GB2251717A - Automatic tracking circuit - Google Patents

Abstract

An automatic tracking circuit for a VCR comprises a first comparator COMP1 for comparing input sampling data with a reference voltage, an integral part 10 for integrating the standardized sampling data from the first comparator COMP1, and a second comparator COMP2 for comparing the sampling data integrated through the integral part 10 with an envelope level. The tracking of a tape is automatically adjusted during the whole operation of the VCR as well as at the beginning of the playback of the tape, by monitoring the envelope level. Thus, users can watch a noiseless fine picture without the need for manual adjustment of the tracking. <IMAGE>

Description

AUTOMATIC TRACKING CIRCUIT This invention relates to automatic tracking circuits for use in automatic tracking alignment of a playback head with a recording medium, and is concerned particularly, although not exclusively with VCRs (Video Cassette Recorders) which pick up an image signal recorded on a video tape by using a head to play back the image signal.

In a VCR, the head has to track the video tape accurately in order to provide a picture without noise. However, in conventional VCRs, users have to adjust the tracking of the tape manually with a tracking control switch whilst watching the reproduced picture, and the tracking has to be adjusted whenever the tape is played back, which is inconvenient to the user.

Recently, a circuit which automatically tracks the tape only at the beginning of playback has been proposed, so that such inconvenience can be somewhat reduced during playback.

However, when the speed of the tape is changed or the signal recorded onto the tape disappears and re-appears, automatic tracking can not be performed, and thus the user has to adjust the tracking manually. That is, a VCR including a circuit which can automatically adjust the tracking only at the beginning of the tape can not adjust the tracking when the speed of the tape is changed or the recorded signal re-appears.

Therefore, in order to obtain a good picture without noise, the user has to adjust the tracking manually, similarly to the conventional case.

Preferred embodiments of the present invention aim to provide an automatic tracking circuit which detects a decrease of video envelope level due to loss of tracking and then performs an automatic tracking operation by driving a servo integrated circuit (IC) if the decrease of the video envelope level exceeds a predetermined value, so as always to provide a good picture even when the speed of the tape is changed or the recorded signal disappears and re-appears, as well as at the beginning of the playback of the tape.

According to a first aspect of the present invention, there is provided an automatic tracking circuit for use in automatic tracking alignment of a playback head with a recording medium, the circuit comprising: input means for receiving input sampling data at a predetermined sampling rate and standardising the sampled data to provide a pulse train at a predetermined amplitude level; waveform shaping means for receiving said pulse train and shaping it to a waveform of triangular form; comparator means for receiving and comparing said triangular waveform with an envelope level of said input sampling data and providing a compared output waveform representing the difference in level between said triangular waveform and said envelope level; and control means for receiving said compared output waveform and providing a control signal to adjust tracking of a respective playback head in response to a parameter of said compared output waveform.

Preferably, said control means is operative to provide a control signal to adjust tracking of a respective playback head when said parameter of said compared output waveform exceeds a predetermined value.

Preferably, said parameter is a pulse width of a pulse train of said compared output waveform, which pulse width represents the difference in level between said triangular waveform and said envelope level.

Preferably, said input means comprises a first comparator for comparing said input sampling data with a reference voltage to standardize said data.

Preferably, said first comparator comprises an operational amplifier for receiving a supply voltage divided by two biasing resistors at a noninverting terminal as said reference voltage, and for comparing said reference voltage with said input sampling data applied to an inverting terminal.

Preferably, said waveform shaping means comprises a capacitor and a resistor for charging or discharging according to the output from said first comparator, a transistor turned on or off according to the charging or discharging of said capacitor and said resistor for generating a gearing waveform.

Preferably, said comparator means comprises an operational amplifier for comparing said triangular waveform applied to a noninverting terminal with said envelope level applied to an inverting terminal, and for applying the compared output waveform to said control means, which comprises a microprocessor.

Said input sampling data may have five pulses during half the period of a head switching clock.

According to another aspect of the present invention, there is provided an automatic tracking circuit for use in automatic tracking alignment of a playback head with a recording medium, the circuit comprising means for detecting a decrease of envelope level of a playback signal due to loss of tracking and means for performing an automatic tracking operation if the decrease of the envelope level exceeds a predetermined value.

Such a tracking circuit may further comprise any one or more of the features disclosed in the accompanying specification, claims, abstract and/or drawings, in any combination.

