GB2239371A

GB2239371A - PAL signal chrominance decoder

Info

Publication number: GB2239371A
Application number: GB9020808A
Authority: GB
Inventors: Heinz Hess
Original assignee: BTS Broadcast Television Systems GmbH
Current assignee: Philips GmbH
Priority date: 1989-09-25
Filing date: 1990-09-25
Publication date: 1991-06-26
Anticipated expiration: 2010-09-25
Also published as: GB9020808D0; DE3931903A1; DE3931903C2; BE1004577A6; GB2239371B

Abstract

In a method and a circuit for separating a PAL signal into a luminance signal and a chrominance signal by line comb filtering, the chrominance signal is derived by a standard PAL decoder 8, 9 from three successive lines if there is no detected 7 colour change between successive lines and is derived by a simple PAL decoder 11, 12 from only a single line if there is such a colour change. <IMAGE>

Description

W 1 & J1 ( ^ METHOD AND CIRCUIT FOR PAL SIGNAL SEPARATION :D::3 7 -1- This

invention relates to a method for separating a PAL signal into a luminance signai and a chrominance signal by line comb filtering.

The processing of pronounced colour jumps between individual lines (e.g. in the colour fading in of characters) leads to problems in a comb filter decoder. In addition to colour blurring, flutter problems occur (6.25 Hz), particularly if there is demodulation preceded by signal separation according to a standard PAL method.

In order to avoid these problems a method is known from GB 2 169 772 in which PAL colour signals derived from successive line delay elements are multiply added together and subtracted from one another prior to the actual demodulation, the processing differing in accordance with the presence of colour transitions under the control of a detector signal. However, this method is relatively complicated.

In addition, EP 0 217 649 A2 discloses a circuit for the separation of a digital NTSC or PAL signal into luminance and chrominance signals, switching taking place as a function of a switching signal influenced by colour changes between the comb or band pass filtered chrominance signal. However, this suffers from the disadvantage that in the PAL operating mode a signal separation operation must follow, 5 which means extra signal delay expenditure.

The object of the present invention is to reduce or remove the aforementioned colour problems in connection with the comb filtering of a PAL signal in a simple and reliable manner.

Accordingly, the present invention provides a method for separating a PAL signal into a luminance signal and a chrominance signal by line comb filtering, in which the chrominance signal is derived by a standard PAL decoder from three successive lines if there is no colour change between successive lines and is derived by a simple PAL decoder from only a single line if there is a colour change between successive lines.

The invention has the advantage that by combining standard PAL decoding with simple PAL decoding in the case of colour changes between successive lines, it is possible to obtain improved results.

The invention has proved to be particularly advantageous if the decoder selection is effected 1 1 by a control signal obtained by the non-additive mixing of a first signal derived by band pass filtering and rectifying the comb-filtered luminance signal and a second signal derived by band-pass filtering and rectifying the difference between the comb-filtered luminance signal and the second of the three successive lines subject to standard PAL decoding.

An embodiment of the invention will now be particularly described with reference to the accompanying drawing, wherein:

Figure is a block diagram of a circuit for performing 15 an embodiment of the invention, Figure 2 is a preferred form of the signal separation circuit in the standard PAL decoder of Figure 1, and Figure 3 is a block diagram of a circuit for producing the control signal used in Figure 1.

Referring to Figure 1, a colour television signal according to the PAL standard is supplied at a terminal 1 to two one-line delay networks 2 and 3. Thus, there are available at points 4, 5 and 6 respectively a non-delayed, a one-line delayed, and a two-line delayed input signal. These signals are supplied both to a control signal production circuit 7 and to a circuit 8 with comb filter action for separating the chrominance signal in the form of the two carrier frequency colour difference signals U and V. The signals U and V are supplied to a standard PAL demodulator 9, at whose outputs are available the video frequency colour difference signals (R-Y) and (B-Y). The one line delayed input signal at point 5 is also supplied across a band pass filter 11 to a simple PAL demodulator 12, at whose outputs there are also available the video frequency colour difference signals (R-Y) and (B-Y).

The signal separation circuit 8 and the standard PAL demodulator 9 together form a modofied standard PAL decoder, whilst the band pass filter 11 and the simple PAL demodulator 12 together form the known simple PAL decoder.These two decoders are described for example in the book "Farbfernsehen Aufgabenstellung und 18sungswege eines optisch-elektronischen Cbertragungsproblems" by Dr. Helmut Schdnfelder, Liebig-Verlag 1965, pp. 93 to 98.

