Time base correction of color video playback signal

Start of patent duration: Longest possible duration: Issued:

Inventor:

1984 02

1984 10 © Dependence:

© Pamphlets considered to delineate the prior art:

AT 376 096

Nr.376096

The invention relates to a circuit arrangement for correcting or compensating the time base error of a color subcarrier type color video signal mixture obtained from a recorded signal, which is understood to mean a color video signal mixture, which has luminance and chrominance components and a horizontal synchronizing signal and in which the chrominance component is converted in frequency relative to a standard subcarrier to a relatively low frequency band lower than that in which the luminance component is recorded, which circuitry comprises a display device with a superposition circuit . which reconverts the squared chrominance component of the recorded signal by a carrier reference signal to provide the chrominance component of the reproduced color video signal mixture with the frequency of the standard subcarrier frequency, a signal generator which outputs the carrier reference signal from the horizontal synchronizing signal of the reproduced color video signal mixture derives and thus includes its time base error, and a time base correcting means which receives the reproduced color composite video signal and the carrier reference signal as a clock signal and compensates for the time base error of the color video signal mixture output from the display device.

Due to the relatively low write or Recording speed and due to the thus relatively low recording bandwidth of the conventional, cheap Bildbandaufzeichnungsgeräte with helical-tracking recording, direct color recording is not possible with such recording devices, which is why the so-called overlay or color subset arrangement for the recording and reproduction of color image signals is used by such image tape recorders. In the recording operation in a typical helical-scan video recorder, a color composite image is decomposed into its chrominance component and luminance component, whereupon the luminance component is used for frequency modulation of a carrier, while the chrominance component in frequency is converted from the standard subcarrier frequency to a relatively low frequency band which is below the frequency band, in which the frequency modulated luminance component occurs. The frequency-modulated luminance component or luminance component and the frequency-reduced chrominance component or Farbartkomponente be recombined for recording in parallel tracks which extend at an oblique angle over a magnetic tape and which are scanned sequentially by means of two magnetic recording heads, which are rotated, for example, at a speed of 30 Umdr / s, in each track a field of Record video signal information.

After a recording has been made on the tape, the recorded composite color video signal can be reproduced by the rotating heads of a corresponding picture signal playback or picture signal playback device. In many cases, the image signal display device may be constituted by a reproducing or reproducing part of the video recorder which was originally used for recording the color image signal. However, as is often the case, the arrangement used to reproduce or play back the recorded color image signal mix may be different than the arrangement used to record the particular color image signal mix. Although servo systems are generally provided to control the rotational motions of the magnetic heads and the rectilinear motion of the tape during the recording and playback operations, variations in the rotational speed of the magnetic heads and / or in the rectilinear velocity of the tape may occur during recording and during recording Play or during the playback of the color image signal mixture occur. Moreover, dimensional changes may occur in the magnetic tape, for example due to tape shrinkage or stretching of the tape after recording. All of the possible changes listed above can result in errors in the frequency and phase of the reproduced color image signal mix. Such frequency and phase errors, known as time base errors, produce undesirable, perceptible effects such as jitter, brightness distortion, and incorrect color reproduction in the finally reproduced color image. It is of particular importance that the time base errors in a reproduced color image signal composite be corrected when this color image signal composite is to be transmitted in the course of a television program.

Nr.376096

The time-base correction means developed so far are used to eliminate or compensate time-base errors introduced in picture or other information signals by their recording and / or reproduction. In such time base correctors, successive lines or other intervals of the reproduced color image signal are written into a memory at a clock frequency which generally varies with the time base errors. Then, the color image signal is read from the memory at a standard clock frequency and is thus largely free of time base errors. An example of such an arrangement is given in US Pat. No. 3,860,952.

When the recorded color composite image signal is of the color subcarrier type, which means that the chrominance component of the relevant color image signal mixture has been down-converted from a standard subcarrier frequency, such as 3.58 MHz in the case of an NTSC signal, to a relatively low frequency band below the frequency band in which the luminance component is recorded, for example by frequency modulation of a suitable carrier, then the playback or playback device for such a recorded color composite signal mix includes a so-called superposition circuit in which the chrominance component of the reproduced signal is superimposed again to the standard chrominance subcarrier frequency. However, the process of frequency converting the chrominance component to a low carrier frequency, such as 688 kHZ, for recording the respective color image signal mix and then re-frequency converting the chrominance component of the reproduced signal to the standard subcarrier frequency of 3.58 MHz, for example, destroys the phase relationship between the horizontal synchronizing signal and the color burst signal. This leads to an increase in difficulty in making the time base correction of the reproduced color image signal mixture.