The invention also extends to apparatus (e.g. a video tape recorder) for playing back a signal recorded on a recorded medium, the apparatus comprising a playback head and an automatic tracking circuit according to any of the preceding aspects of the invention.

According to another aspect of the present invention, there is provided an automatic tracking circuit comprising a first comparator for comparing input sampling data with a reference voltage to standardize, an integral part for integrating the standardized output sampling data from the first comparator, and a second comparator for comparing the output from the integral part with an envelope level to provide such a compared output waveform to a microcomputer.

For a better understanding of the invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: Figure 1 shows one example of an automatic tracking circuit embodying the present invention, for use in a VCR; and Figure 2 shows examples of waveforms of major parts of the automatic tracking circuit of Figure 1.

In Figure 1, in a first comparator COMP1, a reference voltage derived from a supply voltage VCC by two biasing resistors R1 and R2 is applied to a noninverting terminal thereof, input sampling data is applied to an inverting terminal thereof to be standardized, and a triangular waveform shaping part 10 is connected to the output terminal of the first comparator COMP1 to differentiate the sampling data output from the comparator.

The output of the triangular waveform shaping part 10 is applied to a noninverting terminal of a second comparator COMP2, where an envelope level of the input sampling data is applied to the inverting terminal of the second comparator COMP2 to compare the output voltage of the triangular waveform shaping part 10 with the envelope level and provide a compared result to a microcomputer MICOM (not shown).

The triangular waveform shaping part 10 comprises a transistor Q1, two capacitors C1 and C2, and two resistors R3 and R4. Also, a pull-up resistor R5 is connected to the output of the second comparator COMP2.

The sampling data may be either an arbitrary pulse or a video picture search pulse, and a preferable sampling data has about five pulses during half the period of a head switching clock.

In the example of the invention given above, the sampling data which has five pulses during half the period of the head switching clock as shown in Figure 2A is applied to the inverting terminal of the first comparator COMP1 and the supply voltage Vcc is divided by the resistors R1 and R2 to be applied to the noninverting terminal of the first comparator COMP1 as a reference voltage. Then, the first comparator COMP1 provides standardized sampling data as shown in Figure 2B. The output signal of the first comparator COMP1 as shown in Figure 2B is applied to the base of the transistor Q1 in the triangular waveform shaping part 10 to be differentiated by the capacitor C2 and the resister R4 according to the driving of the transistor Q1 and thus, a waveform is applied to the noninverting terminal + of the second comparator COMP2 as shown in Figure 2C.

Therefore, the output waveform of the first comparator COMP1 as shown in Figure 2B is applied to the noninverting terminal + of the second comparator COMP2 as shown in Figure 2C through the differentiation of the triangular waveform shaping part 10.

At the same time, the envelope level shown in Figure 2D is applied to the inverting terminal of the second comparator COMP2 and if the tape is not tracked accurately, the signal recorded on the tape is not picked up normally and thus the envelope level is gradually decreased.

Contrarily, if the tape is accurately tracked, the envelope level is increased, or remains at a suitably high level. In other words, in the inaccurate tracking case, the envelope level is decreased as shown in Figure 2D, while it is increased (or remaining high) in the accurate tracking case.

Then, the second comparator COMP2 compares the differentiated output from the triangular waveform shaping part 10 with the envelope level, where the integrated output from the triangular waveform shaping part 10 has a constant period and the envelope level is changeable, so, when the tracking is not accurate, the envelope level is lowered as shown in Figure 2D.

Thus, as the envelope level decreases due to the inaccurate tracking of the tape as shown in Figure 2D, the second comparator COMP2 provides an output signal as shown in Figure 2E to the MICOM (not shown).

As shown in Figure 2F, the MICOM evaluates a pulse width from the falling edge of the output pulse from the first comparator COMP1 to the falling edge of the output pulse from the second comparator COMP1, and if the evaluated pulse width exceeds a predetermined value, the MICOM drives a respective servo IC (not shown) to adjust the tracking of the tape, regarding the tracking to be incorrect. In order to detect the falling edges of the output pulses as shown in Figure 2B, the MICOM may receive directly the input sampling data as fed to the first comparator COMP1.

On the other hand, if the evaluated pulse width is below the predetermined value, the MICOM does not alter the tracking, regarding the tracking to be correct.

The automatic tracking circuit as described above operates not only at the beginning of the playback of the tape but also during the whole operation of the VCR.

