JPS6214974B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Although the invention has a wide range of applications, it is particularly suited for use in reducing the effects of adjacent channel interference from AM monophonic systems to AM stereo systems, and is particularly suited for use in reducing the effects of adjacent channel interference from AM monophonic systems to AM stereo systems. This will be explained.

Conventionally, there are many methods for reducing the effects of adjacent channel interference, including manually or automatically operated variable selectivity filters and manually adjusted audio or IF notch filters to provide sharp discrimination against interference. means are used. For example, John Willey, New York City, India;
Sands, Incorporated (John Wiley &
See "Radio, Receivers, Design" by K. R. Sturley, published by Sons Inc., pp. 517, 518, 543.

Also, many systems have been launched that automatically change receiver selectivity as a function of interference. Selectivity may vary symmetrically or asymmetrically. A description of such a system can be found on pages 543 and 544 of the Starley article cited above. Changes in selectivity are provided at intermediate frequencies similar to the sensing system.

It is an object of the present invention to provide an inexpensive automatic device that reduces interference while maintaining relatively good frequency response.

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

The present invention also uses the so-called "cocktail party effect" to further discriminate against interference.
The "cocktail party effect" is an effect that allows binaural listeners to separate speech arriving from two different speakers, allowing them to pay greater attention to one speaker. It is.

This refers to the ability of humans, who have both ears, to be able to hear one story while distinguishing it from the other, even when many people are speaking different stories at the same time in one room. This is because the different speeches within a room all originate from different points and therefore reach the listener from different directions.

In an AM stereo receiver of the type designed to receive independent sidewave stereo signals, if the stereo decoder circuit is designed to have excellent stereo separation of approximately 5 KHz or higher, Neighboring station interference caused by a broadcasting station that is +10KHz away from the broadcasting station will be heard mainly only on the right channel, and adjacent station interference caused by a broadcasting station that is -10KHz away will be heard mainly only on the left channel. So, if you try to listen to the desired station, you will hear that the broadcast from this station is mainly coming from the center of the stereo sound system, but adjacent channel interference causes the sound to come from either the left or the right. If we utilize this human characteristic called the cocktail party effect, we can obtain a receiver that can discriminate against adjacent channel interference.

The primary purpose of the present invention is to make the "cocktail party effect" available to listeners of AM monophonic and stereo signals.

The present invention can be used to receive amplitude modulated waves, and in particular,
Suitable for receiving certain types of AM stereo waves. The invention may, for example, (a) measure the level of received interference in an upper sideband stereo channel caused by a jamming channel adjacent to the upper sideband of the desired signal; and (b) measure the level of received interference in the lower sideband of the desired signal. (c) when the stronger interference level exceeds a predetermined level, the measurement in (a) shall be made; two independent sideband (ISB) type AM stereo modulation by automatically reducing the audio frequency response of the stereo channel experiencing the greater interference level according to whether the measurement of (b) is greater. It can be used to reduce adjacent channel interference in wave reception.

In many applications of the invention, it is desirable to attenuate both the channels above and below the desired channel by a greater factor than when the system is operating substantially without interference. However, it is desirable to provide more attenuation to adjacent channels that experience greater interference levels.

The invention can also be used to improve the performance of monophonic reception systems by allowing listeners to utilize the "cocktail party effect". One way to improve adjacent channel interference in a monophonic double-sideband signal receiver is to
The steps include separating and demodulating both the upper and lower sidebands of the desired signal and sending the resulting audio waves to appropriate circuitry to drive separate transducers. The transducer is a stereo speaker, i.e.
The speakers can be installed as speakers spaced apart from each other by about 120 cm to 180 cm. The invention can also be implemented by using a stereo headphone as a transducer.

There are many ways to separate and demodulate the upper and lower sideband channels, including phase-shifting and filter-based systems. Such techniques are well known to those skilled in the art. Systems with full carriers or sideband reduced or suppressed carriers can be used, ie, systems with carrier levels lower than the peak coupling amplitude of the sidebands or full carriers are used.

The monophonic application of the invention also utilizes a device that measures the interference level of channels adjacent to the upper and lower sideband components of the desired signal and reduces the audio frequency response of channels experiencing greater interference levels. be able to. Thus, the listener can not only obtain an improved interference reduction effect based on the Cartel-Party effect, but also a sound system with improved selectivity.

