GB2181016A - Compensating velocity errors in a reproduced video signal - Google Patents

Abstract

In a method for compensating velocity errors in a video signal taken from a magnetic tape in which each field is recorded in several tracks 1 to 4 running obliquely with respect to the longitudinal direction of the tape, a respective error signal corresponding to the velocity error of each line is determined and used for compensating the velocity error of the next line. For compensating the velocity error of the first line of each track, the difference A or B between the error signals of the first line of the present track and the last line of the preceding track are determined and used for compensating the velocity error of the first line of the next or the next but one track. <IMAGE>

Description

SPECIFICATION Method for compensating velocity errors in a video signal The invention relates to a method for compensating velocity errors in a video signal.

When playing back video signals, particularly colourtelevision signals, from a record support such as a magnetic tape, as a result of mechanical inaccuracies of the scanning system and tape transport system, fluctuations occur in the time base playback as compared with those during recording.

Due to the lack of other suitable time marks in the video signal, these time base errors can only be measured at the beginning and end of each line, e.g.

by a phase comparison of the horizontal frequency sync pulses of the played back video signal with horizontal frequency comparison pulses. In known time error correctors, a delay to the video signals in the sense of correcting the time base errors is controlled on the basis of the result in this measurement.

In known digital time base correctors, the video signal taken from the magnetic tape is digitised with clock pulses derived from the video signal, i.e. the clock pulses suffer from the same time base errors, and is written into a memory from which it is read out with clock pulses which are free from time base errors. However, here again, the time base errors can only be measured at the beginning and end of the lines. However, in order to be able to also correct the change to the time base errors during a line, called velocity errors, a number of methods have been proposed. In these methods, the delay of the video signals used for correcting the time base errors during the course of each line is gradually modified in the direction of the expected time base error at the beginning of the following line.For this purpose, in the known methods, the time base errors are measured in successive horizontal blanking intervals and are compared with one another.

The further processing of the velocity error signal obtained through this comparison is a function of the recording process used. In magnetic tape recorders based on the transversal recording process, in which four magnetic heads successively record tracks on the tape which are almost at right angles to the tape edge, and in so-called segmented helical recording processes, velocity error correctors are used in which the velocity errors of lines scanned by in each case the same head and which in each case have the same position with respect to the magnetic tape edge are averaged over several head wheel revolutions. It is also known, mainly in conjunction with other helical recording processes, to determine the velocity error for each line and to delay the video signals by one line, so that the error signal can be used for the same line.

Both known methods suffer from disadvantages, particularly in conjunction with a digital video processor for a magnetic tape recorder operating according to the segmented helical recording process.

Accordingly, the present invention provides a method for compensating velocity errors in a video signal which is reproduced from a record support on which the video signal is recorded a plurality of tracks, wherein a respective first error signal corresponding to the velocity error of each line is determined and used for compensating the velocity error of the next line, and wherein for compensating the velocity error of the first line of each track, a second error signal is determined at the start of each track and used for compensating the velocity error of the first line of the next or next but one track.

The inventive method has the advantage compared with the first aforementioned method that velocity errors both during the first line of a field and also the remaining lines can be corrected in an advantageous manner.

Compared with the other known method for correcting time base errors, the inventive method has the advantage that simple, accurate determination of the time base errors is possible.

Although the main use is in magnetic tape recorders with segmented recording of the fields, it can also be advantageous to use this method in tape recorders recording each field on a single track.

An embodiment of the invention will now be described with reference to the accompanying drawings, wherein: Figure 1 is a voltage-time diagram of an example of velocity errors; and Figure 2 is a block circuit diagram of an arrangement for performing the method according to the invention.

The diagram of Figure 1 shows velocity error as a function of time in the case of a magnetic tape recorder with segmented scanning i.e. in which the information content of each field is recorded over several tracks at an angle to the iongitudinai direction of the tape. In the present case, each segment is assumed to comprise 52 lines. The curve shows the magnitude of the velocity error, to an arbitrary vertical scale, during the scanning of four consecutive segments 1,2,3 and 4. In each case at the start of a segment there is a jump A or B, whilst the velocity error changes only slightly within each 52-line segment.Segments 1 and 3 are played back from a first magnetic head and segments 2 and 4 from a second magnetic head of the recorder, the heads being disposed diametrically opposite one another on the circumference of a head wheel of the recorder.

It has been found that the jumps A and B of the velocity error on passing from one magnetic head to the other remain substantially constant. The velocity error level overall is, however, subject inter alia to statistical variations.