In other words, even when the speed of the tape is changed or the recorded signal disappears and re-appears, the tracking is automatically adjusted, since the envelope level is always detected during the whole operation of the VCR.

Thus, the user can watch a high quality noiseless picture without the need for any manual adjustment of tracking.

By comparing the differentiated waveform with the envelope level when the adjustment of the tracking is needed, for example, in case of the variation of the speed of the tape or the disappearance and re-appearance of the recorded signal, the tracking is accurately adjusted by driving the servo IC if the envelope level is proven to be below the predetermined value by the MICOM, while if the envelope level is above the predetermined value, the tracking is fixed.

Thus, the user can watch a good picture without noise, due to the automatic tracking of the tape.

The reader's attention is directed to all papers and documents which are filed concurrently with or previous to this specification in connection with this application and which are open to public inspection with this specification, and the contents of all such papers and documents are incorporated herein by reference.

All of the features disclosed in this specification (including any accompanying claims, abstract and drawings), and/or all of the steps of any method or process so disclosed, may be combined in any combination, except combinations where at least some of such features and/or steps are mutually exclusive.

Each feature disclosed in this specification (including any accompanying claims, abstract and drawings), may be replaced by alternative features serving the same, equivalent or similar purpose, unless expressly stated otherwise. Thus, unless expressly stated otherwise, each feature disclosed is one example only of a generic series of equivalent or similar features.

The invention is not restricted to the details of the foregoing embodiment(s). The invention extends to any novel one, or any novel combination, of the features disclosed in this specification (including any accompanying claims, abstract and drawings), or to any novel one, or any novel combination, of the steps of any method or process so disclosed.

Claims (13)

1. An automatic tracking circuit for use in automatic tracking alignment of a playback head with a recording medium, the circuit comprising: input means for receiving input sampling data at a predetermined sampling rate and standardising the sampled data to provide a pulse train at a predetermined amplitude level; waveform shaping means for receiving said pulse train and shaping it to a waveform of triangular form; comparator means for receiving and comparing said triangular waveform with an envelope level of said input sampling data and providing a compared output waveform representing the difference in level between said triangular waveform and said envelope level; and control means for receiving said compared output waveform and providing a control signal to adjust tracking of a respective playback head in response to a parameter of said compared output waveform.

2. A tracking circuit according to claim 1, wherein said control means is operative to provide a control signal to adjust tracking of a respective playback head when said parameter of said compared output waveform exceeds a predetermined value.

3. A tracking circuit according to claim 1 or 2, wherein said parameter is a pulse width of a pulse train of said compared output waveform, which pulse width represents the difference in level between said triangular waveform and said envelope level.

4. A tracking circuit according to claim 1, 2 or 3, wherein said input means comprises a first comparator for comparing said input sampling data with a reference voltage to standardize said data.

5. A tracking circuit according to claim 4, wherein said first comparator comprises an operational amplifier for receiving a supply voltage divided by two biasing resistors at a noninverting terminal as said reference voltage, and for comparing said reference voltage with said input sampling data applied to an inverting terminal.

6. A tracking circuit according to any of the preceding claims, wherein said waveform shaping means comprises a capacitor and a resistor for charging or discharging according to the output from said first comparator, a transistor turned on or off according to the charging or discharging of said capacitor and said resistor for generating a gearing waveform.

7. A tracking circuit according to any of the preceding claims, 1, wherein said comparator means comprises an operational amplifier for comparing said triangular waveform applied to a noninverting terminal with said envelope level applied to an inverting terminal, and for applying the compared output waveform to said control means, which comprises a microprocessor.

8. A tracking circuit according to any of the preceding claims, wherein said input sampling data has five pulses during half the period of a head switching clock.

9. An automatic tracking circuit for use in automatic tracking alignment of a playback head with a recording medium, the circuit comprising means for detecting a decrease of envelope level of a playback signal due to loss of tracking and means for performing an automatic tracking operation if the decrease of the envelope level exceeds a predetermined value.

10. A tracking circuit according to claim 9, further comprising any one or more of the features disclosed in the accompanying specification, claims, abstract and/or drawings, in any combination.

11. A tracking circuit substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus for playing back a signal recorded on a recorded medium, the apparatus comprising a playback head and an automatic tracking circuit according to any of the preceding claims.

13. Apparatus according to claim 12, being a video tape recorder.