The present invention particularly applies to dual independent sidebands (ISB) where stereo-related information appears in the upper and lower sideband modulations of a carrier wave.
This is useful for receivers of type stereophonic signals. In one such type of signal, the carrier is amplitude demodulated with stereo total (L+R) intelligibility and simultaneously phase demodulated with stereo difference (L-R) intelligibility. Such stereo waves are described in detail in, for example, US Pat. No. 3,218,393 and US Pat. No. 3,908,090. Such stereo wave receivers typically operate at an intermediate frequency and provide the special characteristics of a phase-shifting sideband separator, such as that provided by a superheterodyne type circuit, to achieve the cocktail party effect. You can configure it to emphasize it. The phase-shifting separation device, unlike a conventional stereo receiver, will provide substantial separation of interfering signal components of adjacent channels, typically above 5 KHz, which is an acceptable high frequency range for stereo separation components. Attenuating the interference component for one sideband at one stereo speaker by connecting the output of the phase-shifting sideband separator to a device that separately amplifies the audio output of the sideband separator. The interference components of adjacent channels of other sidebands will be reduced by the other stereo loudspeaker. The desired stereo component will fall approximately within the range between the two speakers, as is usual for stereo illusions.

In both stereophonic and monophonic reception, a reduction in audio frequency response should only be applied in the worst cases, when sideband channel interference is of sufficient amplitude to cause audibility problems. Typically, the power level of the interference is 10% of the desired sideband power level.
If it is less than a factor of 1, there is no need to reduce the audio frequency response. Typically, the response of a channel experiencing stronger interference levels is reduced to approximately one-third of the total response of the channel. For example, a stereo reception of a 6KHz audio response with medium sound quality would be reduced to 2KHz. In the case of a monophonic, the response would be reduced from 3KHz to 1KHz, for example. The side experiencing less interference will be reduced to a lesser extent, and in some applications of the invention, the side experiencing weaker interference will have no change in frequency response.

In order to better understand the invention and to appreciate other objects of the invention, reference will now be made to the drawings.

FIG. 1 is a block diagram of a receiver embodying the invention. This type of receiver is used to receive a type of AM stereo signal, namely a dual independent sideband (ISB) stereo signal, in which essentially the left stereo channel information is transferred to one sideband. The right stereo channel information is transmitted via the other sidebands. The illustrated circuit can also be used to receive monophonic signals. When used to receive monophonic signals, in addition to providing improved selectivity, this provides the additional advantage over conventional AM monophonic reception of spatially separating interference from the desired signal.

Most of the desired monophonic and stereo signals will be heard coming from a point midway between the left and right speakers. However, interference between adjacent channels above the desired channel will appear to come from a point to the left of the desired signal, and interference below the desired signal will appear to come from a point to the right of the desired signal. As mentioned above,
This allows the listener to discriminate the interference and specifically select the desired signal. It is necessary to examine Figure 1 to understand how this system provides improved selectivity.

An antenna 102 is connected to an RF-IF superheterodyne circuit 104. The LF output of 104 is sent to an amplifier 106. The output of 106 appears on line 107, which is sent to three separate circuits: upper sideband, lower sideband, and carrier channel circuits. The upper circuit is separated by a filter 108 which is connected to an amplifier 114 which in turn is connected to a product demodulator 118. The injection port of product demodulator 118 is connected to carrier channel 110 by line 111. The carrier channel is used to select the carrier signal and provide a clean carrier wave. One circuit adapted for such an operation is described in US Pat. No. 3,973,203.

There are many methods for improving intelligibility and demodulation in the upper and lower sidebands of both sidebands, including phase-shifting systems in addition to the system shown in FIG.

The lower sideband is selected by filter 112;
This filter 112 is connected to an amplifier 116, which in turn is connected to a product demodulator 120. Product demodulator 120 is further connected to carrier channel 110 through line 111. In one form of this system, product demodulators 118, 12
An envelope demodulator may be used instead of 0. Such a configuration would also allow carrier channel 110 to be eliminated. However, in order to minimize distortion and obtain the best signal-to-noise ratio at the fringe area locations, a circuit utilizing a product demodulator as shown in FIG. 1 is superior. Also, if an envelope demodulator is used without a carrier channel circuit, the upper and lower sideband filters will be required to pass the carrier component, which will complicate the specifications.

The output of the product demodulator 118 is supplied to an amplifier 122 and the output of the product demodulator 120 is supplied to an amplifier 124. The output of the amplifier 122 is passed through the high pass filter 13
8 and then to a detector 140.
Similarly, the output of the amplifier 124 is transmitted to the high pass filter 1.
42 and then to a detector 144. filters 138, 142 and detector 140,
144 is used to measure the level of interference adjacent to the desired sideband.

In current broadcasting services, the AM standard broadcast band in the United States is separated by 10KHz. Filter 13
8,142 is used to separate common channel interfering signals. Therefore, the filters 138, 14
2 has relatively little attenuation and passes 10KHz waves.

