JPS58173982A - Channel selection device - Google Patents

Abstract

PURPOSE:To widen the range in the automatic fine tuning, by setting an output middle point voltage of an AFC circuit higher than the 1st threshold voltage and lowering the AFC output voltage lower than the 2nd threshold voltage when no signal exists with a signal detection circuit detecting the presence/absence of a horizontal synchronizing signal. CONSTITUTION:The AFC middle point potential is shifted by making the ratio of load resistors 17, 18 of the AFC circuit unbalance by 1:2 or more. In the balancing voltage of an output of an NAND circuit 23 comprising the synchronizing signal and a horizontal blanking pulse, the horizontal synchronizing signal is lost when the tuning frequency is shifted higher by about 500kHz or more, to set on a transistor 27 and to lower the AFC middle point potential. In comparison with the output voltage of conventional AFC circuits, an inverted S-shaped characteristic which AFC voltage is lowered is obtained, wherein the AFC middle point voltage is increased and the signal is lost.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a channel selection device used in a television receiver,
In particular, the present invention relates to a channel selection device using one type of PLL frequency synthesizer.

In a tuning device using one type of PLL frequency synthesizer, the local oscillation frequency of the tuner is determined according to a frequency division ratio determined by a fixed frequency divider or a variable frequency divider. Therefore, if the desired channel frequency deviates from the regular frequency, for example, when receiving an offset signal such as from CATV, or when connecting a VTR or video game to a TV receiver, this frequency deviation may cause a problem. In such a system, a tuning device is used that has an automatic fine adjustment function that can automatically correct the frequency shift.

However, in the conventional method, the control range is limited by the output voltage characteristics of the AFC circuit, and a sufficiently wide automatic fine adjustment cannot be performed. The purpose of the present invention is to provide a channel selection device that widens the range.

The present invention sets the output midpoint voltage of the AFC circuit higher than the first threshold voltage, and uses a signal detection circuit that detects the presence or absence of a horizontal synchronizing signal to detect the presence of a horizontal synchronizing signal.
The AFC output voltage is lowered so that it is lower than the threshold voltage of , thereby widening the range of automatic fine adjustment.

The present invention will be explained below using FIGS. 1 to 4.

FIG. 1 is a block diagram showing an embodiment of a channel selection circuit according to the present invention. In the figure, 1 is a tuner, 2 is a video intermediate frequency amplifier, 3 is a deflection circuit including a synchronization separation and horizontal oscillation circuit, 4 is an AFC circuit, and 5 is a detection circuit that detects a signal using a horizontal synchronization signal and a horizontal retrace pulse. , 6 has a threshold voltage for detecting an inverted S-shaped H level of the AFC output voltage.A first comparator 7 has a threshold voltage for detecting an inverted S-shaped L level of the AFC output voltage. A second comparator; 8 is a fixed frequency divider that divides the frequency of the local oscillation signal of the tuner; 9 is a variable frequency divider that divides the frequency corresponding to each channel by using the channel selection operation key 10 and is controlled by the comparator 45; , 11 is a reference frequency oscillator,
12 is a phase comparator, and 13 is a low-pass filter that provides a tuning voltage to the tuner. Now, if a desired channel is selected using the channel selection operation key 10, the division ratio corresponding to that channel is set in the variable frequency divider 9, and the band of the tuner is set.

FIG. 2 shows the A F'C circuit 4 shown in FIG.
Detection circuit 5. This is a specific example of comparator 6.7.

14 is an AFC circuit of the receiver, 15.16 is an AFC output driving transistor, 17. ia is a load resistance that determines the AFC midpoint voltage, 19.20 is a filter that removes ripples in the AFC voltage, 21 is a horizontal synchronizing signal input terminal, and 22 is a horizontal retrace pulse input terminal.

23 is a NAND circuit that matches the horizontal synchronizing signal and the horizontal retrace pulse; 24, 25, and 26 are filters that smooth the output voltage of the NAND circuit 23; and 27 is a switch circuit that is turned on and off depending on the output voltage of the NAND circuit.

28 is a resistor for changing the AF'C load resistance ratio, 29°30.31 is a resistor that determines the threshold voltage of the first comparator 62 and the second comparator 34, and 66 is an automatic fine adjustment at H level. A control terminal 34 is a control terminal that controls the automatic fine adjustment frequency division ratio to be lowered when the frequency division ratio is at H level.

The load resistances 17 and 18 of the AFC circuit in Figure 2 are generally selected to have the same value, but in this case, the AFC voltage is symmetrically set above and below the center of the power supply voltage as shown in Figure 3 (a). Reverse S
0 (here is the first threshold)” VH
, M2, the output of each comparator is obtained in a frequency range of fz'+f3' with respect to the AFC center frequency f1 as shown in the figure. From this, the automatic fine adjustment range is 12/~fa', and if there is an offset of the received signal below f2' or above 137, automatic fine adjustment cannot be performed at the center.

Here, in the present invention, by unbalancing the ratio of the load resistances 17°18 of the AFC circuit to 1:2 or more, as shown in FIG.
As shown in a), the balanced voltage of the output of the NAND circuit 26 due to the zero synchronization signal and the horizontal retrace pulse, which can shift the AFC midpoint potential, is approximately 500% when the tuning frequency is high.
If the deviation exceeds kllz, the horizontal synchronization signal disappears, as shown in FIG. 4(d). By turning on the transistor 27 with this signal and further raising the AFC midpoint potential, the dotted line portion can be lowered as shown by the solid line as shown in FIG. 4(a).

As can be seen from this characteristic, compared to the output voltage of a conventional AFC circuit, an inverted S-shaped characteristic is obtained in which the AFC midpoint voltage is raised and the AFC voltage is lowered where the signal disappears.

By selecting the threshold voltage of the first threshold voltage IVu I i 2 as vL, the signal for controlling the automatic fine adjustment frequency division ratio to be increased can be obtained as shown in FIG. 4(b).
Further, a signal for controlling the voltage to be lowered is obtained as shown in (c).

As described above, the conventional control signal can be expanded to f2 to f3 compared to f2' to fa', thereby widening the automatic fine adjustment range.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing an embodiment of the channel selection device according to the present invention, FIG. 2 is a specific circuit diagram of the main part thereof, and FIGS. 6 and 4 are acid pressure waveform diagrams. 17, 1B, 19.28...Resistance 27...Transistor agent patent attorney Usui 1) Profitability Iζ. , ) 5, -, ++ ・ 7 ・ Inspection 1 Figure zth

Claims (1)

[Claims] It has an AFC circuit connected to the tuner, a detection circuit that detects signals based on the horizontal synchronization signal and the horizontal retrace pulse signal, and a first threshold voltage for detecting the H level of the inverted S-shaped output of the AFC circuit. a first comparator, a second comparator having a second threshold voltage for detecting the L level of the inverted S-shaped output; If the voltage is higher than the threshold voltage, the output of the first comparator is used to control the reception frequency of the tuner to be lowered, and if it is lower than the threshold voltage of the second comparator, the reception frequency of the tuner is controlled to be increased. When the voltage is within the threshold voltage range of the 1st and 2nd comparators, the control means keeps the receiving frequency constant, and when the detection circuit detects the signal, the AFC
A channel selection device comprising a correction means for lowering the output voltage of the circuit.