KR830002420B1 - How to reduce adjacent channel interference - Google Patents

Abstract

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Description

How to reduce adjacent channel interference

1 is a block diagram of an exemplary receiver configured in accordance with the present invention.

2 is a schematic block diagram of a switching arrangement for switching low frequency filters to switch to upper and lower sideband channels to provide improved interference performance.

3 is a sketch showing the deep angles seen in the spectral display.

The present invention is directed to an improved method of allowing a listener to directionally select the surroundings in a radio device to reduce adjacent channel interference.

This ability is called the directional attention seletivity effect and uses this term herein. And it is defined as an effect that allows a listener with both ears to distinguish words from two different talkers, thereby giving one more attention to one of these talkers. This selectivity is also improved by measuring adjacent channel coherence levels above and below the desired channel and automatically changing the selectivity characteristics of the receiver according to the measured interference.

The device can be used not only to receive amplitude modulated (AM) stereo signals, but also to improve the performance of monophonic devices.

The present invention can be used in a wide range, but is particularly suitable for attenuating adjacent channel interference for AM monophonic and stereo devices and will be specifically described herein.

Several methods have been developed in the past to reduce the effects of adjacent channel interference, including various selective filters, either passive or automatic, and audible or intermediate frequency (IF) notch filters that are manually adjusted to sensitively identify interference.

In addition, there have been many types of devices developed that automatically convert the selectivity of the receiver to interference. The selectivity can be changed symmetrically or asymmetrically. Such devices are known. The conversion of selectivity is provided at the same intermediate frequency (IF) as in the sensing device.

It is an object of the present invention to reduce interference while maintaining a relatively desirable frequency response as a cheap autonomous device.

Another object is to provide a method for reducing adjacent channel interference, which is particularly suitable for AM stereo reception.

The primary object of the present invention is to allow listeners of AM monophonic and stereo signals to take advantage of the directional attention selection effect.

The present invention can be used to receive AM radio waves and is particularly suitable for receiving AM stereo radio waves. For example, the present invention provides a method for measuring the interference level received in an upper sideband stereo channel resulting from (a) interfering with a channel adjacent to the upper sideband of the desired channel, and (b) a channel adjacent to the lower sideband of the desired channel. Measure the interference level received in the lower sideband stereo channel resulting from interference, and (c) (a) or (b) the audible frequency response of the stereo channel towards the greater interference level when the stronger interference level is above the predetermined level; Can be used to automatically attenuate the adjacent channel interference in the reception of an independent sideband (ISB) type of AM stereo modulation wave.

In many applications of the present invention, it is necessary to attenuate both the desired upper and lower channels by a larger coefficient when operating the device without practical interference, but more attenuation for adjacent channels that are avoided from higher interference levels. It is preferable to provide.

The present invention can be used to improve the performance of the monophonic receiver by allowing the listener to utilize the directional attention selection effect. One method to improve the adjacent channel interference of single-rate dual-sideband signal receivers is to isolate and demodulate both the upper and lower sidebands of the desired signal and then to generate the appropriate audible circuit components to drive the separated transducers. Supplying is included. The transducer may be a stereo loudspeaker installed at intervals of 4-6 feet. It is also possible to practice the invention using stereo headphones as a transducer.

There are many ways to separate and demodulate the upper and lower sideband channels, including the filter device. Such techniques are known to those skilled in the art. The device can be used with a full carrier or dual-sideband reduced carrier, i.e. a wave whose full carrier or carrier level is less than the peak where the sideband amplitudes are summed.

The monophonic application of the present invention can also use a device that measures the interference level in the channel adjacent to the upper and lower sideband components of the desired signal and reduces the audible frequency response of the channel avoided from the larger interference level. As such, the listener will not only enjoy the improved effect of reduced interference caused by the directional attention selection effect, but also the improved selectivity of the device.

For stereo sound and monophonic reception, the reduction in audible frequency response applies only when the worst sideband channel interference is of sufficient amplitude to cause problems with listening. In general, if the power level of the interference is less than 1/10% of the desired sideband power level, there is no need to reduce the audible frequency response. Typically, the response of the channel avoided from the stronger interference level is reduced by about one third of the total response of the channel. For example, in medium stereo, the 6kHz audible response reception is reduced to 2KHz. In monophonic communication applications, the response is reduced to 3kHz-1kHz. Sidebands that are damaged from smaller interference are reduced to a lesser extent, and in some applications of the present invention, the weaker sidebands have no change in frequency response.