When the clock frequency at which intervals of the reproduced color image signal are written in the memory of the time base corrector is determined by an artificial, somewhat out-of-band subcarrier generated in the time base correcting arrangement from only the horizontal synchronizing signal of the reproduced color image signal mixture , then the problem of the lack of phase coherence is solved, however, the time base corrector is made incomplete only because the color burst signal, which provides more accurate information regarding the time base errors, is not used. In view of this, it has already been proposed to apply the so-called two-channel solution in which the subcarrier or carrier reference signal artificially obtained outside the band obtained from the horizontal synchronizing signal of the reproduced color picture signal mixture and which sets the clock frequency for writing in the time base. Determines correction device, is also fed back to the superposition circuit or the processing means of the display device to eliminate the lack of phase relationship. In the two-channel arrangement mentioned above, the overlay processing arrangement of the display device comprises, in particular, an automatic phase-locked loop (APC) which generates or generates the out-of-band signal. obtained artificial subcarrier and outputs a re-frequency conversion signal to a frequency converter, by which the chrominance component of the reproduced signal is converted by superposition again in the standard chrominance subcarrier frequency. Due to the limited feedback gain of the automatic phase locked loop or Phase control loop in the display device and also due to the fact that the feedback gain of the automatic phase locked loop may change from one display device to the other display device, the color burst in the reproduced color composite image signal obtained at the output of the respective display device may not be completely out of sync Bandes artificially derived subcarrier match, which is generated in the time base corrector. Accordingly, the existing two-channel system or the existing two-channel arrangement can not reliably effect a good time base correction or compensation, u.zw. especially in cases where the time base corrector is provided as a component for use in connection with various image tape players or players.

- 4 No.376096

The invention is accordingly an object of the invention to provide an improved circuit arrangement for the correction or compensation of Zeitbasisfehlern in a color image signal, u.zw. in particular in the event that the relevant color image signal is recorded and reproduced with a so-called overlay or color sub-array arrangement. In particular, the color burst contained in a composite color image signal, which occurs as an output of a display device having a superposition circuit, should correspond to an artificially derived subcarrier formed outside of the tape which is formed in an associated timebase correction device, the color burst in question Overlay circuit is due, to set in this the intended for a re-frequency conversion conversion signal. '

Starting from a circuit arrangement of the type mentioned, this object is achieved in that a controllable phase shifter is provided, via which the carrier reference signal is supplied from the signal generator of the superposition circuit, that a gate circuit for deriving a burst reference signal from the output of the display device and a phase Comparator are present, which compares the phase of the burst reference signal obtained from the gate with the phase of the carrier reference signal from the generator and supplies the controllable phase shifter with a control signal determined by this comparison to control the amount of phase shift which the phase shifter gives to the carrier reference signal.

The invention will now be explained with reference to drawings, for example. In the drawings show: Fig. 1 shows schematically the frequency spectrum of a recorded color image signal of the color subcarrier type to be reproduced according to the invention; Fig. 2 shows the reproduced color signal composite as Fig. 1 after being processed in a display device containing a superposition circuit; 3 is a block diagram showing a known circuit structure of a display device comprising a time base corrector and a circuit for providing an out-of-band artificial subcarrier for time base correction, the subject subcarrier also being the superposition circuit of the display device in a two-channel arrangement of known type can be returned, and Fig. 4 in a corresponding block diagram as shown in FIG. 3 a circuit arrangement according to an embodiment of the invention.

Referring to Fig. 1, it will be appreciated that a recorded color composite image signal recorded in accordance with the invention is dependent on the color subcarrier or colorant composite. A color reduction type in which the recorded color image signal includes a frequency modulated luminance component Sp having a center frequency of 3.5 MHz, for example, and a chrominance component S ^ which is in the frequency of a standard subcarrier frequency, for example, 3.85 MHz in the case of NTSC Signal has been converted to a relatively low subcarrier frequency of, for example, 688 kHz in order to occur in a frequency band, which is substantially below the frequency band of the recorded frequency modulated luminance component. In addition, in the recorded color composite image signal, the usual vertical and horizontal synchronizing signals and color burst signals are included. Such a color subcarrier type color image signal mixture as illustrated in FIG. 1 can be realized by a conventional helical-scan type video recording apparatus, ie with so-called oblique scanning, are recorded in parallel tracks which run obliquely over a magnetic tape -T-. At this time, a partial image of the image signal information is recorded in each of the successive tracks.