In the method according to the present embodiment, it is assumed that within a segment the velocity error is determined in known manner by measuring the length of a line and this is used for correction purposes in the following line. However, for correcting the first line in a segment since there is no similar value available from a preceding line, use is made of the level of the jump A or B from the preceding head change, so that the value from the value for the last line of a given segment a conclusion can be drawn regarding the correction value for the first line of the next but one segment.

In certain applications, the amplitude of jumps A and B are substantialiy the same, so that it is sufficient to draw conclusions from one segment to the next.

In some known magnetic tape recorders with segmented scanning, during the playback after the last line of each segment, no signal is available for determining the length of this line. Thus, in a further development of the embodiment, use is made of the velocity error of the penultimate line for determining jumps A and B, as well as for the velocity error of the first line of the following segment.

By means of a numerical example, further details are given hereinafter of the embodiment. A correction value is to be determined for the first line of segment 4 and for this purpose quantity A is added to the measured value for the fifty first line of the preceding segment 3, which is also used as a correction valve in the fifty second line. Quantity A is calculated from the difference of the values determined for the first line of segment 2 and the fifty second line of segment 1, the latter being derived from the length of the fifty first line of the first segment The velocity error essentially determined by the measurement of the line length is supplied to the arrangement of Figure 2 as a 9 bit wide digital signal across a D-register 5 clocked with horizontal frequency pulses H.So that the value determined at the end of the fifty first line is available for determining the jumps A and B and not only for correction purposes during the fifty second line, at the beginning of the first line of a segment the corresponding pulse H is not supplied to the clock input of the D-register.

The digital signals representing the correction values pass through an adder 6 and the values A or B are only added in the first line of each segment. To the adder 6,there is connected a limiter 7, which ensures that the maximum or minimum value given by the 9-position binary number is passed on if there should be an overflow or underflow due to the addition in adder 6.

By means of a register 8, which is clocked with the horizontal frequency pulses H, the correction signals from the limiter 7 can be supplied to a known correcting circuitvia an output 9.

The output signals of limiter 7 are also supplied to a further register 10, which stores the correction value for the fifty second line of each segment until the following correction value obtained through the length measurement of the first line of the next segment is present. The stored signals are removed in inverted form from register 10, so that in an adder 11 there is formed the difference A or B of the correction values of in each case the first line of a segment and the last line of the preceding segment.

These values A and B are separately stored in registers 12 and 13, in each case for the duration of two segments, and are alternately supplied to the adder 6 at the beginning of the next but one segment.

Registers 12 and 13 are clocked with the horizontal frequency pulses H. Their outputs are in each case controlled by means of an OC-input with corresponding signals OC1 and OC2 in such a way that no signal from registers 12 and 13 is supplied to adder 6 during the second to fifty second lines. As stated hereinbefore, under certain conditions the amplitudes of signal jumps A and B can be substantially the same and in this case only one of the registers 12 or 13 is required.

In order to facilitate understanding of the invention, Figure 2 only shows the essential components of a circuit for performing the embodiment of the invention. The supply of the components with clock signals and switching signals is also represented in simplified form. Thus, e.g!, the horizontal frequency pulses H can have different widths and phase positions.

Claims (8)

1. A method for compensating velocity errors in a video signal which is reproduced from a record support on which the video signal is recorded in a plurality of tracks, wherein a respective first error signal corresponding to the velocity error of each line is determined and used for compensating the velocity error of the next line, and wherein for compensating the velocity error of the first line of each track, a second signal is determined at the start of each track and used for compensating the velocity error of the first line of the next or next but one track.

2. A method according to Claim 1, wherein in the video signal the information content of each field is recorded in a plurality of tracks which are successively scanned by a plurality of magnetic heads arranged on a head wheel of a magnetic tape recorder.

3. A method according to Claim 2, wherein the second error signal is obtained by forming the difference between the first error signals for the first line of each segment and the last line of the preceding segment.

4. A modification of the method according to Claim 3, wherein the first error signal for the penultimate line of each segment is used for forming the said difference.

5. A method according to Claim 3 or 4, wherein for correcting each first line of a segment with respect to the first error signal of the last line of the preceding segment, the second error signal from the preceding segment or the segment before that is added thereto.

6. A method according to Claim 1, wherein the second error signal is obtained by forming the difference between the first error signals for the first line of each track and the last line of the preceding track.

7. A method according to Claim 6, wherein for correcting each first line of a track with respect to the first error signal of the last line of the preceding track, the second error signal from the preceding track or the track preceding the latter is added thereto.

8. A method according to Claim 1, substantially as described herein with reference to the accompanying drawings.