
out of the recording mode using record switch 21. It may also happen that, 
when entering the recording mode, the active one of variable resistors 
VA.sub.1 -VA.sub.10 may be improperly adjusted. When this occurs, an 
improperly tuned picture may be displayed for which the user may 
misunderstand the cause and may believe that a malfunction exists in his 
equipment when the problem is merely improper tuning. 
In order to avoid the inconvenience noted in the preceding paragraph, and 
to permit direct comparison of the tuning in the two tuning channels A and 
B, an embodiment of the invention shown in FIG. 2 provides means for 
presetting the pairs of corresponding variable resistors associated with 
the two tuning channels A and B aided by a simultaneous display of video 
signals from both tuning channels. For simplification of the drawing, FIG. 
2 omits channel selecting circuitry 40 and merely indicates the source or 
destination of signals associated with channel selecting circuitry 40. 
As before, the output of antenna 31, split into two equal signals by signal 
distributor 32, is processed in parallel channels by an electronic tuner 
33A, video intermediate frequency amplifier 34A and video detector 35A as 
well as by electronic tuner 33B, video intermediate frequency amplifier 
34B and video detector 35B. Channel tuning is performed by control signals 
from television channel selector 37A applied to electronic tuner 33A and 
from video tape recorder channel selector 37B applied to electronic tuner 
33B. The output of video detector 35A is applied to an input of a 
synchronizing signal separating circuit 52 which applies a horizontal 
synchronizing signal S.sub.H (FIG. 3A) to one input of an AND gate 54, and 
a vertical synchronizing signal S.sub.V (FIG. 3B) to an input of a 
monostable multivibrator 53. Monostable multivibrator 53 has a period 
equal to one half a field period (120th of a second). When triggered by 
the negative-going leading edge of the vertical synchronizing signal 
S.sub.V indicated by a downward pointing arrow (FIG. 3B), monostable 
multivibrator 53 produces an output signal S.sub.M (FIG. 3C) which 
consists of a negative or low signal portion of a period equal to one half 
a field and then resumes its positive or high condition until again 
retriggered. The signal S.sub.M is applied to the second input of AND gate 
54. The output S.sub.C of AND gate 54 (FIG. 3D) is applied to a preset 
terminal P of a preset switch 51. A normal terminal N of preset switch 51 
receives a positive voltage +B. The output of preset switch 51 is applied 
to one input of a NAND gate 55, and a second input of NAND gate 55 
receives the latch output of record switch 21. As is well known, when 
record switch 21 is in the opened condition shown, it has the effect of 
enabling one input of NAND gate 55. Thus, whatever signal is available at 
the second input of NAND gate 55 is inverted in NAND gate 55 and produces 
an inverted output signal S.sub.S (FIG. 3E) which is applied to changeover 
switch 36. 
The output of video detector 35A is also connected directly to a normal 
terminal N of a preset switch 56 and to an input of a limiter 57. Limiter 
57 removes the synchronizing signals from the video from video detector 
35A and applies the remaining video signal to a preset terminal P of 
preset switch 56. The movable contacts of preset switches 51 and 56 are 
ganged for concerted movement thereof between their normal N and their 
preset P fixed terminals. When preset switches 51 and 56 are connected to 
their normal N terminals, the function of the apparatus in FIG. 2 is 
identical to the function of the apparatus previously described with 
reference to FIG. 1. 
When preset switches 51 and 56 are moved to their preset P terminals, 
changeover switch 36 is controlled by signal S.sub.S so that video and 
synchronizing signals are applied from video detector 35B to monitor 
circuits 11 and cathode ray tube 12 during the first half of each field 
and video signals, from which the synchronizing signals have been removed, 
are applied from detector 35A and limiter 57 to monitor circuits 11 during 
the second half of each field. In order to provide synchronizing signals 
to monitor circuits 11 during the second half of each field, changeover 
switch 36 is further controlled by signal S.sub.S to be in contact with 
its terminal S during the video portion and in contact with its terminal M 
during the horizontal synchronizing portion of each line in the second 
half of each field. Thus, during the first half of each field, both video 
and synchronizing signals are derived from video detector 35B. During the 
second half of each field, the video to be displayed on cathode ray tube 
12, and from which the synchronizing signals have been removed by limiter 
57, is derived from video detector 35A, but the horizontal synchronizing 
signals continue to be derived from video detector 35B. This avoids the 
lack of synchronization which could occur if synchronizing signals are 
derived from separate tuning, amplifying and detecting circuits. 
