JPS5838989B2 - The current situation is very difficult. - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to means for expanding the frequency pull-in range of an automatic local oscillation frequency control (hereinafter abbreviated as AFT) circuit of a television receiver.

First, the principles and problems of the conventional AFT method will be explained.

FIG. 1 is a block diagram showing an AFT loop and its surroundings in a conventional television receiver.

High frequency signal received by the antenna (its frequency is approximately 10
(ranging from 0 MHz to 1000 MHz) passes through the high frequency amplification section shown at 1, and then is converted to an intermediate frequency (approximately 58 MHz) at the frequency conversion section shown at 2.
is converted to

This includes a local oscillator, whose oscillation frequency is 4
It is controlled by the output of the frequency discriminator shown in .

The value obtained by subtracting the input high frequency from this local oscillation frequency becomes the frequency of the output intermediate frequency signal.

Now, after this intermediate frequency signal is amplified by the video intermediate frequency amplifying section shown in 3, it is applied to the video detection circuit shown in 5, where a composite video signal and an audio intercarrier signal are obtained as output. .

Of these, the audio intercarrier signal (frequency is 4.5
MHz) is connected to a speaker through an audio detection amplification section 6, and the audio signal is reproduced.

The video signal of the composite video signal is connected to a fluoroun tube through a video amplification section 7 to reproduce an image.

The synchronization signal of the composite video signal passes through a synchronization separation circuit shown at 8 and is taken out as its output, which is connected to the subsequent deflection circuit and used as timing information for scanning and reproducing the image. Ru.

Now, a part of the output of the intermediate frequency amplification section 3 is applied to a frequency discriminator 4; the frequency of the video carrier wave is discriminated here.

As described above, this output controls the frequency of the local oscillator included in the frequency converter 2 to suppress fluctuations in the frequency of the video carrier wave.

In addition, in a so-called electronic tuning type tuner, the tuning frequency of the high frequency amplifying section may be continuously controlled by the output voltage of the frequency discriminator.

FIG. 2 shows the operation of the AFT loop.

In the figure, the horizontal axis is the local oscillation frequency, and the vertical axis is the output of the frequency discriminator, that is, the voltage that controls the local oscillator.

The so-called 8-character characteristic indicated by 12 indicates the discrimination characteristic of the frequency discriminator.

f at the center of the horizontal axis.

means the correct local oscillator frequency, in which case the corresponding intermediate frequency video carrier frequency is the normal 58, 75 MHz
The output of the discriminator is the median value.

On the other hand, straight lines indicated by 13, 14, 15, and 16 indicate the relationship between the input control voltage and the output oscillation frequency of the local oscillator, and the straight line 13 indicates the free oscillation frequency of the oscillator f.

This is the case when the free oscillation frequency is 1.5 MHz lower than f.

This is the case where the deviation is 1.5 MHz higher than that.

These free oscillation frequency deviations are drawn to the intersection with the figure-8 characteristic 12 by the action of the AFT loop, and as a result, the actual oscillation frequency is adjusted to the required accuracy (fo±100 KHz
(within).

Incidentally, the frequency pull-in range of the AFT loop having the characteristics shown in FIG. 2 is limited to within approximately ±1.5 MHz.

The reason is that, as shown by the dotted straight line 15, when the deviation of the free oscillation frequency is large, there are three intersections with the figure-8 characteristic 12, and among these, point b is an unstable point, so it will not stay here. However, since there is a stable point C in addition to point a, which is a preferable stable point, once it is drawn to this point, it remains there.

The same applies when a shift in the free oscillation frequency occurs as shown by 16 in the figure.

Therefore, the pull-in range is limited to ±1.5 MHz.

The free oscillation frequency of the local oscillator fluctuates due to temperature, humidity, changes over time, etc., and all of these are within ±1.
It is technically and economically difficult to keep the frequency within 5 MHz.

In particular, in the case of a television receiver, the number of reception channels is 50 or more, and the frequency extends to nearly 1000 MHz, so it is difficult to maintain the above-mentioned stability.

For this reason, in conventional TV receivers equipped with an AFT loop,
Often, the local oscillator frequency would be significantly out of tune, causing inconvenient situations such as the image disappearing from the screen.

SUMMARY OF THE INVENTION An object of the present invention is to provide a local oscillation frequency control method that can solve the above-mentioned drawbacks of the conventional method and expand the pull-in range of the AFT loop to more than twice that of the conventional method.

In order to achieve the above object, the present invention expands the bandwidth of the 8-character characteristic and provides special correction means to prevent malfunctions associated with this.

