DE2350407C3 - Circuit for automatic horizontal frequency control for a device for generating a still image - Google Patents

Description

The invention relates to a circuit for automatiien Horizontal frequency control for a device for generating a still image by sieving out one image signal from a television signal, storage dos image signal and repeated playback of the stored signal, with horizontal synchronization pulses, which can be separated by a synchronization signal separator, in a phase detector with phase comparison pulses are comparable, which can be derived from a horizontal deflection circuit to a To get control signal that indicates the phase difference between, and this control signal via a Low-pass filter is fed into a horizontal resonant circuit to adjust the phase of the horizontal resonant circuit to control.

ίο There is already a circuit for automatic Horizontal frequency control known (US-PS 27 64 686), which has a switching device to off Filter to select one with a short time constant and one with a long time constant,

is where the switching device according to the present or the absence of a signal that is opened or closed by comparing a Self-vertical synchronizing signal with an external vertical synchronizing signal obtained in a mixing stage

: o ten will. If the self-vertical synchronization signal during operation and the foreign vertical synchronizing signal are in phase, if no positive part of the signal is generated in the mixer, the switching device is closed and the filter with the longer one

:> Time consiants used to make the circuit for automatic horizontal frequency control less sensitive to noise. If, on the other hand, the Eigen-Vertical-Synchroa ^; The control signal and the external vertical synchronization signal are not in phase

ίο are, the switching device is opened and it is the filter is used with the short time constant, resulting in a fast response of the circuit for automatic horizontal frequency control leads.

It follows that known with this

is circuit a faster synchronization can be obtained can when the phases of a vertical synchronizing signal and a horizontal synchronizing signal are simultaneous are shifted, as is the case when two different programs on one television station be replaced. However, the known circuit is not suitable for rapid response in one device of the type mentioned at the beginning for generating a still image by filtering out a field signal a television signal, storing the field signal and

4s repeated playback of the stored signal perform. Namely, in the known circuit, the phase of a horizontal synchronizing signal becomes shifted while the phase of a vertical sync signal is simultaneously maintained, wi «

so this further below with reference to FIG. 2a and 2b sewi is explained. Therefore, if the known circuit for automatic frequency control in a device dei above type is used, the switching device is always closed, so that a fast;

S5 response is not possible.

There is also a circuit for automatic frequency control known (US-PS 32 87 657), in dei compared an input signal in a comparator with an output signal from an oscillating circuit

ho is going to get a difference signal at the output of the compare get chets. This difference signal becomes an oscillating circuit via a low-pass filter with a fixed bandwidth te. corresponding to a fixed time constant, and a further low-pass filter with a variable bandwidth

hs te, according to a variable time constant fed in order to adjust the frequency of the resonant circuit. and the difference signal also becomes the further! Low-pass filter over a high-pass filter, a rectifier

he and an amplifier fed in order to adjust the bandwidth of his further low-pass filter, in this case The circuit for automatic frequency control will depend on the bandwidth of the further filter Controlled amplitude of the difference signal. In particular, the bandwidth of this filter becomes smaller Frequency difference between the input signal and the output signal larger, so that a long Time period is required in order to synchronize between the input signal and the output signal achieve.

If such a circuit for automatic Frequency control in a device for generating a still image by filtering out a field signal a television signal, storing the field signal and repeatedly reproducing the stored signal is used, the circuit will be disturbed by noise. Noise signals appear on the reproduced picture because it takes a long time to achieve synchronization and the bandwidth of the further filter can be kept small even after the blanking period.

Another circuit for automatic horizontal frequency control is known (DT-AS 11 47 257), in which the output connection of a low-pass filter is grounded via a capacitor and a diode. The diode conducts to lengthen the time constant of the low-pass filter and to make the automatic horizontal frequency control circuit less responsive to noise when a horizontal sync signal and a flyback pulse are synchronized. The diode is blocked in order to reduce the time constant of the filter and to achieve fast synchronization in the circuit for the automatic horizontal frequency control. In order to connect the capacitor and the low-pass filter in a stable manner with this circuit, however, when the horizontal synchronization signal and the flyback pulse are synchronized, the time constant of a discharge element, which comprises a further capacitor and makes the diode conductive, should be greater than 10 f / (H. = Horizontal scanning period). Accordingly, there is a relatively long period of time between the time at which the horizontal synchronizing signal and the flyback pulse are out of phase and the time at which the diode becomes conductive, so that the rapid response of the circuit for automatic horizontal frequency control after a relatively long period of time is achieved. Furthermore, in this known circuit, a phase shift between the horizontal synchronizing signal and the flyback pulse is corrected step-by-step when the diode is made non-conductive and the synchronization speed is high. However, the diode is made conductive even if the horizontal sync signal and part of the flyback pulse overlap, so that the fast sync operation ceases before the horizontal sync signal and flyback pulse are fully synchronized, and a relatively long time is required to to achieve complete synchronization between these signals.

