GB2181017A

GB2181017A - Compensating drop-outs in a colour television signal

Info

Publication number: GB2181017A
Application number: GB08622625A
Authority: GB
Inventors: Jurgen Heitmann
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1985-09-21
Filing date: 1986-09-19
Publication date: 1987-04-08
Also published as: DE3533699C2; JPS6268391A; FR2587869A1; GB8622625D0; DE3533699A1; GB2181017B

Abstract

In a method for compensating drop-outs in the case of colour television signals taken from an information carrier, in which the colour television signal is split up 4, 5, 6 into chrominance 10 and luminance 7 signals, the chrominance and luminance signals are delayed in stores 13, 14 as a function of the presence of a drop-out and then recombined 21. The splitting up and recombination are both performed in digital form and the recombined signal is used as the compensated signal irrespective of the presence of a drop-out. Thus, no use is made of a main channel in which no such splitting occurs. The digital delay circuits can have a smaller bit width than corresponds to the analog/digital conversion of the colour television signal. <IMAGE>

Description

SPECIFICATION Method For Compensating Drop-outs in a Colour Television Signal The invention relates to a method for compensating drop-outs in a colour television signal.

When piaying back television signals obtained from an information carrier, particularly a magnetic tape, blemishes in the magnetizable coating of the magnetic tape make themselves disturbingly apparent in the form of drop-outs. It has long been known to detect such drop-outs by suitable circuits and to replace the missing circuits and to replace the missing signal portions by signal portions from preceding lines. In the case of colour television signals, it must be ensured that the chrominance signal with the correct colour carrier phase position is used. The colour television signal is therefore split up into its chrominance and luminance signals upstream of the compensation circuit and then, for the purpose of compensating, a drop-out when it appears, the luminance signal is delayed by one and the chrominance signal by two line periods.

These known methods were initially performed with analog circuits and subsequently with digital circuits. However, in the case of digital circuits for colour television signal error correction and which comprise circuits for compensating signal dropouts, a very high quantization precision of e.g. 9 bits is required in order to meet all demands made in professional television technology. However, this high bit width makes the digital circuits to be used much more expensive. In addition, in the known methods a main signal path is provided for the colour television signal when it is not subject to drop out so that the split up and recombined luminance and chrominance signals, which are only used in the case of drop-outs, are allocated to a second channel or signal path.

The problem of the present invention is to provide a method for compensating drop-outs in the case of colour television signals taken from an information carrier, which can be more economically performed.

Accordingly the invention provides a method for compensating drop-outs in a colour television signal taken from an information carrier, in which the colour television signal is split up into chrominance and luminance signals, and in which the chrominance and luminance signals are delayed as a function of the presence of a drop-out and are then recombined, the colour television signal being both split up and recombined in digital form and the recombined signal being used as the compensated signal irrespective of any drop out.

The invention further provides a method for compensating drop-outs in a colour television signal taken from an information carrier, in which the colour television signal is split up into chrominance and luminance signals, and in which the chrominance and luminance signals are delayed as a function of the presence of a drop-out and then recombined, the colour television signals being present in digital form with a predetermined bit width, the chrominance and luminance signals being transmitted by digital delay circuits with a lower bit width and/or in undelayed form with the predetermined bit width.

The inventive method has the advantage that the technical expenditure may be reduced compared with known methods. A further advantage of the invention is that for delaying the digital signals it is possible to use conventional 8 bit-based digital circuits.

An embodiment of the invention will now be described with reference to the accompanying drawings, in which: Fig. lisa block circuit diagram of an embodiment of the invention, and Fig. 2 is a block circuit diagram of the digital filter of Fig. 1.

A digital colour television signal from a magnetic tape recorder is supplied at 1 to the circuit shown in Fig. 1. The colour television signal is recorded on the magnetic tape in carrier frequency form and is demodulated and has undergone analog/digital conversion prior to being supplied to the circuit of Fig. 1. In order to be able to detect drop-outs, the carrier frequency signal is supplied at 2. With the aid of a known drop-out detector 3 switching signals are produced if the amplitude of the carrier frequency signal is below a predetermined value.