First, in order to widen the bandwidth of the figure-8 characteristic, it is sufficient to simply expand the bandwidth of the filter used in the discriminator circuit 4 in Figure 1, but as described below, malfunction due to audio carrier energy can be increased. There's a problem.

FIG. 3 is for explaining the problems when using wideband 8-character characteristics.

In the same figure, 17 indicates the character 8 characteristic when the video carrier wave is discriminated.

The bandwidth of this is substantially limited by the frequency versus amplitude characteristic of the intermediate frequency amplifying section 3 shown in FIG.

This is shown in FIG. The horizontal axis in the figure shows the frequency, fP
represents the video carrier wave, which is 58, 75 MHz in Japan.
is selected as the normal value.

Since there may be a signal of another adjacent channel in the frequency region of f p + 1.5 MHz or more, a sufficient degree of attenuation is provided so that the energy of this signal does not leak.

On the other hand, the audio carrier wave of the receiving channel exists at the fp 4.5 MHz position, and the signal of another adjacent channel may exist in the frequency region below this.
Sufficient attenuation is provided to prevent this energy from leaking.

As mentioned above, the bandwidth of the intermediate frequency amplification section is f p +
It is limited from L 5 MHz to fp 4.5 MHz.

Therefore, the bandwidth of the figure 8 characteristic 17 in FIG. 3 cannot be made wider than the above bandwidth.

In reality, as described later, the optimal bandwidth of the 8-character characteristic is f
o The lower bandwidth should be selected to be approximately 3.5 MHz.

Now, if we widen the bandwidth of the figure 8 characteristic in this way, when the local oscillation frequency shifts to the higher side, that is, to the right in Figure 3,
As shown by the dotted line, character 8 characteristics depending on the voice carrier appear.

This occurs because the frequency spectrum on the horizontal axis moves to the right in FIG. 4, so the output level of the video carrier wave decreases, while the output level of audio energy increases.

The local frequency is f. If it deviates to +4 or 5 MHz,
The frequency of the audio carrier comes to the position of the regular video carrier.

Generally, the local frequency is f.

At +IMHz or higher, the audio carrier level becomes stronger than the video carrier level, and as a result, in an actual case where both video and audio carrier waves coexist, the figure 8 characteristic of the frequency discriminator is as shown by the dashed dot line 19 in Figure 3. become.

The left and right sides of the dash-dotted line 19 are connected to the figure-8 characteristics shown by the solid line and dotted line in FIG. 3, respectively.

Now, assuming that the control gradient of the local frequency by the discriminator output is as shown at 20 and 21 in Fig. 3,
The frequency pull range of this AFT loop is 20,210
f shown by a straight line.

-3,5MHz to f.

- It is limited to 2MHz. This retraction range is 20
, 210, that changing the slope of the straight line, ie, the frequency control slope, will not improve the results.

That is, although the bandwidth of the figure-8 characteristic was expanded compared to that shown in FIG. 2, the pull-in range was conversely reduced due to the negative influence of the audio carrier wave.

In order to improve this, it is sufficient to remove the unnecessary 8-character characteristic due to the audio carrier wave on the right side of Fig. 3, and the present invention utilizes the information after the output of the video detector 5 of Fig. 1 as a means for this removal. It is something.

The video detector has an envelope detection function, and by this function, it originally has the function of detecting amplitude modulation information of the video carrier wave.

The output waveform of the video detector under normal operating conditions is shown by waveform 22 in FIG.

In the figure, the highest level of the waveform is controlled by the action of a so-called AGC circuit so that it is always a constant value (1.4V in this case).

Now, the local frequency is about 500 K higher than the normal value.
Hz shift, the frequency spectrum in Figure 4 shifts to the right by that amount, and as a result, the level of the audio carrier included in the input to the video detector 50 in Figure 1 becomes comparable to the level of the video carrier. .

Therefore, a small portion of the video signal in the video detection output disappears due to the masking effect of the audio carrier energy.

As a result, the detected output waveform rises as indicated by the dotted line 23 in FIG.

Furthermore, the local oscillation frequency shifts and f.

At approximately +IMHz, the video carrier wave is almost completely masked by the audio carrier wave energy, resulting in a detected output waveform as shown in FIG. 24.

The first embodiment of the present invention focuses on the distortion appearing in the video detection output waveform and utilizes it to correct the character-8 characteristic, and its configuration is shown in FIG.

The difference between FIG. 6 and the conventional system shown in FIG. 1 is that a correction circuit 9 is added, and the other circuits have the same functions.