Therefore, if the circuit known from DT-AS 11 47 257 for automatic frequency control in the The device mentioned at the outset is used to generate a still image from a television signal it is difficult to achieve complete synchronization for the horizontal sync signal within the blanking period to be achieved, as the fast synchronization

drove stops when the horizontal sync signal and part of the flyback pulse overlap. Consequently, in this circuit for automatic horizontal frequency control, the upper part of the reproduced still image curved.

1, 2a and 2b show waveforms of simplified television signals which have odd field television signal parts I, even field television signal parts II and vertical synchronizing signal parts Hl. 1, 2a and 2b, image signal parts 1 to 16, horizontal synchronizing pulses A to and horizontal deflection currents 17 to 21 are provided.

From FIG. Fig. 1 shows that in the presently used television broadcasting system, the length of a field is selected to have a value which is an odd multiple of the horizontal period H of a television signal obtained by interlacing a picture so that horizontal -Synchronization pulses occur at intervals of the horizontal period H. Therefore, in a television receiver that receives a television signal from orderly transmitted odd fields I and even fields II, a circuit for automatic horizontal frequency control (hereinafter also referred to as horizontal frequency controller) with a phase detector for comparing the phase of the horizontal synchronizing pulses with the phase of the flyback pulses and provided with a horizontal oscillating circuit which is controlled by the output signal of the phase detector in order to generate a horizontal deflection current 17 which is synchronous with the television signal arriving at the television receiver television signal described above are very small, the

AC feedback loop gain of the circuit for automatic horizontal frequency control be chosen appropriately small in the television receiver. Therefore, the noise bandwidth can be sufficiently reduced and the noise reduction characteristic in the circuit for automatic Horizontal frequency control in the television receiver can be improved.

However, one circuit commonly used in television receivers is automatic horizontal frequency control not suitable for an Hdb picture memory player in which a still picture can be obtained by sifting out a field picture signal from a television signal, the field picture signal stored and this stored signal is played back repeatedly. The reasons for this will be discussed in more detail below with reference to FIG. 2a and 2b explained.

As explained above, the one field of a television signal for interlace scanning is selected to have a value which is an odd multiple of the horizontal repetition period. Therefore, in any case where the odd field I or the even field II is sifted out from the television signal, the length of one picture signal is equal to mH + 1/2 H, where m is an integer. Therefore, if only the odd field I is screened out, stored and repeatedly reproduced, the interval between horizontal synchronizing pulses P and E in the adjacent region of the repeated image signals is equal to 1/2 H as shown in FIG. 2a emerges. On the other hand, if only the even field II is screened, stored and repeatedly reproduced, the interval is

OU 4U /

between the horizontal synchronizing pulses D and ζ) in the adjacent area of the repeated image signals equal to 3/2 //, as shown in FIG. 2b can be seen. Therefore, if the horizontal synchronizing pulses E to P \ n Fig. 2a or the horizontal synchronizing pulses A to D and Q to W in Fig. 2b are used to generate a horizontal deflection current, controlled by the conventional circuit for automatic horizontal frequency control, which is usually provided in television receivers, the horizontal deflection current thus generated has waveforms as these are designated by 18, 19, 20 or 21 in Figs. 2a and 2b. It is apparent that this current waveform cannot respond quickly to the 1/2 H or 3/2 H phase shift, and its frequency gradually increases or decreases until the current waveform becomes synchronous with the horizontal sync signal in the composite television signal after a period of time corresponding to some horizontal periods.

Such a condition is pictorially shown in Figs. 3a and 3b shown. The upper part of the reproduced Image is bent to the right as shown in Fig.3a is shown when the frequency of the horizontal deflection current is low, during the upper part of the reproduced image is bent to the left, as shown in Fig.3b, when the frequency of the Horizontal deflection current is high.

This shortcoming can be overcome by adding the AC feedback loop gain
the circuit for automatic horizontal frequency control in the field memory playback device is enlarged to a very large value, whereby the tracking speed of the automatic frequency control increases, so that the circuit can respond quickly to the phase shift of 1/2 // or 3/2 H .