For better understanding of the present invention, reference may be made to what is described below in conjunction with the accompanying drawings.

1 is a block diagram of a receiver embodying the present invention. A heterogeneous receiver may be used to receive any AM stereo signal, such as an independent sideband type stereo signal in which left stereo channel information is transmitted over one sideband and essentially right channel information over the other sideband. The visible circuit can be used to receive a monophonic signal. When used to receive a monophonic signal, in addition to providing improved selectivity, it has the advantage of typically receiving AM monophonic by separating away the interference from the desired signal.

The desired monophonic signal and components of many stereo signals will appear to come from the midpoint between the left and right loudspeakers. However, channel interference adjacent to the desired channel top is likely to come from the left side of the desired signal and disturbances below the desired signal are likely to come from the right side of the desired signal.

As described above, this allows the receiver to identify the interference and take the desired signal. In order to understand how this device provides improved selectivity, it is necessary to examine Figure 1. The antenna 102 is connected to a radio frequency-intermediate frequency (RF-IF) superheterodyne circuit 104. The IF output of 104 is sent to amplifier 106. The output of 106 shown in line 107 is transmitted to three separate circuits: the upper sideband, the lower sideband, and the carrier channel circuit. The upper circuit is separated by the filter 108 and passes through the amplifier 114 to the product demodulator 118. The input terminal of the product demodulator 118 is connected to the carrier channel 110 by a line 111. The carrier channel is used to select a carrier signal and to provide a clean carrier. One circuit suitable for such a service is known from US Pat. No. 3,973,203.

The lower sideband is selected by filter 112 and supplied to product demodulator 120 through amplifier 116. The output of the carrier channel 110 also reaches the product demodulator 120 via line 111.

By modifying this device, product demodulators 118 and 120 can be replaced with envelope demodulators. If so arranged, the carrier channel 110 can also be removed. However, circuitry using the product demodulator shown in FIG. 1 is the best to minimize distortion and achieve the best signal-noise operation at the fringe region location.

In addition, if an envelope demodulator is used without a carrier channel circuit, the upper and lower sideband filters are required to pass through the complex carrier components.

The output of the product demodulator 118 is supplied to the amplifier 122 and the output of the product demodulator 120 is supplied to the amplifier 124. The output of the amplifier 122 is supplied to a highpass filter 138 and back to the detector 140. Similarly, the output of amplifier 124 is fed to high pass filter 142 and back to detector 144. Filters 138 and 142 and detectors 140 and 144 are used to measure the level of interference adjacent to the desired sidebands.

In current broadcasters, the carrier frequency on the American standard AM band is separated by 10 kHz. Filters 138 and 142 are used to isolate adjacent channel interference signals. Therefore, filters 138 and 142 must pass 10 kHz waves with relatively small attenuation.

To conveniently detect adjacent channel carrier levels, the filters and sideband filters 108 and 112 in the RF and IF circuits 104 must be wide enough to pass signals at least ± 10 kHz away from the center or desired carrier frequency. do.

The outputs of detectors 140 and 144 are connected to comparison circuit 146 via lines 141 and 145. The comparison circuit 146 evaluates which of the detectors produces the higher level wave. If detector 140 produces a larger output than detector 144, it is believed that interference to the upper sideband is greater than interference to the lower sideband. For example, the comparison circuit 146 is regulated and switched to the lower cutoff frequency of the switchable low pass filter 126. Conversely, if the output from detector 144 shown in line 145 produces a voltage greater than the voltage shown in line 141, then the low pass filter 128 will have greater interference with the lower sideband channel and switch operation. Switch to a lower cutoff frequency than filter 126.

The cutoff frequencies for filters 126 and 128 may be a function of interference level or may have discrete steps. For example, a channel that suffers from a large level of interference can operate at a cutoff frequency of one third of the standard full-wave band used when receiving a signal with relatively little interference. For communication services, the cutoff frequency is 1 kHz. Broadband filters, which result in measuring a small amount of disturbances, when embodied in the present invention, may have reduced bandwidth by less than the sidebands that remain in the broadband characteristics of the radio wave or are damaged by stronger interference.

