JPS639766B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an automatic channel selection device that can be used when a television receiver performs an automatic channel selection operation. It is an object of the present invention to provide a device that can arbitrarily set the electric field strength of a signal for stopping.

Hereinafter, an apparatus according to an embodiment of the present invention will be described in detail with reference to the drawings. The circuit configuration is shown in FIG. In the figure, 1 is a synchronization signal detection circuit,
If there is a synchronization signal in the detected output of the received signal, the detected output will be at a high level. 2 is an AFC voltage detection circuit, and the AFC voltage obtained by discriminating the frequency of the VIF signal output from the tuner is used as the reference voltage V _H and V _L
If the detection output is between the two, the detection output will be at a high level. This AFC voltage detection circuit 2 includes a comparator 3,
4 and NOR gate 5. Comparator 3 is
When the AFC voltage is higher than _VH , the output becomes high level, and when the AFC voltage is lower than _VL , the output of comparator 4 becomes high level. Therefore, if the AFC voltage is between _VH and _VL , the output of the NOR gate 5 will be at a high level. 6 is an AND gate whose output becomes high level only when a synchronization signal is present and the AFC voltage is between V _H and V _L. 7 is a switch for changing the stop sensitivity; when it is on the A side, it stops only when a strong electric field reception signal is detected, and when it is on the B side, it stops even when a weak electric field reception signal is detected.

Since the conventional device was not provided with such a changeover switch 7, the gate 8 directly outputs the output signal of the AND gate 6 as a search stop control signal. If this output signal is applied as a search stop signal to an automatic station search device, the search can be stopped at the center of the AFC voltage, that is, the optimal tuning point of the broadcast reception signal, but even in the case of a weak electric field signal, no discrimination is possible. I ended up stopping the search.

Therefore, in this device, a monostable multivibrator 9 is added to this circuit, and by switching it with a switch 7, it is possible to switch the sensitivity to stop searching. This monostable multivibrator 9 generates a low level output for a certain period of time T after the output of the comparator 3 rises. By changing the time T of this output, the electric field strength limit for stopping automatic station searching can be arbitrarily changed.

That is, when the changeover switch 7 is set to the A side, the gate 8 generates an output only while the output of the monostable multivibrator 9 is at a low level, and therefore the output of the monostable multivibrator 9 is at a high level. At the time of level, even if a high level output is output from the AND gate 6, that is, even if the center of the AFC voltage is detected, the search stop control signal is not output from the gate 8. Therefore, the automatic station searching device will skip the station with the weak electric field.

The output of the monostable multivibrator 9 becomes low level only for a certain period of time after the output of the comparator 3 changes from low level to high level. Therefore, the conditions for stopping automatic station search are:
Even after a certain period of time T has elapsed since the AFC voltage exceeded the predetermined reference voltage value _VH and the output of the comparator 3 became high level, the gate 6 still outputs a high level output. This condition means that when a broadcast signal with a strong electric field is received, the AFC voltage reaches the predetermined reference value _VH.
This only holds true when the time width exceeding T exceeds T.

The time charts in FIGS. 2 and 3 show the state in which the search stop signal is output only when a strong electric field signal is received.

In Figures 2 and 3, a is the AFC voltage from the AFC voltage generation circuit, b is the output of comparator 3,
c is the output of comparator 4, d is NOR gate 5
e is the output of the synchronization signal detection circuit 1, f is the output of
The output of the AND gate 6, g is the output of the monostable multivibrator 9, and h is the output of the gate 8. FIG. 2 shows a state when a broadcast signal with a strong electric field strength is received, and at this time the AFC voltage a is output over a fairly wide frequency range. Since the frequency sweep speed of the station searching operation is constant, the AFC voltage a is generated over a long period of time. Therefore,
This AFC voltage a is converted to the reference voltage V _H by comparator 3.
A band t exceeding this is detected. On the other hand, the output g of the monostable multivibrator 9 has a constant width T, and if t is longer than this, the search stop control signal h is generated from the gate 8.
Search is stopped.

On the other hand, FIG. 3 shows a situation when a broadcast signal with a weak electric field strength is received. In this case, the bandwidth t in which the AFC voltage a is generated becomes narrow, and this becomes the width T of the output g of the monostable multivibrator 9. If it is shorter than , the search stop control signal h is not output from the gate 8, and the search operation is not stopped.

And the output g of monostable multivibrator 9
By arbitrarily setting the width T, the limit of whether or not to stop searching can be arbitrarily determined.
Control can be performed so that station searching is stopped only when a broadcast signal with a desired electric field strength or higher is received.

As described above, according to the present invention, the search stop control signal is generated to stop the search only when a broadcast signal with a desired electric field strength or higher is received, and the search is stopped only when a broadcast signal with a desired electric field strength or higher is received. Since the station search stop control signal is not generated when a signal is received, stations with weak electric fields are skipped, and high quality images can always be obtained. By arbitrarily setting , it is possible to arbitrarily set a limit on whether or not to stop station searching.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram of an automatic channel search and selection device according to an embodiment of the present invention, and FIGS. 2 and 3 are waveform diagrams of each part thereof. 1...Synchronization signal detection circuit, 2...AFC voltage detection circuit, 3, 4...Comparator, 5...
NOR gate, 6...AND gate, 7...switch, 8...gate, 9...monostable multivibrator.

Claims (1)

[Claims] 1. Input an AFC voltage that provides a wide bandwidth when a broadcast signal with a strong electric field strength is received, and a narrow bandwidth when a broadcast signal with a weak electric field strength is received, and
an AFC voltage detection circuit that issues a detection output signal if the voltage is between a first reference voltage and a second reference voltage;
a first gate means for generating a high level output signal when a synchronization signal is present and the detection output signal is generated; A monostable multivibrator that outputs a pulse signal of a predetermined width,
The output of the first gate means and the output of the monostable multivibrator are input, and if the output signal of the first gate means exists within the pulse width period of the output of the monostable multivibrator, a search stop signal is sent. and second gate means that generates a search stop signal if the output signal of the first gate means deviates in time from the pulse width period of the output of the monostable multivibrator. Tuning device.