The output signals of the two decoders are now passed to respective inputs of a switch 13 which is under the control of the control signal produced in the circuit 7. As long as there is no colour change between successive lines, the switch is in the indicated position. that is the output signals of the standard PAL decoder 8, 9 are transferred to the 1 output terminals 14 of the switch 13. If a colour jump occurs the circuit 7 supplies a signal, which switches the switch 13, so that the output signals of the simple PAL decoder 11, 12 are transferred to the output terminals 14. Thus, at the terminals 14 there is available either a colour signal decoded according to the standard PAL method, if the input signal corresponds to uniform coloured surfaces, or a colour signal decoded according to the simple PAL method, if colour jumps occur between successive lines.

Figure 2 shows a particularly advantageous circuit for separating the chrominance signal in the form of the comb-filtered, colour carrier frequency colour difference signals U and V. Between the input 4 and the output 6 of the two line delay th re are two parallel-connected RC networks 21, 22 and 23, 24 acting as phase shifters. The junction point 26 or 27 of each phase shifter is connected to one input of a respective subtraction circuit 28 or 29. The other input of the subtraction circuits 28 and 29 is connected to the point 5 at the output of the one line delay element 2. The carrier frequency, comb-filtered colour difference signals U and V are available at the outputs of the stages 28 and 29.

The circuit for producing the control signal shown 1 I-:

in Figure 3 (corresponding to box 7 in Figure 1) is also connected to the points 4, 5 and 6 of the undelayed and delayed input signals. By addition in the adder 31 of the undelayed signal from point 4 and the two-line delayed signal from point 6, the luminance signalis derived which is available at the junction point 32. This signal, which still contains residual chrominance signal components, is now supplied to the positive input of a sub- traction circuit 33, to whose negative input there is applied from point 5 the one-line delayed input signal. A signal similar to a chrominance signal is available at the output of the circuit 33. The luminance signal from the point 32 is also supplied across a band pass filter 34, a limiter 35 and a rectifier 36 to one input of a non-additive mixing (NAM) circuit 37, to whose other input there is applied the signal from the output of the circuit 33 across a further band pass filter 38, a further limiter 39 and a further rectifier 41.

Thus, in the two branches, the respective signals are first frequency filtered and then amplitude limited, so that only the components of the colour signal in the colour carrier frequency range are used for producing the control signal. These two signals are now interconnect.ed in the NAM circuit 37, so that it is always possible to remove a signal from the output of the circuit 37 if residual spurious signals are detected in the two signal paths. This output signal is passed across a low pass filter 42 to the switching input of the switch 13 in Figure 1 for switching the video frequency colour difference 5 signals (R-Y) and (B-Y).

4 I- - 1

Claims

1. A method for separating a PAL signal into a luminance signal and a chrominance signal by line comb filtering, in which the chrominance signal is derived by a standard PAL decoder from three successive lines if there is no colour change between successive lines and is derived by a simple PAL decoder from only a single line if there is a colour change between successive lines.

2. A method according to claim 1, wherein the selection between the simple and standard PAL decoders is effected by a control signal obtained by the non-additive mixing of a first signal derived by band pass filtering and rectifying the comb-filtered luminance signal and a second signal derived by band pass filtering and rectifying the difference between the comb-filtered luminance signal and the second of the three successive lines subject to standard PAL decoding.

3. A citcuit for performing the method according to claim 1, comprising two parallel-connected RC networks connected between the input and the output of two one-line delay elements, the junction point of each RC network being connected to one input of a respective subtraction circuit whose other input is connected to the output of the first C 4 0 C_ h one-line delay element, whereby carrier frequency colour difference signals are available at the output of the two subtraction circuits, and a first demodulator for converting the difference signals 5 into video frequency colour difference signals (R-Y) and (B-Y), the output of the first one-line delay element being connected across a band pass filter to a further demodulator and that the outputs of the two demodulators being connected to a switching circuit which is switchable by a control signal.

4. A circuit according to claim 3 for performing the method according to claim 2, wherein the line comb filtered luminance signal is connected both across a band pass filter and a rectifier to one input of a non- additive mixing circuit and to the positive input of a subtraction circuit, the negative input of the latter being connected to receive the delayed PAL signal at the output of the first one-line delay element and the output thereof being connected across a further band pass filter and a further rectifier to the other input of the non-additive mixing circuit at whose output the control signal is derived across a low pass filter.

5. A circuit according to claim 4, wherein a respective limiter amplifier is located between each band filter and the respective rectifier.

-1 1 - 10

6. A method as claimed in claim 1,substantially as hereinbefore described with reference to the accompanying drawings.

1 Published 1991 at The Patent Office. State House. 66171 High Holborn, London WC I R 41P. Further coplesrnay be obtained from Sales Branch. Unit 6. Nine Mile Point CwmIelinfach, Cross Keys. Newport. NPI 7HZ. Printed by Multfplex techniques ltd, St Mary Cray. Kent