To conveniently sense the level of adjacent channel carriers, the filters of RF and IF circuit 104 and sideband filters 108 and 112 are wide enough to pass signals that are slightly offset from the center frequency, i.e., the desired carrier frequency by ±10 KHz. It is necessary.

The outputs of detectors 140 and 144 are connected to comparator circuit 146 through lines 141 and 145.
Comparison circuit 146 evaluates which detector generates a large level wave. However, if detector 1
If 40 produces a greater power than 144, then the upper sideband interference may be greater than the lower sideband interference. Comparator circuit 146, for example, switches the lower cutoff frequency adjustment of switchable low pass filter 126. Conversely, if the output from detector 144 appears on line 145 and is greater than the voltage appearing on line 141, there will be more interference in the lower sideband channel and switchable low-pass filter 128 will have a lower cutoff frequency than filter 126. It will be switched.

The cutoff frequencies of filters 126 and 128 can be a function of the interference level and can be jumped in steps. For example, a channel experiencing significant levels of interference may be set to a cutoff frequency that is 1/3 of the normal total bandpass used when the channel is receiving signals relatively free of interference. For communications services, this cutoff frequency is 1KHz. In the first embodiment of the present invention, the wideband filter that measures the amount of lower interference is either left with its full wideband characteristic, or its bandwidth is reduced by an amount slightly less than the sidebands that suffer from stronger interference. Leave it for a while.

The reason it is advantageous in some cases to reduce the frequency response of both sidebands is that if either sideband is experiencing interference, the desired signal will generally be weakly indicated. This bandwidth reduction therefore helps overcome bad noise conditions.

Switchable or controllable low pass filter 12
The outputs of 6 and 128 are fed to amplifiers 130 and 132, respectively, and then to, for example, a left transducer fed by the upper sideband and a right transducer fed by the lower sideband. .

In typical applications of this device, the transducer is a conventional loudspeaker, although stereo headphones are often also used.

FIG. 3 shows a typical spectral condition for a standard AM broadcast system, where the desired carrier frequency is designated F _c . The adjacent channel 10 KHz above the desired carrier frequency is designated F _c +1. Also, the adjacent channel on 20KHz is designated F _c +2. Similarly, the lower portions are indicated by F _c -1 and F _c -2.

The aim of the invention is to improve the reception performance in this type of interference.

Even the best AM broadcasting stations have sideband components of 5KHz or higher, so it should be considered that there is considerable sideband overlap. This sideband component from the common channel can therefore be included in the passband of the desired signal. In this case, a rare type of interference called "monkey chatter" occurs, which can be extremely audible and impair intelligibility.

Therefore, even if the receiver completely eliminates the carrier component of adjacent channel interference, it still suffers from interference from the sidebands of adjacent channel signals. By significantly attenuating nearby signals within 10 KHz of the desired carrier, the present invention not only helps eliminate 10 KHz heterodyne noise, but also greatly attenuates sideband interference or monkey chatter. In the example shown in FIG. 3, the interference from adjacent channel interfering signals above the carrier frequency is considerably stronger than the interference from lower sideband interfering waves. It is therefore important that the sensitivity of the receiver is substantially reduced for upper sideband components.
If the receiver is located at a point where the lower sideband experiences a large amount of interference, an additional selection state for the lower sideband must be switched.

It is true that frequency allocation is such that in a given area there is little interference from adjacent channel signals 10 or 20 KHz away from the assigned station. However, if a listener tunes in to a distant station, there is a high possibility that the listener will experience adjacent channel interference.

As described above, the present invention utilizes the so-called cocktail party effect to enhance the adjacent channel interference rejection characteristics of the receiver.

In order to realize the cocktail party effect, the apparent location of the desired sound source must be spatially separated from the apparent source of the interference waves. Therefore, the present invention
In both the AM stereo embodiment and the monophonic example, it is important that the sideband separation be effective for all components at the high effective frequency limit of the audio passband. However, for the lower frequency portion of the audio passband it is not important that this separation be large. This is because the interfering components of adjacent channels are generally not close to the carrier of the desired signal. Thus, in the monophonic example, the circuitry providing sideband separation need not be particularly effective for frequencies below, for example, 1 or 2 KHz. Therefore, if a phase-shifting system is used for upper and lower sideband separation, the network need not provide accurate phase correction in the lower frequency range and may have fewer components.

In the case of AM stereo, separation is required for both stereo effects and interference reduction effects. Therefore, the circuitry used for AM stereo requires adequate isolation, eg, at least 300 Hz or less.

