Output signal switching circuit for a video tape recorder

An output signal switching circuit for a video tape recorder which has an RF signal changeover switch which is automatically connected to the output terminal of an RF converter during reproduction mode and the video tape recorder is prevented from being automatically switched when a connecting line is connected to a composite video or base band video signal output terminal, thus assuring that the video tape recorder is easier to operate than prior art systems.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates in general to output circuits for video tape 
recorders and particularly to an output signal switching circuit which 
supplies an output signal from a video tape recorder to other apparatus 
such as a television receiver. 
2. Description of the Prior Art 
In commercially available video tape recorders (VTR), during reproduction, 
a video signal and an audio signal which are reproduced from magnetic tape 
are remodulated to form a composite RF signal according to a broadcasting 
modulation system. Such RF signal has a frequency band of a vacant 
channel, that is a channel where no standard broadcast signals are being 
received, and such RF signal is supplied to an antenna terminal of a 
television receiver. The television receiver is capable of receiving not 
only standard broadcast signals, but also receives the described 
reproduced signal and operates with the circuit arrangement illustrated in 
FIG. 1. 
FIG. 1 illustrates in block an example of a prior art system wherein an 
input terminal 51 of a VTR receives an RF signal from an antenna, not 
shown. The signal applied to the input terminal 51 is supplied to a VTR 
tuner 52 which is tuned to receive a desired broadcast signal. The tuner 
52 produces video and audio signals by demodulating the incoming signal 
and supplies the video signal to a video processor circuit 53 and the 
audio signal to an audio processing circuit 54 of the VTR. The video 
circuit 53 and the audio circuit 54 include tape head systems for 
recording and reproducing signals. 
The video signal and the audio signal from the video and audio processor 
circuits 53 and 54 are supplied to an RF converter 55 in which they are 
formed into a signal which complies with the broadcasting system so as to 
produce an RF signal with the carrier of the vacant channel. The RF signal 
output of the RF converter 55 is connected to a first fixed terminal 56a 
of an RF signal changeover switch 56. 
The RF signal from the input terminal 51 is supplied to a second fixed 
terminal 56b of the switch 56 as illustrated. The moveable contact 56c of 
the switch 56 when connected to terminal 56a supplies the RF output signal 
to a RF output terminal of the VTR which is connected to an antenna input 
terminal of a television receiver 70. Switch 56 is designated a TV/video 
changeover switch. 
According to the prior art device during reproduction, the signal is 
reproduced, for example, from the magnetic tape are or reformed as signals 
which comply with the broadcasting system by the video processor circuit 
53 and the audio processor 54 and are converted to an RF signal at the 
frequency of the vacant channel by the RF converter 55 and then are fed 
through the fixed terminal 56a of the switch 56 to the television receiver 
70. During other modes other than the reproduction mode, the moveable 
contact 56c of the switch 56 is connected to the fixed terminal 56b so as 
to supply the signal which is applied to the input terminal 51 to the 
television receiver 70. Thus, the signal reproduced from the VTR can be 
received and reproduced by the television receiver 70 and also the 
standard broadcast signals can be received by the television receiver 
under this condition. 
In the circuit arrangement of the prior art, however, when the VTR is 
placed into the reproduction mode and the television receiver 70 is tuned 
to the vacant channel to allow watching of the reproduced signal on the 
television receiver, in the event the user forgets to move the moveable 
contact 56c to the fixed contact 56a at the output of the RF converter 55, 
a reproduced signal from the converter 55 will not be supplied to the 
television receiver 70 and thus a picture and sound will not be reproduced 
on the television receiver and only noise signals will occur which is 
undesirable. 
Thus, in the prior art circuit arrangement, a switching control circuit 57 
is usually provided to automatically control the switch 56. In such 
system, during reproduction mode, a signal is generated when the VTR is in 
a reproduction mode and such reproduction mode signal is applied to an 
input terminal 58 of a control circuit 57 so as to automatically cause the 
moveable contact 56c of the switch 56 to be moved to the fixed contact 56a 
to connect the output of the RF converter 55 to the television receiver 
during the reproduction mode of the VTR. If the switch 56 is controlled by 
the control circuit 57 so as to automatically switch the moveable contact 
56c to the fixed the contact 56b which is connected to the input terminal 
51 at the instant when the reproduction mode ends, when the television 
receiver 70 is tuned to the signal of the vacant channel, the television 
receiver will be supplied with the signal from the antenna which results 
in a signal having a high noise level being reproduced by the receiver 70. 
So as to prevent this mode of operation, the switch 56 is usually 
controlled by the control circuit 57 so as to connect its moveable contact 
56c with the fixed terminal 56a at the output of the RF converter 55 only 
when the reproduction mode is again detected and the switch 56 is 
controlled by the control circuit 57 so that the moveable contact 56c is 
connected to the fixed terminal 56b which is connected to the input 
terminal 51 by supplying a manual operating signal to an input terminal 59 
of the control circuit 57. The signal to the input terminal 59 may be 
supplied by a television/video switching control button or by a switch, 
not shown. 
