GB1586875A - Reducing noise in the playback of recorded video signals - Google Patents

Description

(54) REDUCING NOISE IN THE PLAYBACK OF RECORDED VIDEO SIGNALS (71) We, VEB ELEKTROTECHNIK EISENACEI, KOMBINAT STERN-RADIO BERLIN, of Alstadtstrasse 27, 59 Eisenach, German Democratic Republic, a Corporation organised and existing under the laws of the German Democratic Republic, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to a switching arrangement for noise reduction during playback of recorded video signals, in video transmission or for other signals.

Audio noise reduction techniques are known but these are mostly adapted to the acoustic physiology of the human ear and are not suitable for use in video signal technology.

An arrangement is known using a controllable low pass filter in which the cut-off frequency is shifted downward when higher frequencies are absent. This suppresses high frequency noise.

According to this invention there is provided an arrangement for effecting noise reduction in video signals comprising feeding the video signal through switch means to either a wide band pass filter to pass the complete band-width of the signal or to a narrow band pass filter wherein the pass band centre frequency is variable and set through first control means according to the instantaneous input signal frequency, the switch means being operable through further control means to connect the wide band pass filter when the rate of input signal frequency change exceeds a pre-set value, and the narrow band pass filter when the rate of input frequency change is below said value.

In the presence of a frequency modulated input signal. the arrangement uses preferably a limiting amplifier and a demodulator connected in series and feeding a low pass filter connected to a control input of the narrow band pass filter to effect control of the centre frequency of same in accordance with low frequency changes of the input signal, the pass band centre frequency of the filter arrangement is displaced in accordance with the slow displacement of the carrier, without a greater band width being required.

A comparator may be connected to the demodulator output directly and through a delay line so that, in the case of fast frequency changes of the input signal, a signal occurs at the comparator output, which after full-wave rectification switches in the further filter to produce greater band widths.

A half wave rectifier is not sufficient as negative signals at the comparator output would be clipped, without affecting the necessary control of the filter for greater band widths.

In accordance with the invention, the signal processing arrangement is comprised of a narrow band pass filter, which in the case of slow frequency changes in the input signal is controlled proportionally as far as deviation of the pass band frequency is concerned, and one or more wider band pass filters connected for fast frequency changes.

An embodiment in accordance with the invention is shown by way of example in the accompanying drawing showing a block schematic for reduction of noise in a frequency modulated video signal.

In the drawing a signal input l is connected to a filter unit 2 through a delay line 1 and a switch 141. The filter unit has a band pass filter 3, in which the pass band centre frequency can be shifted through a control input, and a fixed band pass filter 4. The outputs of the two filters are connected through a second switch 142 to the output 0.

Either one or other of filters 3 or 4 is selectable according to the state of switches 141 and 142. The input I is further connected to a limiting amplifier 6 and demodulator 7 the output of which controls filter 3 through a low pass filter 8. The demodulated signal from 7 is fed to one input 10 of a comparator 9 directly and to the other input 11 through a signal delay line 12. The output from comparator 9 is subject to full wave rectification at 13 and the output signal is used to effect control of switches 141 and 142.

In the case of slow frequency changes of the recorded frequency modulated video signal to be processed fed to input I from a playback apparatus, such as occur with slow half-tone changes in the picture, the pass band centre frequency of the filter 3 is shifted by the signal from low pass filter 8 in accordance with half-tone change.

The limited band width of the filter 3 following the changes of the limited input frequency range effects a reduction in unwanted noise, so that noise in the picture elements with less luminance change is effectively suppressed where normally it has a particu- larly disturbing effect.

In the case of rapid frequency changes, which occur, for example, in high contrast black-to-white transitions, the full band width is required in the signal path and this is achieved by switching in filter 4. The comparator 9 produces a signal when such changes by virtue of the delay line 12 and when a sufficiently fast transition occurs the changeover to the full band width through switches 14 takes place. Thtis means less noise reduction, but this is not so disturbing with such fast black-to-white transitions.

The advantages of this arrangement is the small transmission band widths achievable for the case of the steady or slow changing carrier frequency, which is found during slow partial tonal changes in the picture.