However, in commonly used circuits for automatic horizontal frequency control with a low-pass filter including a pass band (single-pole single-zero low-pass filter), the following relationship between the AC feedback loop gain m ■ / o and the noise bandwidth // w holds:

'"2.7 y

with m = a / b, with a = angular frequency at the corner or limit point of the low-pass filter and 6 = angular frequency at the zero point of the low-pass filter and ^ ⁰ with A = direct current feedback loop gain of the circuit for automatic horizontal frequency control.

As can be seen from the equation (1), an increase in the AC feedback loop gain m · k causes an increase in the noise bandwidth / λ / λ /, and the aimed noise suppression characteristic cannot be obtained. Such a circuit for automatic horizontal frequency control has the disadvantage that jaggies appear in the reproduced images or unstable images which are unsuitable for practical reproduction because of the noise.

To overcome the disadvantages outlined above, it is the object of the present invention to provide a circuit to specify the automatic horizontal frequency control of the type mentioned, in which the phase of the horizontal resonant circuit can respond quickly to the phase of the horizontal synchronization signal that follows the vertical sync signal and in which the noise bandwidth during the vertical scanning period can be reduced to improve the noise suppression characteristic.

According to the invention, this object is achieved by a pulse generator for generating control pulses synchronously with vertical synchronizing pulses, and by a switching device that is connected to the between the Phase detector and the horizontal resonant circuit lying low-pass filter and connected by the Control pulses can be operated to the low-pass filter partially or completely from the phase detector for a predetermined limited time within a vertical blanking period after the vertical sync signal turn off so that the phase of the horizontal resonant circuit quickly changes to a phase of the horizontal synchronizing signal is addressable.

In the circuit according to the invention, the phase of the horizontal resonant circuit can quickly match the phase of the Responding horizontal synchronizing signal, and there are no noise signals on the reproduced Image even if a fast synchronizing operation accompanied by the generation of noise signals in the circuit for automatic horizontal frequency control for the vertical blanking period is carried out.

The invention is explained in more detail below with reference to the drawing. Show it

Fig. 1, 2a and 2b waveforms of a simplified television signal and a horizontal deflection current,

3a and 3b still images obtained by storing a field part of the television signal and a repeated display of such a signal part can be obtained while the horizontal synchronization with the above-mentioned circuit for automatic Horizontal frequency control is carried out,

F i g. 4 shows a block diagram of a circuit according to the invention for automatic horizontal frequency control and

F i g. 5 shows a practical embodiment of the in FIG. 4 shown circuit.

In FIG. 4 has a circuit according to the invention for automatic horizontal frequency control a synchronization signal input 31, a phase detector 32, a low-pass filter 33, a horizontal oscillation and deflection stage 34, a pulse generator 35 which generates switching pulses which have a particular duration and are synchronous with vertical synchronizing pulses, and a switching device 36, which is dependent on the Infeed of the actuating pulse from the pulse generator 35 is open and short-circuited during the rest of the time is held

In the circuit shown in Figure 4, the feeds Pulse generator 35 sends a control pulse to the switching device 36 in order to move it into its open position bring, and holds this switching device 36 in such a position! For a certain period of time, the follows the vertical sync signal interval, whereby the low-pass filter 33 is separated from the horizontal frequency controller in order to reduce the AC feedback loop effect to enlarge during this period. This time period is preferably selected to precede the vertical blanking period corresponds to the arrival of the image signal corresponding to the next field

Switching device 36 closed to the low-pass filter 33 reconnect to the horizontal frequency controller, thereby increasing the AC feedback loop gain and the noise reduction characteristic is improved.

In FIG. 5 showing a practical embodiment of the in FIG. As shown in the block diagram shown in FIG. 4, the horizontal frequency regulator comprises a power supply connection 41, a synchronizing signal input 42, the horizontal synchronizing signal and the Vertical synchronization signal receives, for example a phase comparator pulse input, a flyback pulse input 43, a voltage output 44, a phase detector 46, a low-pass filter 47 with a normally grounded terminal 45, a transistor

48 for amplifying the synchronizing pulses obtained by integrating the vertical synchronizing signal are, two transistors 49 and 50, which form a monoflop, a drive pulse pass diode 51, two Diodes 52 and 53 that go along with the transistors

49 and 50 form a switching device, a resistor 54 and a capacitor 55 for Determination of the switching duration of the switching device, a capacitor 56, the diodes 52 and 53 switches off during the switching period, a discharge resistor 57 for the capacitor 56 and a Resistance 58.