Further, in this example, it is desirable to provide relatively good isolation for all high frequency responses of the system so that interference is isolated and the cocktail party effect is fully exploited. However, this separation need not be so great that a listener can discern the interference. Generally, 10db separation is appropriate.

Furthermore, if an additional wave of adjacent channel interference is applied in the practice of the present invention, then measuring the level of the interference wave alone is sufficient for separation. for example,
If the carrier frequency of the adjacent channel interference is known, ie, 10 KHz from the desired carrier, only separation that is adequate at 10 KHz is required. Therefore, it is unnecessary to give a full cocktail party effect to these frequency components if they are strong enough to cause a disturbance;
It may be removed or at least greatly attenuated by a low pass filter such as that shown in FIG. In reality, as long as the network is restored to 10KHz, the separation can be roughly configured, for example, over several tens of KHz. The simplicity of this specification allows for a substantial reduction in cost and complexity of the means for providing separation of upper and lower sideband components.

FIG. 2 shows the switch arrangement used to provide the desired improved selection effect. The electronic switch 200 is 202, 206, 208, 212
It reduces the upper sideband response from the total response (position 2) to 2KHz (position 1), and reduces the lower sideband response from the total response (position 2).
Used to change the frequency response of the system to reduce it to 2KHz (position 3). When this switching circuit is applied to FIG. 1, it is controlled by comparison circuit 146 and is used in place of filters 126 and 128. Switches 208 and 212 are used to change the frequency response of the system to lower sideband intelligibility. All switches are interlocked and change position together. In the position shown in FIG. 2, the upper sideband has a frequency response limited to 2 KHz by low pass filter 204, and the lower sideband is limited to 3 KHz by low pass filter 210. If severe interference is in the lower sideband, and the switch is in the third position, the lower sideband will be limited to a 2KHz response and the upper sideband will be limited to a 3KHz response.

When there is no interference or its level is low, the switch is operated to the second position, which disables the filter and provides maximum frequency response.

In some systems, it is desirable to switch on only one filter and control the other sidebands to work over the entire response. Such a switch configuration is applied to the filter 210 in the circuit of FIG.
Remove the lead from the input of this filter 210 and connect the lead from the output to the second filter.
This can be easily obtained by modifying the diagram.

For this field, audio filter 2
04 and 210 can be placed at many other points, and combinations of these filters can be used to achieve the desired selectivity improvement. The characteristics of the filter may be discontinuously stepped or continuous. The filter may also be passive, using designs such as those found in many documents. You can also use an active audio filter.

In all embodiments, the configurations described above are merely illustrative of possible specific embodiments. Of course, many variations can be made without departing from the spirit of the invention.

[Brief explanation of the drawing]

FIG. 1 is a block diagram of a typical receiving system constructed in accordance with the present invention, and FIG. 2 is a schematic block diagram showing a switching arrangement for switching the low pass filter to upper and lower sideband channels to improve coherence. The diagram, Figure 3, is a switch showing severe interference that can appear on a spectral display. 102... Antenna, 104... RF-IF superheterodyne circuit, 106... Amplifier, 10
7... Line, 108... Filter, 110...
Carrier channel, 111... line, 112...
...filter, 114, 116...amplifier, 11
8,120... Product demodulator.

Claims (1)

[Claims] 1. A method for reducing adjacent channel interference in receiving an independent sideband AM stereo signal, comprising: (a) a received adjacent channel in a demodulated upper sideband of said stereo signal; (b) measuring the level of received adjacent channel interference in the demodulated lower sideband of said stereo signal; (b) determining that the level of interference of each adjacent channel is at a predetermined level; A method of reducing adjacent channel interference comprising automatically reducing the audio frequency response of a stereo channel once the frequency response of that stereo channel is exceeded. 2. The method of claim 1 including the additional step of reducing the audio frequency response of a stereo channel having a weaker level of interference of an adjacent channel by an amount less than the reduction applied to the other stereo channel. 3. The method of claim 1, wherein said predetermined level is approximately 0.1% of the power level of the stereo signal sidebands. 4. The audio frequency response of a stereo channel subjected to stronger adjacent channel interference levels is reduced to approximately one-third of the total response of that stereo channel.
The method described in section. 5. The method of claim 1, wherein the audio frequency response of a stereo channel subjected to stronger adjacent channel interference levels is reduced to approximately 2 KHz. 6 The frequency response of a stereo channel subject to stronger interference levels of adjacent channels is approximately
2. The method of claim 1, wherein the stereo channels are reduced to approximately 3KHz and the other stereo channels are reduced to approximately 3KHz.