JPH01296873A - Dropout detecting circuit - Google Patents

Abstract

PURPOSE:To detect a dropout as specified with a set value by connecting a diode in parallel with the load resistance of a differential amplifier to detect the dropout. CONSTITUTION:A video signal is inputted through a both-wave rectifying circuit 2 to a dropout detecting circuit. The dropout detecting circuit consists of a buffer circuit 3, a detecting circuit 4, a differential amplifying circuit 5 and an output stage circuit 7. The differential amplifier 5 has transistors(TRs) Q2 and Q3, and a diode D is connected in parallel with a load resistance R3. Thus, the gain of the differential amplifier 5 is decreased, and the error of a detection level is reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a dropout detection circuit for detecting dropouts occurring in a magnetic tape when reproducing a video signal recorded on the magnetic tape.

2. Description of the Related Art Video signals generally include a color signal for adding color to the screen and a luminance signal for representing the brightness of the screen. Among these, when recording the luminance signal on a magnetic tape, the frequency is 3.4 to 4.
．． It is frequency modulated and recorded at 4 MHz. next,
When reproducing a magnetic tape, the frequency modulated signal is demodulated to reproduce the original signal. However, if there is a dropout on the magnetic tape on which signals are recorded, noise will occur on the screen due to the loss of some signals even when the magnetic tape is reproduced. Therefore, when reproducing a magnetic tape with dropouts, a method is used in which the dropouts are detected and the signals reproduced before the dropouts are used to suppress screen noise. For this reason, it is necessary to accurately detect dropout.

A conventional dropout detection circuit used for this purpose is shown in FIG. 2, and will be described with reference to FIG.

This circuit consists of a double-wave rectifier circuit 2 to which an input terminal 1 is connected, a buffer circuit 3 consisting of a transistor QI and a current source 1, and a detection circuit 4 consisting of a resistor R1 and a capacitor C.
, a dropout detection differential amplifier 5 consisting of transistors Q2 and Q3, resistors R2 and R3, current source 2 and voltage source V1, transistors Q5 and Q6, resistor R4,
Current source! 3, an output stage circuit 7 formed by a differential amplifier consisting of a voltage RV 2 and an output terminal 6;

The operation of the conventional dropout detection circuit configured as described above will be described below.

First, the signal Vina shown below is input to the input terminal 1.

Vina= As in ωt −
(1) A: Amplitude ω of input signal; Angular frequency t; Time input signal V inn passes through the double-wave rectifier circuit 2 (the circuit gain is set to 1), and the transistor Q of the buffer circuit 3! is transmitted to the emitter of

The signal at the emitter of transistor Q1 is V e I a
Then, the signal V E I a is expressed by the following equation using Fourier series expansion.

V E l m-(2A/K)-(4#) x mountain t
cos2nIIIt/ (2n-1) (2n+1)
f...(2) Next, the signal V E l s is transmitted to the base of the transistor Q2. If the signal here is VB2m, it is expressed by the following equation.

VB2a-(2A/z)-(4/x) x 1 11/J1+(2nwr+-c)211cos
2rvt/(2n-1)(2n+1)fn+l+ (3) Resistance value C of single resistor R1: Capacity of capacitor C Here, from the left side of equation (3), the DC component of signal VB21 is 2A/π, the AC component of the signal VB2a is −(4/) x ”f, I 1/I l+(2nwrl −C
)2ttcos2niit/(2n-1)(2n+1
) 1n fable 1.

Next, the output voltage of the differential amplifier 5 is the transistor Q5.
The base potential of is shown below. Signal VB shown by equation (3)
2a and the reference voltage v1 of the voltage source V1 is V B2@>>
V + +7), all of the current from current source 2 flows to the collector of transistor Q2, and the base potential VB5aH of transistor Q5 is expressed by the following equation.