The digital colour television signal is supplied from input 1 to a signal splitting circuit comprising a digital low-pass filter 4, a delay circuit 5 which compensates the delay time of the signals in the digital low-pass filter 4, and a subtracting circuit 6.

The digital low-pass filter 4 and the delay circuit 5 are supplied with a clock signal C, which has roughly three times the colour carrier frequency.

The low frequency components of the digital colour television signal are present at the point 7 and essentially correspond to the luminance signal.

The higher frequency components, essentially the chrominance signal, is present at the point 10, i.e., at the output of the subtracting circuit 6.

If there is no drop-out, pulses are supplied to outputs 11, 12 of detector circuit 3, which are applied to the WE (write enable) inputs of two random-access memories 13 and 14. Randomaccess memories 13 and 14 are in each case controlled by a respective address counter 15 and 16, to which are supplied clock signals C and horizontal frequency pulses H. Random-access memory 13 has a capacity of one line, whilst random-access memory 14 has a capacity of two lines.

The pulses applied to the WE inputs ensure that the input/output gates I/O of the random-access memories 13, 14 each operates as an input during one part of each clock period as an output during the remaining part. Switches 17, 18 are controlled by means of inverting stages 19, 20 in such a way that signals are only supplied to the I/O gates of the random-access memories 13, 14 when the I/O gates are operating as inputs. During the remainder of each clock period, those signal values written in during the first part of the clock period are read out.

Thus, there is no effect on the luminance and chrominance signals, apart from this very small delay.

However, the chrominance signal must be inverted, for which purpose switch 24 is brought into the lower position by detector 3 when there is no drop-out. The two luminance and chrominance signals are then recombined in an adding circuit 21 and are available for further use at output 22.

Whereas in known drop-out compensating circuits a so-called main channel is provided, in which the colour television signal is not split up into chrominance and luminance signals, and a replacement signal is only used when drop-outs occur, in the above technique according to the invention the colour television signal which has been split up into its luminance and chrominance components and then recombined is still transmitted on even when there is no drop-out. No quality reduction occurs as a result of the signal splitting, because the 8-bit wide output signal of low-pass filter which is subtracted from the delayed input signal in circuit 6 is simply added again in circuit 21 to give a 9-bit wide output signal at 22 equivalent to the original signal an input 1.

If a drop-out occurs, the pulses supplied to the WE inputs of the random-access memories 13, 14 are stopped. Thus, no signals are written into the random-access memories 13, 14. Thus, the already stored signals are read out again after one cycle, i.e.

after one line in RAM 13 and after two lines in RAM 14, switches 17 and 18 remaining open.

The signal paths from input 1 across delay circuit 5, subtracting circuit 6, switch 24, inverter 25 and adder 21 are designed for a bit width of nine, whilst low-pass filter 4 and random-access memories 13, 14 only have a bit width of eight, which constitutes a significant simplification, because many of the commercially available components are designed for a bit width of eight. If there are no drop-outs, the signal is transmitted with a nine bit width because, as stated hereinbefore, the output signal of the digital low-pass filter is ineffective due to subtraction and subsequent addition.

With regards to its capacity of two lines, the random-access memory 14 is designed for the PAL and SECAM television systems. A capacity of one line is adequate for compensating drop-outs in NTSC colour television signals.

Fig. 2 shows a block circuit diagram of the digital low-pass filter 4. The digital signals at input 1 are successively passed with a bit width of 8 across eight D-registers 31 to 38. The D-registers are controlled with a clock signal, which corresponds to the sampling clock of the digital colour television signal, so that in each D-register the signal is delayed by one sample period. As is known, in transverse filters, the differently delayed signals are summated weighted coefficients of different magnitudes. In the present low-pass filter, the coefficients are symmetrical to a sample value with an average delay. Thus, prior to a multiplication by the particular coefficient, in each case two sample values can be added.