The reason it is advantageous in some situations that reduce the frequency response of both sidebands is that if one sideband is damaged from interference, it usually indicates that the desired signal is weak. As such, reducing the large bandwidth removes weak noise conditions.

The outputs of the low pass filters 126 and 128 that can be switched or controlled are sent to the respective amplifiers 130 and 132 which, in turn, are left transducer and lower sidebands transmitted by the upper sidebands. Is sent to the right converter which is sent by.

In typical applications of this device, stereo headphones are used in many cases. The transducer is typically a loudspeaker.

If the sideband isolated by filter 108 and demodulated by block 118 represents the upper sideband and the interference above the desired channel is greater than the bottom, the signal filter 126 is a cutoff switch at a lower frequency than the filter 128. The interference will be attenuated. It is also possible to continuously reduce the high frequency cutoff of the low pass filter, which is a function of interference, rather than using the switchable filter shown at the bottom of FIG.

In another embodiment of the present invention, both filters 126 and 128 are controlled by reducing their cutoff frequency, but the lowest frequency is provided to the channel that is affected by a greater amount of interference.

Instead of connecting the high pass filter to the outputs of the amplifiers 122 and 124, a band pass filter connected at the output of the amplifier 106, that is, the line 107, may be used. However, such filters are slightly more expensive than the block-type filters i.e., the high frequency filters 138 and 142 in FIG. 1, if they are to provide the ideal selectivity in them.

The outputs of the switchable low pass filters 126 and 128 are supplied to amplifiers 130 and 132 and to the converter circuit. The transducer may be a loudspeaker or a stereo headset. Properly placed loudspeakers (eg, 4-6 feet apart for home installations and close proximity to speakers in cars or other confined areas) or stereo earphones with the same information in the upper and lower sidebands When used to receive a phonic double sideband signal, the directional attention effect can be used to identify adjacent channel interference.

For example, adjacent channel interference below the desired channel: the channel adjacent to the lower sideband of the desired channel appears to the left of the listener, and adjacent channel interference above the desired channel: the channel adjacent to the upper sideband of the desired channel. Appears to the right. The desired upper and lower sideband information is supplied to the speaker producing the desired signal on one side between the two speakers.

This process allows the listener to use the so-called directional attention selection effect and to identify disturbing signals that will appear on the listener's side.

Some of the same effects exist in the AM stereo embodiments described above of the present invention (except that the lower sideband disturbance falls on the right side of the listener and the top falls on the left side), however, the point where the desired signal component is not centered and close to the speaker. As can be seen, the benefit of the smaller directional attention selection effect will be provided.

3 shows a typical spectral position for a standard AM broadcaster, where the desired carrier frequency is denoted by Fc. The adjacent channel 10kHz above the desired carrier is denoted by Fc + 1. Adjacent channel carriers above 20 kHz are represented by Fc + 2. Likewise, adjacent low frequency channels that interfere with the signal have carrier frequencies at Fc-1 and Fc-2.

Most AM stations have a detectable sideband component higher than 5 kHz, so be aware that there is a detectable amount of sideband overlap. Thus, sideband components from broadcast stations of adjacent channels can be introduced into the passband of the desired signal. This creates noise that is hard to understand and very painful to listen to.

In this way, even if the receiver completely removes the carrier component of the adjacent channel interference, it is damaged by the interference from the sidebands of the adjacent channel signal. The present invention significantly attenuates the signal closer than 10 kHz from the desired carrier, separating the 10 kHz heterodyne miracle and attenuating much of the noise due to sideband interference.

In the example shown in FIG. 3, the interference from adjacent upper channels disturbing the carrier frequency signal is stronger than the interference from the lower sideband. Therefore, it is important to obtain a receiver frequency response in which the upper sideband components are substantially reduced. If the receiver is located at a point where its lower sideband is damaged from a greater amount of interference, it is necessary to switch to additional selectivity for the lower sideband.

It is common practice to assign frequencies so that there is little interference in a given area from channel signals of adjacent stations 10 or 20 kHz away. However, if the listener is tuned to a distant station, the listener is likely to be harmed by an adjacent channel.

As described above, the present invention uses the so-called directional attention selection effect to enhance the adjacent channel interference cancellation characteristic of the receiver.