VBSaH= vcc r 2×12°
------- (4) Vcc; power supply voltage r2; resistance value of resistor R2 +2; current value of current source 2. Next, the relationship between the base signal VB2a of transistor Q2 and the reference voltage v1 is VB2. If << v I, that is, if there is dropout, the current of current source I2 (12
) all flow to the collector of the transistor Q3, and the base potential VB5aL of the transistor Q5 becomes the following equation.

VBSsL=VCC-(r2+r3)X+2...
(5) r3: resistance value of resistor R3 Next, V which is the output voltage of the output stage circuit 7. This is shown for uta. The maximum and minimum values of the input signal to the output stage circuit 7 are determined by VBSaH and VB as shown in equations (4) and (5).
5 a L. The differential amplifier of the output stage circuit 7 must operate with this input signal. Therefore, voltage source 'V2 (7) reference voltage v 2 and VBSa) I+ VB5
aL requires the following conditions.

V B511L < V 2 < V B5aH++
++ (6) The following description will be made assuming that the condition of equation (6) is satisfied.

Assuming that the input signal of the output stage circuit 7 is VBSa, V
In the state of B5aL < VB5s < V2, the current (i3) of current source I3 flows entirely to the collector of transistor Q5, and the output voltage V. uta becomes the following formula.

Vouta = i3 x r 4
......('7) r4; Resistance value of resistor R4 Next, base potential V of transistor Q5 of output stage circuit 7
When B5a is in the state of sV2<VBSa≦VBSH, all the current (i3) of current source I3 flows to the collector of transistor Q6, and the output voltage V. uta becomes zero.

As a result of the above, if there is a dropout, output terminal 6
A high level of output is detected.

Problems to be Solved by the Invention In the above conventional configuration, the signal V B 2 a at the base of the transistor Q2 of the differential amplifier 5 is equal to the reference voltage v1.
(VB2aΣv1), that is, when a signal in the vicinity determined as dropout is input, there is a problem as shown below. transistor Q
The signal VB2a transmitted to the base of 2 is expressed by equation (3). When VB2a=VI, it is necessary to consider the output of the differential amplifier 5 by also considering the alternating current component shown in equation (3). First, assuming that the gain of the differential amplifier 5 is Aha, it is expressed by the following equation.

^5s=r3/1(k-T/q-ig2)+(kT/q
-igs)l...(8)k; Boltzmann constant r3; resistance value T of resistor R3; absolute temperature q; elementary charge igz; emitter current ig3 of transistor Q2;) emitter current of transistor Q3 where: , transistor Q2
Assuming that the characteristics of and Q3 are equal, and from the relationship between VB2a and v1, I E2= I E3= i E °
°°°°19). From equation (9), equation (8) becomes as follows.

ASa= r3/ 2 r6...
-・(to)ro ;ni@T/q@iI! Normally, r3>>r, so the gain Ass is A5*>>1
becomes. Therefore, the collector potential of the transistor Q3 of the differential amplifier 5, that is, the base potential V of the transistor Q5
B 5 s is expressed by the following formula.

Vssa-Vcc-i21rz+<r3/2)1-(4
/ni)・Ass・”t (1/71”(2niirhic)
)2)l cos2nwt/(2n-1)(2n+1)1...
(11) From equation (11), when a signal near the dropout is input, the signal input to the output stage circuit 7 contains a large AC component.

Therefore, there is a problem in that the alternating current component in equation (11) results in an error in an actual circuit when compared with the signal level determined as dropout.

The present invention solves the above-mentioned conventional problems by reducing the alternating current gain of the differential amplifier for detecting dropouts to a smaller value than that of the conventional circuit. The purpose of this invention is to provide a dropout detection circuit that can perform the following steps.

Means for Solving the Problems To achieve this object, the dropout detection circuit of the present invention connects a diode in parallel with the load resistance of a differential amplifier that detects dropouts in a conventional dropout detection circuit. be.

Effect: According to this configuration, the gain of the differential amplifier is reduced compared to the conventional dropout detection circuit, and errors in the detection level can be reduced.

