JPH0249060B2 - - Google Patents

Description

[Detailed description of the invention]

This invention is based on the APC installed in the VTR playback system.
The present invention relates to a variable oscillation circuit suitable for application to circuits, etc. In a VTR, the carrier color signal (chroma signal) of the color video signal is low frequency converted and then superimposed on, for example, an FM modulated luminance signal and recorded.
The reproduction system is provided with a frequency conversion circuit for converting the chroma signal to its original frequency, and this frequency conversion circuit is supplied with a carrier signal for frequency conversion in addition to the reproduction chroma signal. This carrier signal for frequency conversion is formed by an APC circuit 10 as shown in FIG. In the figure, terminal 1 has a burst signal S _B (3.58MHz) separated from the frequency-converted reproduced chroma signal.
This is phase-compared with the reference output S _R (3.58MHz) obtained from the reference oscillator 2 in the phase comparator 3, and the comparison output is converted into a control voltage V _C in the low-pass filter 4, and this control The voltage V _C is supplied to the variable frequency oscillator 5, and a frequency (3.58 MHz + jitter component) corresponding to the control voltage V _C is obtained. This is the signal S _L for low frequency conversion (688K in this example)
Hz) and is supplied to the frequency conversion circuit 6, where it is converted into a carrier signal S _C having a predetermined frequency (4.27MHz), and this is converted into a reproduced chroma signal (688K
Hz) is supplied as a carrier signal to a frequency conversion circuit (not shown) provided on a transmission line. In addition, 8 is an oscillator (variable type) that obtains the low frequency signal S _L
It is. Now, in the APC circuit 10 configured as described above, both the reference oscillator 2 and the variable oscillator 5 use crystal oscillators 2a and 5a as oscillating elements.
When a is used, the control sensitivity of this oscillator is generally low, so when the hue has changed significantly, it is difficult to correct it to the original hue in a very short time. On the other hand, relaxation oscillators (excluding crystal oscillators) used in the low-frequency oscillator 8 generally have high control sensitivity and can vary the frequency up to several tens of kHz; It is conceivable to use it as the oscillator 5, but using this would cause the problem of too high control sensitivity and the problem of oscillation frequency drift. The problem with the former is that since the frequency variable range is several tens of kHz, there is a lock point for the APC loop every f _H (15.75 kHz). The problem with the latter is that the oscillation frequency fluctuates significantly (on the order of several 100 kHz) due to factors such as variations in C and R, which are oscillation elements such as a relaxation oscillator, and the temperature characteristics of the oscillator. A device whose frequency fluctuates in this manner cannot withstand use as the variable oscillator 5 described above. For the former, there is no particular problem because the control sensitivity will be lowered if the control voltage V _C is supplied after being appropriately attenuated. On the other hand, the latter problem cannot be solved so easily. Therefore, the present invention solves the problem of frequency drift that occurs when a variable oscillator with high control sensitivity is used with a relatively simple configuration. Next, a case in which an example of the present invention is applied to a VTR employing the above-mentioned recording method will be described with reference to FIG. 2 and subsequent figures. FIG. 2 is a system diagram showing an example of the variable oscillator circuit according to the present invention, in which the variable oscillator 5 includes an oscillator other than the crystal oscillator (a variable relaxation oscillator such as an emitter-coupled multivibrator),
(variable oscillator using ceramic filter, etc.)
is used. The control sensitivity is adjusted so that the maximum value of the oscillation frequency varied by the control voltage V _C supplied to the variable oscillator 5 does not exceed ±15 kHz. For example, it is adjusted to about ±10kHz. In the present invention, a detection circuit 20 is provided to detect the difference in frequency between the reference output S _R and the oscillation output S _O of the variable oscillator 5. As shown in FIG. 3, this detection circuit 20 has a detection output (voltage) V _D proportional to the frequency difference Δf.
The detection output V _D is the control voltage V _C
The output _{voltage V} It is added to the above-mentioned control voltage V _C and then supplied to the variable oscillator 5. Now, when this variable oscillation circuit 10 used as an APC circuit is configured in this way, the relationship between the oscillation frequency f _V of the variable oscillator 5 and the control voltages V _C and V _X is as shown in equation (1), Further, the relationship between the detection output V _D of the detection circuit 20 and the frequency difference Δf is as shown in equation (2). _{f V = R R + K 1 V C} + _{K 2 V} Modulation sensitivity of the variable oscillator 5 seen from the terminal of the voltage V _C K ₂ : Modulation sensitivity of the variable oscillator 5 seen from the terminal of the control voltage V _X , K ₂ >K ₁ K ₃ : Demodulation sensitivity of the detection circuit 20 f _e : Due to drift Error frequency (drift frequency) From formula (1), V _C = 1/K ₁ (f _V −f _R −K ₂ V _X −f _e ) ………(3) Also, the differential output V _Y is V _Y = K ₄ (V _C − V _D ) ......(4) Here, K ₄ : Gain of differential amplifier 21 Substituting equations (2) and (3) into equation (4) and rearranging, (5) It becomes like the formula. V _Y = K ₄ (1/K ₁ - _{K 3} ) (f _V - f _R ) - K ₄ /K ₁ (K ₂ V _X + f _e ) ......(5) Here, modulation sensitivity K ₁ and demodulation If the product of sensitivity K ₃ is selected as K ₁ · K ₃ = 1 ...... (6), equation (5) becomes V _Y = -K ₄ /K ₁ (K ₂ V _X - f _e )... ...(7) becomes. Also _, in terms _of direct current _{, V X} = _{V Y ,} so _V (8)