The correction circuit 9 is required to have a low-frequency p-wave function, a threshold discrimination function, and an amplification function.

An example of a specific circuit having these functions is shown in FIG.

The configuration of the figure will be explained in detail.

The resistor 25 and capacitor 26 have a low-frequency p-wave function, and by selecting their time constant to be larger than the period of change of the video signal (16 msec), the average DC level of the video detector output is obtained at both ends of the capacitor 26.

The potential difference between the base and emitter of the transistor 270 is approximately 0.7V, so by inserting a 0.4V potential source 28 in series on the emitter side, the overall threshold voltage is set to 1.1V. It is.

When designing the threshold voltage value, it should be selected to be lower than the synchronization head level (1,4V) and higher than the pedestal level (1,OV) in FIG.

Therefore, when the average input DC level is 1.1V or less, the transistor 27 is in a cutoff state, and the circuit does not operate.

On the other hand, when the input average DC voltage exceeds i, iv, the transistor 27 becomes conductive, and as a result, the collector current increases, so that the output voltage of the discriminator 4 shown in FIG. 6 is lowered.

As a method for making the output of this circuit act on the discriminator 4, it is possible to simply connect the collector of the transistor 27 in parallel to the output terminal of the discriminator. In this case, the resistor 29 and power supply 30 in FIG. 7 can be omitted. ) may be added to a part of the amplifier in the discriminator.

FIG. 8 shows how the collector voltage of the transistor 27 in the circuit of FIG. 7 responds to a shift in the local frequency.

In FIG. 8, the horizontal axis shows the deviation of the local frequency, and the vertical axis shows the collector voltage of the transistor 27.

Since the corrected voltage curve 31 as shown in this figure is obtained, the third
The character 8 characteristic shown by the dashed line in the figure is modified as shown at 32 in FIG.

The frequency pull-in range of the AFT loop when using the corrected figure 8 characteristic shown in Figure 9 is fo±3, 5 MHz.
It will be understood from the control characteristics shown in FIGS. 33 and 34 that

This is more than twice the improvement compared to the conventional system shown in FIG. 2, and four times the improvement compared to 19 (conventional broadband type) shown in FIG.

As described above, according to the present invention, it is possible to expand the frequency pull-in range, and therefore it is possible to avoid the inconvenient situation due to the abnormal pull-in phenomenon, which is a problem with the conventional method, and it is possible to always obtain a stable image.

Next, a second embodiment is shown in FIG.

The difference between this figure and FIG. 6 is that the correction circuit indicated by block 10 has been moved from the output section of the video detector 5 to the output section of the synchronous separation circuit 8.

The correction circuit 10 must have an AND gate function between the horizontal blanking signal and the output synchronization signal from the synchronization separation circuit, a low frequency wave function, a threshold discrimination function, and an amplification function. It is.

A specific circuit example of this correction circuit 10 is shown in FIG.

Before explaining the operation of FIG. 11, the output waveform of the synchronous separation circuit will be explained with reference to FIG. 12.

The horizontal axis in the figure is the time axis, and the horizontal period (63 μsec
), a horizontal synchronizing signal as shown in waveform 45 in a of the same figure is obtained.

The width of this pulse is approximately 5 μsec, which corresponds to approximately 8% of the total horizontal period.

The local frequency is f.

lower and f. If it is higher than -3.5MHz, you can get a normal synchronization signal like this, but if the local frequency is f
When the frequency exceeds 6 + I MHz, the output signal of the video detector is masked by the audio signal as described in Figure 5, making it impossible for the subsequent synchronization separation circuit to correctly separate the synchronization signal. Become.

As a result, the synchronization separation output level disappears, or else a very narrow noise-like pulse appears as shown at 45 in FIG. 12.

The pulse indicated at 46 in FIG. 12 is connected to terminal 38 of FIG.
A horizontal blanking signal is added to the synchronization signal, which is well known and is generated in a deflection circuit following the synchronization separation circuit, and is matched in timing with the synchronization signal.

The operation of FIG. 11 will be explained below.

The diodes 36 and 37 and the resistor 39 perform an AND gate between the horizontal synchronizing signal from the sync separation circuit 8 and the horizontal blanking signal from the terminal 38.

A resistor 40 and a capacitor 41 constitute a low frequency wave generator.

VBE between base and emitter of transistor 420 0
．． 7V and the 0.4V of the voltage source 43 on the emitter side, the total voltage is 1. The level of IV is raised to the threshold level.