Recently, television receiver 70 have been designed to have line input 
terminals in addition to an antenna input terminal for the RF signals 
wherein composite video baseband video and baseband audio signals can be 
directly applied to the receiver so as to directly supply the video and 
audio signals in a baseband level. In such receivers since the video 
signal and the audio signal at the input of the RF converter 55 are 
respectively supplied to line output terminals 61 and 62 and such signals 
are supplied to the input terminals of the television receiver 70 and, 
thus, there is a less risk that such signals will be deteriorated by 
conversion and such signals can be accurately reproduced and received at 
the television receiver. During such operation, the antenna terminal of 
the television receiver 70 should also be connected to the RF output 
terminal 60 of the VTR so as to receive the RF signal from the input 
terminal 51. 
In other words, the optimum RF signal should be supplied to the VTR when it 
is intended to record a signal. For this purpose, it is conventional that 
the television receiver 70 is connected to the RF output terminal 60 of 
the VTR and a distributor is provided at the stage before the tuner 52 of 
the VTR. 
However, according to this described connection when the VTR is once placed 
in the reproduction mode, although the television receiver 70 is not 
conditioned to receive the signal of the vacant channel, the moveable 
contact 56c of the switch 56 is automatically connected to the fixed 
contact 56a at the output of the RF converter 55. Thus, the user must 
operate the television/video switching control button again so as to 
receive any selected broadcast channel with the television receiver after 
the reproduction mode has ended. Thus, the operating characteristic of the 
apparatus is not satisfactory. 
According to the prior art described above, when the base band video and 
audio line output terminals of the VTR are connected to the line input 
terminals of the television receiver and the RF output terminal of the VTR 
is connected to the antenna terminal of the television receiver, if the 
VTR is set to the reproduction mode, the RF output terminal will 
unnecessarily be switched to the RF converter output. As a result, when 
the reproduction mode of the VTR ends the user has to manually perform an 
extra operation so as to obtain the original state of the switch which 
considerably deteriorates operation of the system. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved output 
signal switching circuit for a video tape recorder. 
Another object of the invention is to provide an output signal switching 
circuit for a video tape recorder which makes the video tape recorder 
easier to operate. 
According to one aspect of the invention, there is provided an output 
switching circuit for a video tape recorder which comprises: 
(a) a switching means for selectively supplying a first RF signal from an 
antenna and a second RF signal produced by an RF converter of a VTR to an 
RF output terminal; 
(b) a control means for detecting the reproduction mode of the video tape 
recorder and controlling the switching means so as to connect it to the 
second RF signal produced by the RF converter; 
(c) means for supplying an input signal to the RF converter to a line 
output terminal; 
(d) detecting means for detecting when the connection line is connected to 
the line output terminal; and 
(e) inhibiting means for inhibiting switching of the switching means when 
the reproduction mode is detected by using a signal from the detecting 
means. 
Other objects, features and advantages of the invention will be readily 
apparent from the following description of certain preferred embodiments 
thereof taken in conjunction with the accompanying drawings although 
variations and modifications may be effected without departing from the 
spirit and scope of the novel concepts of the disclosure and in which:

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
An embodiment of the output signal switching circuit according to the 
invention is illustrated in FIGS. 2 and 3. FIG. 2 is a block diagram 
illustrating the embodiment of the invention and FIG. 3 is an electrical 
schematic illustrating the detecting circuit shown in FIG. 2. 
In FIG. 2 an input terminal 1 is connected to an antenna which supplies an 
RF signal. The signal supplied to the input terminal 1 is supplied to a 
VTR tuner 2 which allows a desired broadcast signal to be received. The 
video signal and the audio signal received are demodulated by the tuner 2 
and are respectively supplied to a video processor circuit 3 and to an 
audio processor circuit 4. The video and audio processor circuits 3 and 4 
each include a head-tape system for recording and reproduction. The video 
signals from the video processor circuit 3 and the audio signals from the 
audio processor circuit 4 are supplied to an RF converter 5 wherein they 
are modulated on an RF signal so as to form a signal which complies with 
the broadcasting system and wherein the signals thus generated is 
converted to an RF signal for a vacant channel. The RF signal which is 
converted is supplied to one fixed terminal 6a of an RF signal changeover 
switch. 
The RF signal which is supplied to the input terminal 1 is also directly 
supplied to the other fixed terminal 6b of the switch 6. The moveable 
contact 6c of the switch 6 is controlled by a switching control signal 
from a control circuit 7. The control circuit 7 can be supplied with a 
reproduction mode detecting signal from a terminal 8 through a switch 9 
and can also be supplied a signal from a television/video switching 
control button through terminal 10. A signal developed at the moveable 
contact 6c of the switch 6 is delivered to an RF output terminal 11 which 
is connected to the RF input terminal 60 of a television receiver 70 such 
as illustrated in FIG. 1. 