_{The formula is V _ _ _ _ _ _} +f _e =f _R +K ₁ V _C (10) Thus, fluctuations in the oscillation frequency f _V due to drift are removed. Note that when the condition of equation (6) is slightly shifted, the center frequency of the variable oscillator 5, 3.58 MHz, is stabilized with a slight shift. In this way, if the detection output V _D is determined based on the frequency difference Δf between the oscillation output S _O of the variable oscillator 5 and the reference output S _R , and the variable oscillator 5 is controlled so that this becomes equal to the control voltage V _C , the variable Fluctuations in the oscillation frequency _fV due to the drift of the oscillator 5 can be removed. In other words, when the control voltage V _C is constant, if the oscillation frequency f _V changes by Δf _V due to drift, then
The detection output V _D increases by ΔV _D in proportion to Δf _V , and therefore the differential output V _Y decreases by −ΔV _Y. Along with this decrease, the control output voltage _VX also decreases by _-ΔVX , so that the variation _ΔfV in the oscillation frequency _fV becomes zero. If the control voltage V _C changes in this state, the oscillation frequency f _V will be controlled in accordance with the change, as is clear from equation (10). Furthermore, when the control voltage V _C includes an AC component that is faster than the response speed of the detection circuit 20, the output V _Y of the differential amplifier 21 will not become zero, but the time constant of the low-pass filter 22 should be selected to be long. For example, no disturbance is applied to the variable oscillator 5. Then, the control voltage _VX is controlled so that the average values of the control voltage _VC and the detection output _VD match, and thereby the oscillator frequency _fV is always equal to the reference frequency _fR . By the way, by configuring the detection circuit 20 for detecting the frequency difference described above as shown in FIG. 4, a detection output V _D having characteristics as shown in FIG. 3 can be obtained.
can be obtained. In this FIG. 4, the flip-flop circuit 3
The oscillation output S _O (Figure 5B) is supplied to the 0 set terminal S, and the reference output S _R (Figure 5A) is supplied to the reset terminal R.
is supplied to form the pulse output Pa of C in the same figure,
This is smoothed by a low-pass filter 31, and its output Pb (D in the figure) is supplied to a differentiating circuit 32. The polarity of the differential pulse Pc is different when the oscillation frequency _fV is lower than the reference frequency _fR and when it is higher. The differential pulse Pc is supplied to the first slice circuit 33A selected at the positive slice level L ₁ to form _the slice output Pd ₁ shown in FIG. A slice output Pd ₂ of G in the figure is formed in the slice circuit 33B. These are pulse outputs of a predetermined width from the monostable multivibrators 34A and 34B, respectively.
After the outputs Pe ₁ and Pe ₂ (H and I in the figure) are synthesized, the outputs Pe ₁ and Pe 2 are synthesized in the synthesis circuit 35 according to the levels of the outputs Pe 1 and Pe ₂ . Set the reference level of pulse output Pe ₁ and Pe ₂ to
If Eo is assumed, the composite output Pf will be as shown in J of the same figure. Therefore, if this composite output Pf is smoothed by the low-pass filter 36, the oscillation frequency _fV and the reference frequency
DC output (detection output) corresponding to the frequency difference with f _R
V _D (J in the same figure) is obtained. With the configuration shown in FIG. 4 as described above, a frequency difference-detection output characteristic as shown in FIG. 3 can be obtained. In addition, in this FIG. 4, the multivibrators 34A and 34B may be omitted. 6 and 7 show specific examples of the synthesis circuit 35. FIG. FIG. 6 shows a case where the circuit is configured as a current circuit, in which a pair of unit current sources 40 and 41 are connected in parallel, and an output terminal 35a is led out from one of them.
The pair of pulse outputs Pe ₁ and Pe ₂ are then supplied to a decoder 43 for current source control, and the on/off of the current sources 40 and 41 is controlled as desired by this decoded output. For example, as shown in Fig. 5, in order to make the detection output V _D maximum when f _R > f _V and minimum when f _R < f _V , when f _R > f _V , the current source 40 , 41 are both on, and when f _R < f _V , both are off, and f _R =
A logic circuit may be constructed so that only one of the current sources 40 and 41 is turned on when _fV . FIG. 7 shows a case where the synthesis circuit 35 is configured as a voltage circuit, and has a reference voltage source 50 that outputs three values, for example, 4V-6V-8V, and a switching circuit 51, and this switching circuit 51 is configured as a voltage circuit. , a flip-flop circuit 56, an OR circuit 57, and a decoder 58. An example of logical operation (Table-
Shown in 1).