When determining the output voltage of the low-pass p-wavelength converter when the synchronization signal is normal, first, the average DC level of the synchronization signal in Fig. 12 is O, S V, and the diode 360 base,
The emitter-to-emitter VBE is added for a total output voltage of 1.5V.

Therefore, in this case, the transistor 42 becomes conductive, and its collector potential becomes approximately 0.4V.

Therefore, in this state, the next stage transistor 44 is in a cut-off state, so that it does not affect the operation of the frequency discrimination circuit 4 in any way.

On the other hand, the local oscillation frequency is f. When moving above +IMHz,
At that time, the synchronization separation output waveform becomes a waveform as shown in Figure 12, and when this is AND gated with the horizontal blanking signal, the average DC level of the output is approximately 0.7 V, so the transistor 42 is cut off. , therefore, the transistor 44 becomes conductive, and thereby the eighth-order characteristic can be corrected, as in the first embodiment.

Although the second embodiment described above has a slightly more complicated circuit configuration than the first embodiment, it has several performance advantages.

In other words, if the received electric field strength is extremely weak when the local oscillator frequency is shifted higher than about IMHz, the signal energy is insufficient in the video detection output waveform shown in Figure 5. , the output DC level may not exceed 1.1v.

In this case, the correction circuit does not work in the first embodiment, but in the second embodiment, even in such a case, the synchronization separation output waveform is in an abnormal state as shown in Figure 12, so this method This has the advantage that the correction circuit operates normally even in a weak electric field reception state.

Furthermore, during the AND gate output of the horizontal synchronization signal and the horizontal blanking signal in FIG. As a result, good 8-character correction information can be obtained.

Furthermore, if this output is used as timing information for the subsequent deflection system circuit, it will have the effect of killing two birds.

Note that this type of deflection circuit stabilization method is already known, and television receivers that employ such a method already exist.

When applying the present invention to such a television receiver,
The increase in the number of parts can be kept to a minimum, and manufacturing costs can be lowered.

Next, a third embodiment of the present invention is shown in FIG.

The correction circuit indicated by 11 in the figure is a feature of this embodiment, and the necessary requirements for this correction circuit are a function to detect the amplitude of the audio intercarrier signal, a low frequency wave function, a threshold discrimination function, and a width increase function.

FIG. 14 shows a concrete example of this circuit.

The operation will be explained below.

First, the audio intercarrier signal amplification section (frequency is 4.5
The configuration is such that when the local frequency is normal, an audio intercarrier signal of approximately 0.3 vP-P appears at the output of MHz)6.

Then, under normal conditions, transistors 47 and 48 are in a cut-off state.

The same applies even when the local oscillation frequency is below the normal value.

Next, assume that the local oscillation frequency has moved upward by approximately 500 KHz.

In such a state, as described with reference to FIGS. 4 and 5, the level ratios of the video and audio plane carrier waves being output from the intermediate frequency amplifying section 3 are approximately equal.

As a result, the level of the audio intercarrier signal also increases,
Quantitatively, it increases by about 30 dB from the normal value.

As a result, an audio intercarrier signal of approximately ±1.5 VP-P or more appears on the input base electrode side of transistor 47 in FIG. 14.

Therefore, the transistor 47 becomes conductive and is detected to obtain a DC level of approximately 0.8 V or more on its output side.

Since the capacitor 48 is placed here, it also serves as a low frequency wave function.

This DC voltage is applied to the base of transistor 48,
As a result, transistor 48 becomes conductive.

In this way, the amplitude of the audio intercarrier signal is detected, and when it exceeds a certain threshold level, the character-8 characteristic is corrected.

However, it should be noted that in this embodiment, unlike the above-mentioned example, the correction effect disappears if the local oscillation frequency deviates by about 15 MHz or more.

This pattern is shown in FIG. The horizontal axis of the figure is the deviation of the local frequency, and the vertical axis is the transistor 48 in FIG.
is the base voltage of

Therefore, the effect of the third embodiment is that the deviation of the local frequency is f (, + 0.5 MHz or f □ + I M
It has the function of correcting the figure-8 characteristic only in the Hz region.

As already mentioned, in the first and second embodiments, the character-8 characteristic correction effect is achieved when the local oscillation frequency is f.

Appears near +IMHz and above.

Therefore, by combining the third embodiment with the first or second embodiment, the character-8 characteristic correction effect can be increased.

If such a configuration is adopted, Figs. 3 and 9
In the lower band of the figure 8 characteristic shown in the figure, f O-3,5MH
From z to f.