Embodiments Below, embodiments of the dropout detection circuit of the present invention will be described with reference to the circuit diagram shown in FIG.

This circuit consists of a double-wave rectifier circuit 2 connected to an input terminal 1, a transistor whose base is connected to the double-wave rectifier circuit 2, whose collector is connected to a power supply terminal 8, and whose emitter is connected to a ground point via a current source 1. A buffer circuit consisting of Q+, a detection circuit in which a resistor R+ and a capacitor C are connected in series between the emitter of the transistor QI and the ground point, and the output is the common connection point of the resistor R1 and the seven capacitors C, and a detection circuit whose base is the resistor R+ and a transistor Q2 whose collector is connected to the power supply terminal 8 through a resistor R2 and whose emitter is connected to the ground point through a current source I2, to the common connection point of the capacitor C and the transistor Q2, whose collector is connected to the resistor R2 through a resistor R3, A transistor Q3 whose base is connected to a constant voltage source v1 and an emitter connected to a current source ■2, and a resistor R3.
A differential amplifier circuit consists of a diode D connected in parallel to the transistor Q3, the base is connected to the collector of the transistor Q3, the emitter is connected to the power supply terminal 8 via the current source 3, and the collector is connected to the output terminal 6 and the resistor R4. an output stage circuit consisting of a transistor Qs connected to the ground point through the .

The operation of the dropout detection circuit of this embodiment configured as described above will be explained below.

First, input the following signal to input terminal 1.

Next, if V;nb=A s i nω and the signal passed through the double-wave rectifier circuit 2 and transmitted to the emitter of the transistor Q1 is VElb, then by expanding the double-wave rectification of the sine wave into a Fourier series, It is shown by the following formula. (Here, assume that the gain of the double-wave rectifier circuit is l) VEtb-(2A/x)-(4#)・11cos2nw
t/(2n-1)(2n+1)1l (12) Next, in order to convert this VElb signal into a DC component, it is transmitted to the detection circuit 4 composed of a resistor R1 and a capacitor C. Ru. Here, first, when the transfer function G of this detection circuit 4 is determined, the following equation is obtained.

G= 1/ (1+ j ωc rl) −・”
(13)j; imaginary unit ω; angular frequency C; capacitance value rl of capacitor C; resistance value of resistor R1.
The transistor Q
If this signal is V B2b, the following equation is obtained.

B2b- (2A/to) - (4/to) ・'f, 11/ + nwr+c tlcos
2nwt/(2n-1)(2n+1)i-l (I4) The collector potential Vc3b of the transistor Q3, which is the output of the differential amplifier 5, is the signal V shown in equation (14). B
2b and the reference voltage Vl of the constant voltage source v1. First, when VEtb>>vl, all of the current from current source 2 flows to the collector of transistor Q2.

In this case, the collector voltage of transistor Q3 is set to V c
3H, it is expressed by the following formula.

Vc3)1=vCCr2・i2・”
(15) VCCi power supply voltage r2; resistance value 12 of resistor R2; current value of current source I2 Next, if VEtb<<y, that is, if there is dropout, all the current of current source I2 is transferred to the transistor Flows to the Q3 collector. In this case, if the voltage at the collector of transistor Q3 is VC3L, it is expressed by the following equation.

VC3L"vccr2・i2VD
−=・(16) vD; diode anode・
Voltage between cathodes Next, this high level signal VC3H or low level signal VC3L is transmitted to the output stage circuit 7 forming a differential amplifier. Here, this differential amplifier is VC3H
In order to operate with VC3L and the reference voltage v2 of the constant voltage source v2, the following conditions are required between the constant voltage source v2 and the reference voltage v2.

VC3L < V2 < VO2)1
- (17) First, if there is no dropout, that is, if a high-level signal V C3H is input to the base of transistor Q5, then all the current supplied from current source I3 flows to the collector of transistor Q6. flows to Therefore, the output voltage V. utb=0. next,
If there is dropout, i.e. transistor Q5
When a low level signal VC3L is input to the base of the transistor Q5, all of the current <I3> supplied from the current source I3 flows to the collector of the transistor Q5.