After a recording has been made on the tape -T-, the recorded color composite image signal can be reproduced by means of a corresponding image signal reproducing or playback device -10- (Figure 3), which is used by a reproducing or playing part of the original one used for recording the color image signal Picture tape recorder can be formed. Regardless of whether the display device -10- is separate from or part of the image tape recorder used to record the color image signal, such an arrangement may comprise two magnetic heads -11a and 11b- which are appropriately rotated to alternate in succession helical tracks

No.376096 recorded on the tape -T- scan and reproduce color image signals when the relevant tape -T- is moved in a suitable longitudinal direction.

The color image signal composite reproduced from the tape -T- by means of the rotary heads -11a and 11b- is supplied to a luminance separating circuit -12- and a chrominance separating circuit -13- in the usual manner. The luminance separation circuit 12 may be constituted by a high-pass filter which allows the frequency band of the frequency-modulated luminance component (FIG. 1) to be transmitted. The output of the high pass filter -12-, ie the frequency-modulated luminance component is supplied to a frequency demodulator -14-, from which the original luminance component Sy (Fig. 2) of the color image signal is obtained. The chrominance separation circuit -13- may be formed by a low-pass filter which passes the frequency band of the frequency-lowered chrominance signal (Fig.l). The frequency-reduced chrominance component passing through the filter 13 is applied to a superposition circuit 15 -10- of the playback device -10- in which the chrominance component of the reproduced signal is superimposed on a signal at the standard chrominance subcarrier frequency. In the conventional playback device -10- according to Figure 3, the superposition circuit -r-15- is shown in particular as a device which includes a frequency converter -16-, in which the output signal of a low-pass filter -13- from a frequency re-transmission signal which the conversion of the subcarrier causes the recorded chrominance component to be at the original frequency or standard subcarrier frequency. For example, if the frequency-reduced chrominance component of the recorded signal has a subcarrier frequency of about 688kHz, then the frequency re-conversion signal applied to the frequency converter 16-16 may have a frequency of about 4.27 MHz to achieve in the Transducer -16- to act in such a way that the subcarrier of the reproduced chrominance component of the frequency of 688 kHz is converted back to the standard frequency of 3.58 MHz. The demodulated luminance component from the demodulator 14 and the frequency converted chrominance component from the converter 16 are applied to an adder or mixer 17 which outputs the reproduced color composite signal at an output terminal 18 of the display device. of the playback device -10-.

In order to provide the frequency re-transmission signal serving for re-frequency conversion to the converter -16-, the superposition circuit -15- of the device -10- further typically comprises a phase locked loop or a phase lock loop consisting of a burst gate circuit -19, a phase comparator -20, a local oscillator or an external reference signal source -21-, a voltage-controlled oscillator (VCO) -22- and a frequency converter or mixer circuit. consists. The burst gate circuit -19- is connected to the output of the frequency converter -16- so as to transmit the color burst signal contained at the beginning of the respective horizontal line interval in the frequency-converted chrominance component. As usual, the burst signal frequency is equal to the frequency of the subcarrier to which the chrominance information is modulated. Accordingly, in the example described with reference to Fig. 3, each color burst signal supplied through gate circuit 19 to one input of the phase comparator -20- has a frequency of about 3.58 MHz. In contrast, the other input of the phase comparator -20- is connected to a movable contact -24a of a switch -24- to be connected to the local oscillator -21- when the movable contact -24a is connected to a fixed contact -24b- of the switch -24- is applied. The phase comparator -20- compares the phase of the burst signal from the gate circuit -19- with a constant phase of the reference signal occurring at a frequency of 3.58 MHz or a corresponding oscillation supplied by the local oscillator -21-. The phase comparator -20- generates a signal which is proportional to the detected phase difference between the signals applied to the inputs of the respective phase comparator. Such a signal is supplied as a control voltage to the voltage-controlled oscillator -22- to set the oscillation frequency of this oscillator. In the case that the frequency down

Nr.376096

For example, if the chrominance component of the recorded color image signal has a carrier frequency of 688 kHz and is to be converted back to the standard subcarrier frequency of 3.58 MHz, the output signal of the voltage controlled oscillator -22- has a center frequency of 688 kHz. This output signal is applied to the one input of the Frequenzumsetz- or mixing circuit -23-. At another input, the circuit -23- picks up the reference or local oscillation signal at the frequency of 3.58 MHz. The upper sideband of the mixed output signals from the local oscillator -21- and the voltage controlled oscillator -22-, this upper page sideband has a frequency of about 4.27 MHz (3.58 MHz + 0.688 MHz), is considered to be that for a re-frequency conversion provided Frequenzumsetzsignal the frequency converter -16- supplied. It will be appreciated that the phase comparator -20- controls the voltage controlled oscillator -22- in response to the phase difference between the burst signal coming from the gate circuit -19- and the oscillation or reference signal coming from the local oscillator -21-, u.v. , such that the center frequency of the output signal output by the voltage-controlled oscillator -22- is changed. In this way, an automatic phase locked loop or Phase Synchronization Loop (APC) for the frequency converter supplied -16- serving a re-frequency conversion conversion signal created.