In order to realize the directionalist selection effect, it is necessary for the distinct position of the desired sound source (sound source) to be separated from the distinct circle (circle) that interferes with the wave. Thus, sideband separation, both on the AM stereo sphere and on the monophonic sphere of the present invention, is effective for all components that fall within the effective high frequency limits of the audible pass band. However, the large separation of the audible passband for the low frequency part is not as important as the fact that adjacent channel interference components generally do not fall close to the carrier of the desired signal. Therefore, in monophonic embodiments, circuit components that provide sideband separation need not be particularly effective for frequencies below 1 or 2 kHz.

For AM stereo, separation is required for stereo performance and interference reduction. Therefore, AM stereo networks require effective separation up to at least 300 Hz. That is up to 10kHz. The reason for the lack of circuitry to provide separation for higher frequencies is that there is little stereo information provided by high-frequency sound when enough information to locate is in the lower and middle ranges. Therefore, for economic or other reasons, separation is limited to about 200-5000 as above and this roughly corresponds to the separation characteristics of the stereo transmitter.

In the present invention, it is desirable for the device to provide a relatively good separation for high frequency response in order to isolate the interference and fully utilize the directional attention selection effect. However, the separation does not have to be large enough for the listener to discern the interference. In general, a 10db isolation is appropriate.

Moreover, when embodiments of the invention are used that contain additional filters for adjacent channel interference, the separation only needs to be sufficient for the device to measure interference and level. For example, when the carrier frequency of adjacent channel interference is used in a known system, i.e. in a standard AM broadcast: separation should be ideal at 10 kHz where 10 kHz from the desired carrier. As such, if it is strong enough to interfere, it is not necessary to provide a complete directional attention selective effect for those frequency components that are removed or at least greatly attenuated by the low pass filter provided in FIGS. In fact, the network can be configured so that the separation is weak as long as the signal wave appears at 10 kHz, i.e. above several kHz. This document actually reduces the cost and complexity of the device for the separation of the upper and lower sideband components.

2 shows a switching arrangement that is desired and can be used to provide improved selectivity effects. Electronic switch 200, consisting of portions 202, 206, 208, and 212, changes the frequency response of the device from a complete response (2 positions) to bring the upper sideband response up to 2 kHz (1 position) and a lower sideband response. Used to attenuate from (2 position) to 2 kHz (3 position).

Applying to FIG. 1, the switching circuit is controlled by the comparison circuit 146 and used in the filters 126 and 128. Switch portion 208 and portion 212 are used to change the frequency response of the device with respect to the lower sideband information. All switch parts are connected so that they change position at the same time.

As shown in FIG. 2, the upper side band is limited to the frequency response to 2 kHz by the low pass filter 204, and the lower side band is limited to 3 kHz by the low pass filter 210.

If the switch is adjusted to the 3 position in case of severe interference on the lower sideband, the lower sideband is limited to 2 kHz response and the upper sideband to 3 kHz.

When there is no or low interference, the switch is controlled to switch the filter in two positions and provide maximum frequency response.

In some devices, it is desirable to switch on only one filter and to operate the other sidebands in full response. Such a switching arrangement is possible because it changes the circuit of FIG. 2 by removing the filter 210 and connecting the input and output wires of the filter 210 directly.

It will be apparent to those skilled in the art that the audible filters 204 and 210 can be located at a number of other points or combinations of filters can be used to achieve the desired improved selectivity.

The characteristics of the filter can be changed discontinuously or continuously. As will be apparent to those skilled in the art, as is known, the filter may be passive using a design process. An active audio filter can also be used. Design processes for active filters are also known.

In all cases, the arrangements described above merely depict particular embodiments that function in the present invention. Arrangements in various other forms can readily be made in accordance with the principles of the invention without departing from the scope of the invention.

Claims (1)

Demodulate the upper and lower sidebands of the AM signal, direct the demodulated upper and lower sidebands to one side of the binaural channel, and receive adjacent channel interference levels within the demodulated upper sideband of the received signal. An audible high frequency of at least one of the binaural channels having adjacent channel interference according to the measured adjacent channel interference level exhibited in the channel and measuring the adjacent channel interference level received within the demodulated lower sideband of the received signal. A method for reducing interference of adjacent channels in receiving AM signals reproduced in a two-channel binaural output signal type, characterized by reducing the response.

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