GB2071958A

GB2071958A - Filter arrangement

Info

Publication number: GB2071958A
Application number: GB8005546A
Authority: GB
Original assignee: Plessey Co Ltd
Current assignee: Plessey Co Ltd
Priority date: 1980-02-19
Filing date: 1980-02-19
Publication date: 1981-09-23

Abstract

The invention described relates to an arrangement for controlling a number of adjustable frequency response filters which are used to filter respective signals which have a predetermined frequency relationship between them. A phase comparator 4 is used to phase compare a first of the signals with a second signal which is derived from the first via a filter 5 which forms one of the number of filters. An integrator 9 integrates phase compared output signals from the phase comparator 4 and provides a control signal which is fed to control the frequency response of each of the filters 5, 11, 13 and 15. The invention is particularly useful for controlling the filters in a television receiver. A band-pass filter 5 centred at 6 MHz separates the sound carrier, a band-pass filter 13 centred at 4.43 MHz separates the chroma, and a band-stop filter centred at 4.43 MHz separates the luminance. <IMAGE>

Description

SPECIFICATION Filter arrangement This invention relates to a filter arrangement which includes a plurality of controilable filters. The invention is particularly although not exclusively applicable to a colour television receiver in which a plurality of filters are provided to separate sound, chroma and luminance signals from a composite detected signal.

According to this invention there is provided a filter arrangement comprising a plurality of adjustable frequency response filters for filtering respective signals which have a predetermined frequency relationship therebetween, a phase comparator for phase comparing a first of the signals with a second signal derived from the first signal via a first adjustable filter of the plurality; an integrator for integrating phase compared output signals fed from the phase comparator to provide a control signal which is fed to control the frequency response of each filter of the plurality.

The filter arrangement may be a television receiver filter arrangement and in one embodiment is a colour television receiver filter arrangement for providing sound, luminance and chroma signals from a common input signal.

The common input signal may be a composite detected video signal including sound, luminance and chroma components.

The first adjustable filter of the plurality may be adjustable by the control signals so that the phase comparator provides an output signal which is the detected sound signal, further filters of the plurality providing respectively the detected luminance and chroma signals.

The filter arrangement may conveniently be formed as an integrated circuit.

An exemplary embodiment of the invention will be described with reference to the accompanying single figure drawing which shows a filter arrangement in accordance with the invention.

Referring to the drawing there is shown a filter arrangement which forms part of a colour television receiver for the separation of detected sound chroma and luminance signals. An input terminal 1 of the arrangement receives a composite video signal which is a detected signal containing sound, luminance and chroma component signals. This composite video signal is the first detected signal available after the IF stages of the receiver.

The composite video signal received at the termin allies fed via a preset band-pass filter 2 and a signal level limiter 3 to one input of a comparator 4 provided by a quadrature phase detector. A second input for the phase comparator 4 is obtained from the output signal fed from the limiter 3 via an adjustable frequency response band-pass filter 5 which may be fine tuned by means of a control signal fed to a control input 6 of the filter. In the present example the filter 5 has a typical centre frequency of 6 mhz.

The phase comparator 4 provides an output signal in dependence upon the relative phase of the two input signals and this output signal from the phase comparator is fed to an output terminal 7. Output signals from the phase comparator 4 are also fed via a resistor 8 to a high gain comparator 9 across which is connected a capacitor 10. The resistor 8, comparator 9 and capacitor 10 form an integrator in which input signals fed thereto are integrated and compared with a preset reference level, in this case zero volts.

Integrated output signals obtained from the high gain comparator 9 are fed as a fine tuning control signal to the control input 6 of the filter 5. The control signal fed to the control input 6 of the filter 5 adjusts the filter 5 until a quadrature phase shift exists through the filter. At this point the control loop for the filter 5 formed by the phase comparator 4 and the integrator 8,9,10 will reach a steady state condition and output signals presented at the output terminal 7 will be filtered detected sound signals.

In order to separate out the chroma and luminance signals the composite video signal is also fed through an adjustable frequency response band stop filter 11 centred on 6 mhz. The filter 11 is fine tuned by means of a fine tuning control signal applied at a control input 12. Output signals from the filter 11 are fed via an adjustable frequency response filter 13to an output terminal 14 and via a further adjustable frequency response filter 15 to an output terminal 16.

The filter 13 is a band-pass filter centred on 4.43 mhz and is fine tuned by means of a fine tune control signal applied to a control input 17 whilst the filter 15 is a band stop filter centred on 4.43 mhz and is fine tuned by means of a fine tune control signal applied to a control input 18.

The output terminal 14 provides the filtered chroma signals whilst the output terminal 16 provides filtered luminance signals. The filters 11, 13 and 15 are fine tuned by means of the same fine tuning control signal that is applied to the control output of the comparator 9. This fine tune control signal is applied to the fine tune control inputs 12, 17 and 18 of the filters 11,13, and 15 respectively.

The invention therefore provides a simple filter arrangement in which a number of matched controllable active filters may be used to filter a composite video signal to provide separate sound luminance and chroma signal and in which the filters are controlled in synchronism with one another by means of a common control signal. Separate manual adjustment of the filters to achieve the desired response is not necessary. Any incorrect setting of the filters is automatically compensated by means of the control signals.

Further the invention may easily be incorporated in a single integrated circuit package.

Although the invention has been described particularly with reference to filtering in a colour television receiver this is not essential. The invention may be utilized in any filter arrangement in which a plurality of controllable filters are required to filter respective signals which have a predetermined frequency relationship between one another.

Claims

1. A filter arrangement comprising a plurality of adjustable frequency response filters for filtering respective signals which have a predetermined frequency relationship therebetween, a phase comparator for phase comparing a first of the signals with a second signal derived from the first signal via a first adjustable filter of the plurality; an integrator for integrating phase compared output signals fed from the phase comparator to provide a control signal which is fed to control the frequency response of each filter of the plurality.

2. A filter arrangement as claimed in claim 1 in which the filter arrangement is a television receiver filter arrangement.

3. A filter arrangement as claimed in claim 2 in the form of a colour television receiver filter arrangementfor providing sound, luminance and chroma signals from a common input signal.

4. A filter arrangement as claimed in claim 3 in which the common input signal is a composite detected video signal including sound, luminance and chroma components.

5. A filter arrangement as claimed in claim 3 or 4 in which the first adjustable filter of the plurality is adjustable by the control signal so that the phase comparator provides an output signal which is the detected sound signal, further filters of the plurality providing respectively the detected luminance and chroma signals.

6. Afilter arrangement as claimed in any preceding claim and formed as an integrated circuit.

7. Afilter arrangement substantially as herein described with reference and as illustrated in the drawing.