The signals from the video processor circuit 3 and the audio processor 
circuit 4 are also respectively supplied to line output terminals or jacks 
12 and 13 to which the TV set 70 can be connected. A detecting circuit 14 
for detecting if a connection line is connected to the line output 
terminal 12 is connected between the video processor circuit 3 and the 
line output terminal 12. The switch 9 is controlled by the detector 
circuit 14 so that it is opened when a signal is produced by the detecting 
circuit 14. 
In the circuit arrangement of FIG. 2, when the moveable contact 6c of the 
switch 6 is connected to the fixed terminal 6b which is connected to the 
input terminal 1, the RF signal applied to the input terminal 1 is 
supplied through the switch 6 to the line output terminal 11 and to the RF 
input terminal of the television receiver 70. When the VTR is placed into 
the reproduction mode and a connecting line is not connected to terminal 
12, the switch 9 is closed by the detector 14 so as to allow the 
reproduction mode signal which is detected and applied to terminal 8 to be 
supplied to the control circuit 7 so that due to operation of the control 
circuit 7 the switch 6 will have its moveable contact 6c connected to 
fixed terminal 6a at the output of the RF converter 5 so as to supply the 
signal from the RF converter 5 to the line output terminal 11. When the 
connection line of the TV receiver is connected to the line output 
terminal 12, the switch 9 will be opened by the detected signal from the 
detecting circuit 14 so that the control circuit 7 does not operate even 
during reproduction mode and thus leaves the moveable contact 6c of the 
switch 6 connected to fixed terminal 6b which is connected to the input 
terminal 1. 
In this manner, the RF signal at terminal 1 is supplied to the output 
terminal 11 and the video signal and the audio signals are respectively 
supplied to the line output terminals or jacks 12 and 13. 
With the described circuit arrangement, when the line output terminals 12 
and 13 of the VTR are connected to the line input terminals of the 
television receiver 70 and the RF output terminal of the VTR is connected 
to the antenna terminal of the television receiver, even if the VTR is 
placed into the reproduction mode, the RF signal changeover switch is 
prevented from being unnecessarily switched and the user does not have to 
accomplish extra operations at the end of the reproduction mode and the 
operation of the system is considerably improved. 
FIG. 3 illustrates an electrical schematic of the detecting circuit 14 of 
FIG. 2. In FIG. 3, an input terminal 21 receives the video signal from the 
video processor circuit 3 of FIG. 2. The input terminal 21 is grounded 
through a capacitor 22 and resistors 23 and 24. The junction point between 
the resistors 23 and 24 is connected to a terminal 25 which is connected 
to a central conductive terminal of a jack of the line output terminal 12. 
A terminal 26 is connected to the outer conductive member of jack 12 and 
is grounded. The jack 12 has a terminal 27 which is normally connected to 
the terminal 25 to short these terminals out, but which is disconnected 
from the terminal 25 when a male plug of a connecting line is inserted 
into the jack 12. The terminal 27 is connected through a resistor 28 to a 
power supply terminal 29 which receives a voltage Vcc. The terminal 27 is 
also connected to the positive input terminal of a comparator 30. A 
reference voltage source 31 of a reference voltage Vth is connected to the 
negative reference input terminal of the comparator 30. The output 
terminal 32 of the comparator 30 is connected to the control section of 
the switch 9 shown in FIG. 2. 
With the circuit arrangement shown in FIG. 3, when the plug of the 
connection line is not inserted into the jack 12, the DC voltage Vcc from 
the power supply terminal 29 is divided by resistors 28 and 24 and is then 
supplied to the comparator 30. Under these conditions, since the divided 
voltage is selected so as to be lower than the reference voltage Vth, the 
voltage or potential at the output terminal 32 will be low and the switch 
9 shown in FIG. 2 will remain closed. When, on the other hand, the plug or 
jack is inserted into the jack 12, the path between the terminals 27 and 
25 is disconnected or opened so that the voltage Vcc from the power supply 
terminal 29 is supplied to the terminal 30 through the resistor 28. Under 
this condition, since the reference voltage Vth is selected to be lower 
than the voltage Vcc and the condition Vcc greater than Vth is satisfied, 
the potential at the output terminal 32 will become high and the switch of 
FIG. 9 will be opened. 
In this manner, the connection of the connection line to jack 12 is 
detected and the switch 9 is controlled. The switch 9 may be an electronic 
switch such as a transistor or the signal from terminal 32 may be supplied 
to a relay which actuates a mechanical switch 9. 
According to the present invention, when it is detected that the connection 
line is connected to the line output terminal 12 of the VTR, the RF signal 
changeover switch (TV/video changeover switch) is prevented from being 
automatically switched during the reproduction mode such that when the 
television receiver is connected to the line output terminal unnecessary 
switching of the RF switch is prevented thus resulting in a video tape 
recorder which is easier to operate. 
The above description is given for a single preferred embodiment of the 
invention, but will it be apparent that many modifications and variations 
could be effected by one skilled in the art without departing from the 
spirit or scope of the novel concepts of the invention and the scope of 
the invention should be determined only by the appended claims.