As will be evident from the preceding, when preset switches 51 and 56 are 
in the preset condition, the picture displayed on cathode ray tube 12 
includes a portion on the upper half screen containing video from video 
detector 35B adjacent to a portion on the lower half screen containing 
video from video detector 35A. Thus, direct comparison of the video in the 
two channels can be made to ensure that the same channel is being tuned by 
the active ones of variable resistors VA.sub.1 -VA.sub.10 and VB.sub.1 
-VB.sub.10 and to permit accurate comparative tuning of both active 
variable resistors to optimize the displayed signals. 
Referring now to FIG. 4, there is shown an embodiment of the invention in 
which one tuner is used for feeding a signal to a television receiver 
independently of a second tuner which is used for feeding a signal to a 
video tape recorder. Video detector 35A applies the signal tuned by 
electronic tuner 33A (not shown in FIG. 4) to a terminal T of a switching 
circuit 58. The output of video detector 35A is also connected to an input 
of synchronizing signal separating circuit 52 which, in turn, produces a 
horizontal synchronizing signal S.sub.H and a vertical synchronizing 
signal S.sub.V. Horizontal synchronizing signal S.sub.H is applied to one 
terminal of AND gate 54. Vertical synchronizing signal S.sub.V is applied 
to an input of monostable multivibrator 53 which, as previously described, 
produces an output signal S.sub.M (FIG. 3C) which is applied to the second 
input of AND gate 54. 
An output of video detector 35B, which is tuned by electronic tuner 33B 
(not shown in FIG. 4) is connected directly to video tape recorder 20 and 
to a limiter 59. Limiter 59 removes the synchronizing signals from the 
video signal from video detector 35B and applies the resulting video 
signal to terminal V of switching circuit 58. 
The output of AND gate 54 is applied to a preset terminal P of preset 
switch 51. A positive voltage +B is applied to the normal terminal N of 
preset switch 51. The movable terminal of preset switch 51 is connected to 
switching circuit 58. 
When preset switch 51 is in its normal condition with its movable terminal 
in contact with normal terminal N, switching circuit 58 is in the 
condition shown in which the signal from video detector 35A is applied 
through terminal T of switching circuit and monitor circuits 11 to cathode 
ray tube 12. The output of video detector 35B is always connected directly 
to video tape recorder 20. 
When it is desired to compare the tuning condition of signals in the 
television receiver 10 and video tape recorder 20 channels, preset switch 
51 is moved into contact with its preset terminal P. Control signal 
S.sub.C (FIG. 3D) at the output of AND gate 54 controls switching circuit 
58 in a manner which is substantially the inverse of that previously 
described for changeover switch 36 in FIGS. 1 and 2. More particularly, 
the output of video detector 35A is applied directly through switching 
circuit 58 and monitor circuits 11 to cathode ray tube 12 during the first 
half of each field, and the video from video detector 35B, with its 
synchronizing signals removed by limiter 59, is applied through switching 
circuit 58 and monitor circuits 11 to cathode ray tube 12 during the 
second half of each field. In the second half of each field, signal 
S.sub.C momentarily changes over the moving contact of switching circuit 
58 from terminal V to terminal T in order to employ the horizontal 
synchronizing signal from video detector 35A throughout each field. 
In the previous embodiments, the top-half and bottom-half division of a 
displayed television picture between the video from video detectors 35A 
and 35B may be changed to a side-by-side division by employing a 
monostable multivibrator similar to monostable multivibrator 53 but 
triggered by horizontal synchronizing signal S.sub.H to control switching 
circuit 36 or 58 to provide video from one of the sources during the first 
half of each horizontal line and from the other of the sources during the 
second half of each horizontal line. Since horizontal synchronization can 
be performed in this embodiment with the output of one of the video 
detectors 35A or 35B, limiter 57 or 59 is not required. In addition, 
monostable multivibrator 53, triggered by vertical synchronizing signals 
S.sub.V is also not required. 
Having described specific preferred embodiments of the invention with 
reference to the accompanying drawings, it is to be understood that the 
invention is not limited to those precise embodiments, and that various 
changes and modifications may be effected therein by one skilled in the 
art without departing from the scope or spirit of the invention as defined 
in the appended claims. For example, although the preceding description 
has used a system having a television receiver and a video tape recorder, 
it is equally applicable to a system having two video tape recorder 
channels.