DE3509968A1 - Automatic noise gating device for audio-frequency signals - Google Patents

Abstract

When amplitude-modulated signals are received, pulse-shaped interference of different types becomes increasingly noticeable, particularly in the case of long-distance reception, which can completely prevent the reception of such transmissions. A noise gating device is therefore required which constantly operates well independently of the input signal level and the type of noise pulses. For this purpose, the mean signal level is kept constant by picking up the actual value of the controlled variable after the gating out of the noise signal and the gating pulses generated by a dual comparator being passed via a high-pass filter before they affect the gating out. To enhance the effectiveness of the noise gating, particularly in the case of higher noise pulse frequencies, the actual value of the controlled variable can be conducted via a band-pass filter before it reaches the comparator.

Description

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Automatic noise blanker for low frequency signals

The invention relates to an automatically operating noise blanker for low-frequency signals with simultaneous Keeping the average signal level constant, especially for use in sound receiving systems that use Process amplitude-modulated signals.

In sound reception systems, when amplitude-modulated signals are received, pulse-shaped interference occurs different amplitude, duration and repetition frequency. It is especially with remote reception required, which is then in the case of weak reception signals to eliminate increasingly noticeable interference, as they even temporarily prevent the reception of such transmissions can make completely impossible.

It is known to meet these requirements the To limit interfering signals to a certain value. For this purpose, a running averaging of the respective Total signal level is carried out and all levels above this mean value are blocked. (Grundig, Fürth, service and operating documents for the 3400 Professional satellite) With this arrangement, short glitches are well limited (not blanked), longer and before especially periodically recurring interference pulses due to the then higher mean value, but at all not.

It is also known to blank the signal for the period of the interference pulse, i.e. the signal transmission to interrupt what without noticeable loss of information is possible. Here, the overall signal is routed over a signal delay path before it is sent to the Blanking stage arrives while an instantaneously controlled double comparator sends the signal to

Henrik Haverkamp

Ringstrasse 37

Biokznbach . * \, "\ * ''

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Blanking supplies. The signal delay ensures that the part of the Interfering pulse is blanked, which is still below the response threshold (of the double comparator). (CQDL magazine No. 9, year 1983, H.Tiefenthaler, P.428 to 430)

The disadvantage here is that the response threshold of the double comparator corresponds to the respective Signal level must be adjusted, which requires additional attention when using a receiving system requires.

The invention is based on the object of developing a noise blanker for low-frequency signals, which works consistently well regardless of the incoming signal level and the type of interference pulses.

This object is achieved according to the invention in that the one for keeping the mean signal level constant The actual value of the controlled variable to be used is taken after the interfering signal has been blanked and in that the blanking pulses coming from a double comparator are passed over a high-pass filter are used before they are used to control the blanking level.

To increase the effectiveness of the noise blanking, especially in the case of glitches with higher repetition frequencies in a further embodiment of the invention, the regulation with respect to such interference pulses made insensitive by the blanking stage incoming actual value of the controlled variable is passed over a bandpass filter before it reaches the comparator.

The advantages achieved by the invention exist for

One is that by keeping the mean signal level constant, the best response sensitivity is always achieved for the blanking of interfering signals is achieved. On the other hand, they lie in the fact that the Signal level only a function of the - interference-free useful signal level is, which is why the amplitude, duration and repetition frequency of the interference pulses are almost irrelevant for the scheme are. This makes it possible to also blank out interfering signals that are only a few dB above the Peak values of the useful signal lie and the useful signal at repetition frequencies of several 10 Hz can make completely unrecognizable.

An embodiment of the invention is shown in the drawing and will be described in more detail below described.

The low-frequency input voltage comes first from E to an amplifier (1) which can be controlled in its gain and which is also the actuator for the Represents keeping the mean signal level constant.

Then the signal branches: On the one hand, it reaches the blanking stage via a signal delay section (4) (9), a switch that works as crack-free as possible, which consists of a controllable resistor with sufficiently slowly changing switch-on and switch-off edges can exist. On the other hand, the signal arrives to a double comparator (5) which, when a suitably predetermined threshold is exceeded, both for the negative as well as for the positive part of the alternating voltage for the duration of the exceedance supplies a positive output signal via a high-pass filter (7) or a monostable multivibrator (12) and a retriggerable monostable multivibrator (8) takes place Control of the blanking level (9). That of interfering signals

Henrik Haverkamp Ringstrasse 37 Bickenbach

The freed signal is fed to output A via a low-pass filter (11) and, since it represents the actual value of the controlled variable, via a bandpass filter (10) and a half-wave rectifier (6) to the input of a comparator (3), the output of which is given the gain of the controllable amplifier (1) is determined in the usual way via a low-pass filter (2). In order to improve the sensitivity to interference signals with repetition frequencies of 50 to 100 Hz, a suitable bandpass filter (10), which only allows the useful signal spectrum to pass, is interposed between the blanking stage (9) and the half-wave rectifier (6).
In the event of a sudden increase in the level of the input signal, the blanking stage (9) would be activated without further measures and thus block the signal: the control loop would be open and the gain of (1) would be increased to the limit. This can be done by the high-pass filter following the double comparator (5)

(7) avoid. By appropriately dimensioning this stage, it is possible to ensure that only individual blanking pulses with the practically occurring time periods and repetition frequencies for Blanking stage (9) can reach. Accordingly, a non-retriggerable monostable multivibrator can be used Use (12). The retriggerable monostable multivibrator (8), however, causes a minimum blanking pulse duration.

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Claims (3)

Henrik Haverkamp Ringstrasse 37 Blukonbach. **,. **. * '* · ■> claims 1. Automatic noise blanker for low frequency signals while keeping the average signal level constant, especially for use in sound receiving systems that use process amplitude-modulated signals, characterized in that the one to be used to keep the mean signal level constant The actual value of the controlled variable is taken after the interfering signal has been blanked (9) and characterized in that the blanking pulses coming from a double comparator (5) via a High-pass filter (7) are performed before they are used to control the blanking stage (9). 2. Automatically working noise blanker after Proverb 1, " characterized in that the actual value of the rule ^ size is passed over a bandpass filter (10) before it reaches the comparator. 3. Automatically operating noise blanker according to claim 1, characterized in that the blanking pulses coming from a double comparator (5) via a non-retriggerable monostable multivibrator (12) are performed before they are used to control the blanking stage (9) can be used.