Thus, the undelayed signal and that delayed by eight sampling periods are supplied to an adding circuit 41, whilst the signal delayed by one sampling period and that delayed by seven sampling periods are supplied to an adding circuit 42. Adding circuit 43 is used for adding the signals delayed by two and six sampling periods. Adding circuit 44 adds the signals delayed by 3 and 5 sampling periods.

By means of D-registers 45,46,47 and 48, the output signals of adding circuits 41, 44 are supplied to multipliers 51,52, 53 and 54, from which they are read out with further D-registers 55, 56, 57 and 58.

The further adding circuits 60, 61 and 62, which are interconnected with D-registers 63 and 64, are used for adding the weighted sample values.

However, for the completion of the output signal, the sample value of the average delay is still missing, i.e. in the case of the circuit according to Fig. 2 the signal at point 49. Due to the preferred pairwise addition of the other sample values, there are already delays in their signal paths, so that in the circuit according to Fig. 2 the signal is not taken from point 49 but instead the signal delayed by 3 sample periods compared therewith is taken at the point 50. This is supplied to a multiplier 65, by which it is multiplied by a coefficient and passed from a Dregister 67, 66 to an adder 68, to which is supplied across a D-register 67 the weighted sum of the other sample values. The output of the circuit of Fig. 2 corresponds to the point 7 of the circuit of Fig. 1. The following coefficients for the multipliers given in the table have proved particularly advantageous in a 625 line PAL colour television signal; Multiplier Coefficient 51 0.04476444 52 0.295265535 53 0.3131685 54 0.307442 65 0.629889 During the starting phase of a magnetic tape recorder, it may happen that sync pulses from the tape occur due to the lack of locking to the reference sync signal, and consequently incorrectly synchronize the following equipment. In the circuit of Fig. 1, a black limiter 26 is positioned in the path of the luminance signal and has a threshold at approximately 10% below the blanking value. This prevents so-called sync puncturing.

Claims

1.A A method for compensating drop-outs in a colour television signal taken from an information carrier, in which the colour television signal is split up into chrominance and luminance signals, and in which the chrominance and luminance signals are delayed as a function of the presence of a drop-out and are then recombined, the colour television signal being both split up and recombined in digital form and the recombined signal being used as the compensated signal irrespective of any drop out.

2. A method for compensating drop-outs in a colour television signal taken from an information carrier, in which the colour television signal is split up into chrominance and luminance signals, and in which the chrominance and luminance signals are delayed as a function of the presence of a drop-out and then recombined, the colour television signals being present in digital form with a predetermined bit width, the chrominance and luminance signals being transmitted by digital delay circuits with a lower bit width and/or in undelayed form with the predetermined bit width.

3. A method according to claim 2, wherein the bit width of the signals transmitted by the digital delay circuits is smaller by 1 than the predetermined bit width.

4. A method according to claim 3, wherein the predetermined bit width is 9.

5. A circuit for performing the method according to claim 2, including a digital low-pass filter with a bit width of eight, a delay circuit and a subtracting circuit each with a bit width of nine, the filter, delay circuit and subtracting circuit being arranged to split up the colour television signal present as a 9 bit signal, and with a plurality of line memories with a bit width of eight.

6. A circuit for performing the method according to claim 1, comprising a digital low-pass filter, a digital delay circuit and a digital subtracting circuit for splitting up the digital colour television signals.

7. A circuit according to claim 6, wherein the digital low-pass filter is a transverse filter in which several successive sample values are weighted with different coefficients and summed.

8. A circuit according to claim 7, wherein the coefficient for an average sample value is 0.629889, whilst the coefficients for the further sample values with the following time intervals with respect to the average sample value have the following values: -4 scanning periods 0.04476444 -3 scanning periods 0.29526535 -2 scanning periods 0.3131685 -1 scanning periods 0.307442 +1 scanning periods 0.307442 +2 scanning periods 0.3131685 +3 scanning periods 0.29526535 +4 scanning periods 0.04476444

9. A circuit according to claim 5, further including a black limiter in the path of the luminance signal.

10. A circuit according to claim 9, wherein the black limiter has a threshold at 10% below the blanking value.

11. A method for compensating drop-outs substantially as described herein with reference to the accompanying drawings.