Although servo systems can be provided for controlling the rotations of the heads -11a and 11b- and the rectilinear movement of the tape -T- during recording and playback, variations in the rotational speed of the heads and / or in the rectilinear velocity of the tape may nevertheless occur so that such speeds do not coincide with the corresponding values ​​in the recording and reproducing operations. In addition, dimensional changes may occur in the magnetic tape, for example due to shrinkage or stretching of the tape after recording. Due to the above-mentioned possible variations, frequency and phase errors known as time base errors may occur in the reproduced color picture signal composite appearing at the output terminal -18- of the player -10-.

In order to remove or eliminate such time base errors, the prior art arrangement shown in Fig. 3 further includes a time base correction circuit 25-25 with an associated circuit 26-2 for generating a subcarrier or carrier reference signal out of band. This signal contains the time base errors of the reproduced color image signal mix and is thus suitable as a clock trigger signal in the time base correction circuit -25-. The circuit -26- is particularly shown to include a horizontal synchronizing signal separating circuit -27- which separates the horizontal synchronizing signal from the reproduced color picture signal composite obtained at the output terminal -18-. The horizontal synchronizing signal, which has a frequency of 15.75 kHz, for example, is the one input of a phase comparator -28- supplied, which is part of a phase locked loop or a phase-locked loop -29-, which further includes a voltage-controlled oscillator -30- and a frequency divider -31-. In the case that an NTSC color image signal is processed, the voltage-controlled oscillator -30- may have a center frequency of 7.16 MHz, while the frequency divider -31- receives the oscillation output signal of the voltage-controlled oscillator -30- and this output signal of frequency division subjected by 455, so that at the output of the frequency divider -31- occurs a signal with a frequency of 15.75 kHz, when the output of the voltage-controlled oscillator -30- appears at its center frequency. The output signal of the frequency divider -31- is supplied to another input of the phase comparator -28-, which compares the signal in question with the separated horizontal synchronizing signal supplied by the separating circuit -27- and in response to the occurrence of a phase difference between the compared signals an appropriate Supply control voltage to the voltage-controlled oscillator -30-, whose output signal is adjusted by this control in a corresponding manner. The output signal of the voltage-controlled oscillator -30- is further supplied, as shown, a frequency divider -32- which makes a frequency division by 2 in the relevant signal and thereby outputs an output signal at about the standard subcarrier frequency of 3.58 MHz. The one with this frequency

No.376096 is applied to the fixed contact -24c- of switch -24- and also to a clock enable signal input to the time base correction circuit -25-. It will be appreciated that the output of the frequency divider -32-, which represents the aforementioned carrier reference signal produced by the circuit -26-, contains the time base errors of the reproduced color image signal mix, as reflected by the horizontal synchronizing signal from the divider circuit -27- becomes.

In the known circuit arrangement according to Fig. 3, the time base correction circuit -25- is shown to contain an analog-to-digital converter or AD converter 33, which receives the reproduced color image signal mixture from the output terminal -18- of the playback device -10-. In addition, the correction circuit '-25' includes a write clock generator -34- which receives the out-of-band subcarrier signal generated by the frequency divider -32- and generates the write clock pulses at a relatively high frequency of, for example, about 10.7 MHz. The value of this frequency is three times the standard color subcarrier frequency for NTSC signals and also three times the frequency of the output signal of the frequency divider -32-. The write clock pulses occurring at a frequency of about 10.7 MHz are applied by the generator 34 to the A / D converter 33 to control the number of times that this transducer samples the composite color signal from the output terminal 18 and implements the relevant signal from its original analog form into a digital form. The write clock pulses output by the generator 34, which occur at a frequency which is varied with the time base errors in the reproduced image signal, further controls or determines the frequency at which the sampled image signal is received in digital form from the converter 33 - is written into a digital memory -35-. As previously indicated (U.S. Patent No. 4,063,284), the digital memory 35 may consist of a number of register units into which the image signal information converted into digital form is written in a sequence which is controlled by a sequencer. which is also activated by the write clock pulses from the generator -34-. After a short-term storage in the register units of the memory -35-, the digital image signal information is read from these register units in an order which is also determined by the control means -36-. However, the readout of the digital image signal information from the memory -35- is effected at a standard clock frequency as determined by a read clock generator -37-. The read clock pulses from the generator -37- are also supplied to a digital-to-analog converter or DA converter -38- which receives the digital image signal information read from the memory -35- and converts it back into the original analog form. The color image signal resulting in analog form is supplied to an output terminal -39-. It will be appreciated that in the time base correction circuit, successive line intervals of the input reproduced color image signal mix are written into the memory at a clock frequency which generally varies in accordance with the time base errors of the reproduced signal. In addition, it will be appreciated that the image signal is read out of memory 35 at a standard clock frequency so that the color image signal obtained at output terminal 39 is substantially free of any timing errors.