-4 MHz, and the frequency pull width of the entire AFT loop can be expanded to ±3.5 MHz (in the case of the first embodiment), and to ±4 MHz in the third embodiment.

In the embodiment shown in FIG. 6, it has been described that the output DC level of the video detector 5 is detected, but if the video amplifier circuit 7 is an amplifier that transmits or regenerates the DC component, the output section outputs the It is also possible to detect the DC level.

Furthermore, in color television receivers, it is common to extract the audio intercarrier signal using an envelope detector separate from the video detector, but the output DC level of this detector may also be detected.

Furthermore, the circuit examples and representative examples of each constant described in each embodiment are examples of those that satisfy the above-mentioned requirements constituting each block, and the present invention is intended to have a specific circuit layout as a limiting condition. isn't it.

Furthermore, the present invention detects the frequency of the video intermediate frequency carrier wave to obtain the figure-8 characteristic, and thereby controls the local oscillation frequency.
Although the description has been made based on the AFT method shown in the figure, it is also possible to apply the well-known AFT method to a configuration in which the level of the audio intercarrier signal is made constant.

The configuration of this type of AFT method will be explained in FIG.

The only difference between FIG. 1 and this figure is the method of detecting the 8-character characteristic as described above, and instead of the frequency discriminator 40 in FIG. 1, a frequency discriminator 49 is connected as shown.

The figure 8 characteristic of the frequency discriminator 49 is shown by a curve 50 in FIG. 17, as is well known.

This figure 8 characteristic is similar to the curve in Figure 15, but
The polarity of the detection is reversed vertically and the method of selecting the threshold during amplitude detection is different.

(The threshold is small in the graph of Figure 17) 1st
It is clear that when using the characteristic 50 of FIG. 7, the frequency pull width of the AFT loop cannot exceed 1.5 MHz.

If the first embodiment or the second embodiment of the present invention is applied to this type of conventional system, a state in which the local oscillation frequency is shifted higher by 1 MHz or more, as shown by the dashed line 51 in FIG. It is easy to see that the 8-character characteristic can be corrected.

[Brief explanation of drawings]

FIG. 1 is a block diagram showing a conventional AFT method. FIG. 2 is a diagram showing the 8-character discrimination characteristic of the conventional method. FIG. 3 is a diagram showing problems with the conventional method. FIG. 4 is a diagram showing the frequency characteristics of the video intermediate frequency amplifying section. FIG. 5 is a diagram showing the output waveform of the video detector. FIG. 6 is a block diagram showing a first embodiment of the present invention. FIG. 7 is a diagram showing a specific circuit example of the main part of FIG. 6. FIG. 8 is a diagram illustrating the operation of the circuit shown in FIG. 7. FIG. 9 is a diagram showing the effects of the present invention. FIG. 10 is a block diagram showing a second embodiment of the present invention. FIG. 11 is a diagram showing a specific circuit example of the main part of FIG. 10. FIG. 12 is a diagram illustrating the operation of the circuit shown in FIG. 11. FIG. 13 is a block diagram showing a third embodiment of the present invention. FIG. 14 is a diagram showing a specific circuit example of the main part of FIG. 13. FIG. 15 is a diagram illustrating the operation of the circuit shown in FIG. 14. FIG. 16 is a diagram showing another form of the conventional system. FIG. 17 is a diagram showing the effect of the present invention. 4, 47---, -frequency discrimination circuit, 5...
Video detector, 6...Audio detection amplification section, 7...
...Video amplification section, 8...Synchronization separation circuit, 9,
10, 11... Correction circuit.

Claims (1)

[Claims] 1 Automatic local oscillation frequency that includes a frequency discriminator for detecting the video intermediate frequency of the video intermediate frequency circuit and controls the oscillation frequency of the local oscillator using the output signal of this frequency discriminator to suppress fluctuations in the local oscillation frequency. In a television receiver equipped with a control circuit, a circuit for calculating an AND of a horizontal synchronization signal output from a synchronization separation circuit and a horizontal blanking signal, and means for obtaining an average DC voltage of the output of this circuit; The average DC voltage is subjected to threshold-hold discrimination and negative feedback to the frequency discriminator is provided, and the output voltage of the frequency discriminator is controlled by detecting whether the average DC voltage is larger or smaller than a predetermined value, and local oscillation is generated. An automatic local oscillation frequency control method characterized by applying a predetermined control voltage to a local oscillator to reduce frequency errors and expanding the pull-in range without adversely affecting frequency discrimination characteristics due to audio carrier energy.