The output voltage V of the output terminal 6 at this time. utb is expressed by the following formula.

Voutb:! 3 @r 4 I4 ; Resistance value of resistor R4 Next, when the base signal V B2b of transistor Q2 is almost equal to the reference voltage v1 (vB2b2V+), that is, a signal with a magnitude near the set dropout detection signal is input. Find out if this happens. First, the differential amplifier 5
If the gain of is Asb, then the following formula % Formula % iD ; Current flowing through diode D iH2 ;) Emitter current of transistor Q2 iH3 ; Emitter current of transistor Q3 Here, the characteristics of transistors Q2 and Qs are equal,
When iH=1Ez=ig3, the gain Asb is given by the following equation.

Asb=(1/2)・(ig/(ig-Vo/r3)
t...-(19) Here, i E >> VD/
When the condition of r s is satisfied, the gain Ash becomes as follows.

Asb=1/2 (2
0) Therefore, the reference voltage v1 is applied to the base of transistor Q2.
The collector voltage VC3b of the transistor Q3 when an input signal substantially equal to is inputted is expressed by the following equation.

Vc3b=Vcc-r29i2-V.

-(4/に)・1・0s(l/1r-];1:3"hi;1171ko1l-tl cos2n&It/(2n-1)(2n+
1) 1... (21) Therefore, according to the present invention, it is possible to reduce the harmonic components that cause malfunctions when the dropout detection circuit operates at the threshold.

Effects of the Invention According to the dropout detection circuit of the present invention, conventional circuits amplify harmonic components that cause malfunctions, but with the present invention, the gain can be reduced to a small value of 1/2, thereby preventing malfunctions. As a result, dropout detection accuracy is increased and reliability is improved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing an embodiment of the dropout detection circuit of the present invention, and FIG. 2 is a circuit diagram of a conventional dropout detection circuit. 1...Input terminal, 2...Double wave rectifier circuit, 3...Buffer circuit, 4...Detection circuit, 5...Difference dynamic amplifier, 6...output terminal, 7...output stage circuit, 8...power supply terminal, Ql, Q2. Qs, Q4. Qs, Qe・
...Transistor, R1, R21R3, R4...
...Resistance, D...Diode, rl,
I2. 13...Current source, Vl+V2...
... Constant voltage source. Name of agent: Patent attorney Toshio Nakao and 1 other person - Input terminal 3 - Buffer circuit 4 - Palm cutout 5 - Differential amplifier circuit 6 - Output terminal 7 - Equation throw circuit Qt, Qz, Qs, Ch, Qs, Q6- trunksge F-, Ji'z, f? s, f? 4.-Resistance C
---Gonten length Vt, Vt- constant electric power source

Claims (1)

[Claims] A double-wave rectifier circuit to which an input terminal is connected, a base to the double-wave rectifier circuit, a collector to the power supply terminal, and an emitter to the first
a buffer circuit including a first transistor connected to a ground point via a current source; a first resistor and a capacitor connected in series between the emitter of the first transistor and the ground point; a detection circuit whose output is a common connection point between the first resistor and the capacitor, a base connected to the common connection point between the first resistor and the capacitor, a collector connected to the power supply terminal via a second resistor, and an emitter connected to the power supply terminal. a second transistor connected to a ground point via a second current source; a collector connected to the second resistor via a third resistor; a base connected to the first constant voltage source; and an emitter connected to the first constant voltage source. a differential amplifier circuit configured with a third transistor connected to the second current source and a diode connected in parallel with the third resistor;
A fifth transistor whose base is connected to the collector of the third transistor, whose emitter is connected to the power supply terminal via the third current source, and whose collector is connected to the output terminal and the ground point via the fourth resistor.
and a sixth transistor whose emitter is connected to the third current source, whose base is connected to the second constant voltage source, and whose collector is connected to the ground point. Features a dropout detection circuit.