It should be noted that, when in the known arrangement according to Figure 3, the switch -24- is set such that its movable contact -24- is connected to its fixed contact -24b-, the fixed or local vibration signal from the Oscillator -21- is delivered to the mixing circuit -23- in the superposition circuit -15- of the playback device -10-. In addition, the artificial out-of-band subcarrier or carrier reference signal obtained from the time base correction frequency divider -32- in the timebase correction circuit -25- is derived only from the horizontal synchronizing information, ie, from the horizontal Synchronizing signal supplied from the separating circuit -27-. The above arrangement provides relatively poor time base correction because more accurate information regarding the time base errors included in the color burst signal is not used.

When the switch -24- is switched so that its movable contact -24a

No.376096 is connected to its fixed contact -24c-, as shown in Figure 3, then a so-called two-channel arrangement is achieved, in which the artificially obtained, out-of-band subcarrier signal from the frequency divider -32- ago the overlay circuit -15- is fed back in the video tape recorder or player -10-. In such a two-channel arrangement, the phase comparator -20- of the automatic phase locked loop in the superimposing circuit -15- compares in particular the artificially obtained out-of-band subcarrier signal from the frequency divider -32- with the color burst signal supplied from the gate circuit -19- becomes. In addition, the respective phase comparator -20- controls the output of the voltage controlled oscillator -22- accordingly. Accordingly, the mixing circuit outputs the frequency conversion signal for frequency conversion to the frequency converter -16- from the output of the voltage-controlled oscillator -22- and from the artificially obtained out-of-band sub-carrier signal. The purpose of the above operation is to eliminate the phase mismatch between the horizontal synchronizing signal and the color burst signal in the reproduced color composite image signal obtained at the output terminal -18- of the reproducing apparatus -10-. Due to the limited loop gain of the automatic phase locked loop in the display device or in the reproducing apparatus -10- and due to the fact that the loop gain of such an automatic phase locked loop may change from one reproducing apparatus to another, it may happen that the color burst signal in the reproduced color image signal mixture obtained at the output terminal -18- is not completely coincident with the reproduced one obtained subcarrier signal occurring outside the band, which · ίη the time base correction circuit is formed, ie is provided at the output of the frequency divider -32-. Accordingly, the previously known two-channel system or the existing arrangement in the position of the switch -24- shown in FIG. 3 can not reliably achieve or achieve a good time base correction or compensation, u.zw. especially when the time-base correction circuit -25- and the associated circuit -26- are provided for providing the artificially derived out-of-band subcarrier signal as a component for use with various image recording or reproducing apparatuses.

Referring now to Fig. 4, the various components of the arrangement shown in Fig. 3 are indicated by the same reference numerals as in Fig. 3. It will be apparent from Fig. 4 that the above described in connection with the known two-channel Arrangement according to Figure 3 described problem eliminated or avoided by the invention, u.zw. by providing a controllable phase shifter -40- over which the artificially derived out-of-band subcarrier signal is output from the frequency divider -32- to the contact -24c of the switch -24-, i.zw. for the purpose of delivery via this switch to the phase comparator -20- and to the mixing circuit -23-. According to the invention, the controllable phase shifter 40 is controlled by a control signal or by a voltage provided by a phase comparator 41 which detects the phase of the out-of-band subcarrier signal or carrier reference signal from the frequency divider 32 the phase of a continuously occurring subcarrier signal from a burst gate circuit -42- connected to the output terminal -18-. The burst gate circuit -42- derives each color burst signal from the reproduced color composite signal at the output port -18- and converts that signal into a continuous subcarrier signal for the remainder of the next horizontal interval. Such sub-carrier signal from the gate circuit -42- is compared in the comparator -41- with the out-of-band sub-carrier signal from the frequency divider -32- to suitably control the variable phase shifter -40-. Due to the above arrangement, the out-of-band subcarrier signal which is fed back to the superimposing circuit -15- of the reproducing apparatus -10- via the phase shifter -40 and the switch -24- is set to coincide with the signal Color burst signal from the output of the playback device -10- coincides. In this way, accurate time base correction is thus easily achieved.

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