Control of the band width of the pass band of filter 3 can be effected through an automatic control system according to the signal frequencies present at any time.

i WHAT WE CLATM IS:- 1. An arrangement for effecting noise reduction in video signals comprising feeding the video signal through switch means to either a wide band pass filter to pass the complete band-width of the signal or to a narrow band pass filter wherein the pass band centre frequency is variable and set through first control means according to the instantaneous input signal frequency, the switch means being operable through further control means to connect the wide band pass filter when the rate of input signal frequency change exceeds a pre-set value, and the narrow band pass filter when the rate of input frequency change is below said value.

2. An arrangement in accordance with Claim 1, wherein the input signal is fed to the filters through a delay means and directly to the first control means and to the further second control means.

3. An arrangement in accordance with Claim 2, wherein the first and further control means are fed through a limiter amplifier and a demodulator, the further control means comprising a comparator and a fullwave rectifier operating the switch means.

4. An arrangement in accordance with Claim 3, wherein one input of the comparator is connected directly to the output of the demodulator and the other input of the comparator is connected to the output of the demodulator through a second signal delay means.

5. An arrangement in accordance with any one of Claims 2 to 4, wherein the first control means comprises the limiter amplifier, the demodulator and a low pass filter through which the pass band of the narrow pass band filter is set.

6. An arrangement according to any preceding claim, wherein for frequency modulated signals the pass band of the narrow band pass filter is controlled, for narrow band width signals, by shifting same to follow the input signal.

7. An arrangement for effecting noise reduction in signals substantially as herein described with reference to and as shown in the accompanying drawing.

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (7)

**WARNING** start of CLMS field may overlap end of DESC **. the recorded frequency modulated video signal to be processed fed to input I from a playback apparatus, such as occur with slow half-tone changes in the picture, the pass band centre frequency of the filter 3 is shifted by the signal from low pass filter 8 in accordance with half-tone change. The limited band width of the filter 3 following the changes of the limited input frequency range effects a reduction in unwanted noise, so that noise in the picture elements with less luminance change is effectively suppressed where normally it has a particu- larly disturbing effect. In the case of rapid frequency changes, which occur, for example, in high contrast black-to-white transitions, the full band width is required in the signal path and this is achieved by switching in filter 4. The comparator 9 produces a signal when such changes by virtue of the delay line 12 and when a sufficiently fast transition occurs the changeover to the full band width through switches 14 takes place. Thtis means less noise reduction, but this is not so disturbing with such fast black-to-white transitions. The advantages of this arrangement is the small transmission band widths achievable for the case of the steady or slow changing carrier frequency, which is found during slow partial tonal changes in the picture. Control of the band width of the pass band of filter 3 can be effected through an automatic control system according to the signal frequencies present at any time. i WHAT WE CLATM IS:-

1. An arrangement for effecting noise reduction in video signals comprising feeding the video signal through switch means to either a wide band pass filter to pass the complete band-width of the signal or to a narrow band pass filter wherein the pass band centre frequency is variable and set through first control means according to the instantaneous input signal frequency, the switch means being operable through further control means to connect the wide band pass filter when the rate of input signal frequency change exceeds a pre-set value, and the narrow band pass filter when the rate of input frequency change is below said value.

2. An arrangement in accordance with Claim 1, wherein the input signal is fed to the filters through a delay means and directly to the first control means and to the further second control means.

3. An arrangement in accordance with Claim 2, wherein the first and further control means are fed through a limiter amplifier and a demodulator, the further control means comprising a comparator and a fullwave rectifier operating the switch means.

4. An arrangement in accordance with Claim 3, wherein one input of the comparator is connected directly to the output of the demodulator and the other input of the comparator is connected to the output of the demodulator through a second signal delay means.

5. An arrangement in accordance with any one of Claims 2 to 4, wherein the first control means comprises the limiter amplifier, the demodulator and a low pass filter through which the pass band of the narrow pass band filter is set.

6. An arrangement according to any preceding claim, wherein for frequency modulated signals the pass band of the narrow band pass filter is controlled, for narrow band width signals, by shifting same to follow the input signal.

7. An arrangement for effecting noise reduction in signals substantially as herein described with reference to and as shown in the accompanying drawing.