In the case of the FIG. 5 are the transistors 49 and 50 forming the monoflop, respectively usually conductive and non-conductive. Therefore, diodes 52 and 53 are in across resistors -57 and 58 Forward biased, and the terminal 45 of the low-pass filter 47 is through the diode 52 and the Transistor 59 grounded. Depending on the supply of a vertical synchronization pulse of positive polarity in the base of transistor 48, a negative polarity pulse occurs at the collector of transistor 48 to be in the Base of the transistor 49 to be fed through the diode 51. At this point in time the transistor is 49 turned off, and the transistor 50 conducts, so that the diodes 52 and 53 by the collector voltage of the Transistor 49 and the voltage across capacitor 56 are therefore reverse biased the terminal 45 of the low-pass filter 47 is completely electrically isolated from the ground potential

It can be seen that, in the present invention, the low pass filter 47 is during the vertical blanking period is separated from the horizontal frequency controller. Therefore, the AC feedback loop gain increases of the horizontal frequency controller to a level which is essentially the same is the level of the DC feedback loop

40

50 fen gain, so that the horizontal frequency control changes in the phase of the Horizontal Synchroni siersignals in the range between the adjacent

Can follow image signals.

The length of time that the AC feedback loop gain increases is due to the resistance of resistor 54 and the capacitance of the capacitor 55 assigned to the monoflop is determined. More precisely, depending on the Line of the transistor 50, the positive electrode of the capacitor 55 is grounded, and a negative voltage is at the base of transistor 49 to turn it off. The transistor 49 conducts again when the The charge stored in the capacitor 55 is discharged through the resistor 54 until the base voltage of the Transistor 49 is approximately "0". Therefore, the duration of the AC feedback loop gain is increasing by the resistance of resistor 54 and the capacitance of the capacitor 55 determined. When the charge stored in capacitor 55 is discharged to the base voltage of transistor 49 is approximately "0", transistor 49 conducts, and transistor 50 is again turned off to connect the low-pass filter 47 to the horizontal frequency controller, so that the

AC feedback loop gain of the horizontal frequency controller is reduced again and the noise bandwidth is also reduced. A similar process is repeated whenever a Vertical sync pulse is fed into the base of transistor 48, and therefore the Response of the horizontal frequency regulator without deterioration of its noise suppression characteristic can be improved during the vertical sampling period. In particular, the present invention has the following advantages:

1. The horizontal frequency controller can operate without deteriorating its noise suppression characteristic respond quickly to a phase shift occurring after the Vertical sync occurs because of the AC feedback loop gain of the horizontal frequency controller during the period (vertical scanning period) during which the Image signal occurs on the length of time (vertical blanking period) is changed by means of the switching device while the image signal is not present

2. The response time of the horizontal frequency controller can be set appropriately, as the switching duration is freely selectable

For this purpose 3 sheets of drawings

609 635/277

3718

Claims (2)

Patent claims: 1. Circuit for automatic horizontal frequency control for a device for generating a still picture by sifting out a field signal from a television signal, storing the field signal and reproducing the stored one Signal, with horizontal synchronizing pulses, which can be separated by a synchronization signal separator, in a phase detector Phase comparison pulses are comparable, which can be derived from a horizontal deflection circuit to to receive a control signal that represents the phase difference indicates in between, and this control signal via a low-pass filter in a horizontal resonant circuit is fed in to control the phase of the horizontal oscillating circuit, marked by a pulse generator (35) for generation of actuating pulses synchronized with vertical synchronizing pulses, and by a switching device (36) connected to that between the phase detector (32) and the horizontal resonant circuit (34) lying low-pass filter (33) and connected by the control pulses is operable to the low-pass filter (33) partially or completely from the phase detector (32) for a predetermined limited time within a vertical blanking period after the vertical sync signal turn off, so that the phase of the horizontal resonant circuit (34) quickly to a Phase shift of the horizontal synchronizing signal is responsive (F i g. 4). 2. Circuit according to claim 1, characterized in that the switching device (36) is a monoflop (49, 50), which can be controlled by the actuating impulses and in such a state for a a predetermined period of time is held, and at least two diodes (52, 53) in series between the output connections of the monoflop (49, 50) lie that the low-pass filter (33) consists of a Resistor and a capacitor in a series circuit (47), with one connection of the low-pass filter (33) with the connection point between the two diodes (52, 53) in the Switching device (36) and the other connection to the connection point between the phase detector (32) and the horizontal resonant circuit (34) is connected, and that the switching device (36) for the predetermined limited period of time within the vertical blanking period after the actuating pulses is controlled depending on the feed of the control pulses from the pulse generator (35) to thereby the To separate the low-pass filter (33) from the phase detector (32) during such a period of time, so that the Phase of the horizontal oscillating circuit (34) quickly to a phase shift of the horizontal synchronization signal can address that occurs in each field (F i g. 4,5).