[Table] Figure 8 shows the case where the present invention is applied to the APC circuit of the reproduction system of a VTR, and terminal 1 receives a burst signal S _B ′ separated from the color video signal to be recorded.
is supplied. Carrier signal S _C obtained at terminal 7
is supplied to a frequency conversion circuit (not shown) provided on the transmission path of the chroma signal (3.58 MHz), and the chroma signal is converted to a low frequency as desired. Even in this case, the drift frequency can be reduced to 1 even if the variable oscillator 5 is used or an oscillator is used.
It can be suppressed to ~2kHz. As explained above, the present invention provides a direct current component, that is, a voltage V _C from the output V _D of the frequency difference detection circuit 20.
, remove only the frequency fluctuation, amplify it, and add it to the voltage V _C.
Since it is configured to be supplied to the VCO, it has the effect of stabilizing the oscillation frequency of the VCO by eliminating even small frequency fluctuations due to variations in circuit constants or temperature characteristics. Also, even if a crystal oscillator is not used as the variable oscillator 5,
APC circuit can be configured. for that,
Since the control sensitivity of the variable oscillator 5 can be made several times higher than when using a crystal oscillator, the pull-in time is shortened, and even if the hue changes significantly, it can be corrected in an extremely short time. be able to. That is, when used, for example, in a signal forming circuit for frequency conversion of a carrier color signal of a VTR, the effect is remarkable. Moreover, according to this configuration, the number of crystal oscillators used can be reduced by one, and since there is little drift, there are no mislock points in the APC loop that exist every 15 kHz. According to this configuration, the control sensitivity of the variable oscillator 5 can be freely selected.

[Brief explanation of drawings]

Figure 1 is a system diagram showing an example of a conventional APC circuit.
2 and 8 are system diagrams showing an example of the case where the oscillation circuit according to the present invention is applied to an APC circuit of a reproduction system and a recording system, FIG. 3 is a characteristic diagram of a frequency difference detection circuit, and FIG. FIG. 5 is a system diagram showing an example of a frequency difference detection circuit, FIG. 5 is a waveform diagram for explaining its operation, and FIGS. 6 and 7 are connection diagrams showing specific examples of a combining circuit. 2 is a reference oscillator, 5 is a variable oscillator, 6 is a frequency conversion circuit, 20 is a frequency difference detection circuit, and V _C and V _X are control voltages.

Claims (1)

[Claims] 1. (a) A variable oscillator 5 whose oscillation frequency is controlled based on an input control voltage and whose oscillation frequency fluctuates more easily than a crystal oscillator; (b) An output signal from this variable oscillator and a reference. (c) taking the difference between the input control voltage and the detected output voltage from the detection circuit according to the frequency difference;
(d) an adder circuit 24 that adds the output voltage from this differential amplifier circuit and the above input control voltage; and (e) the output signal from this adder circuit is A variable oscillation circuit that is supplied to a variable oscillator to determine an oscillation frequency based on the input control voltage and to correct fluctuations in the oscillation frequency of the variable oscillator.