KR20010012983A - AM quality detector - Google Patents

Abstract

During automatic scanning of the AM frequency band, scanning needs to be stopped only when an available audio signal is received. To achieve this normally, the receiver checks the (RF) signal level and the IF frequency of the received signal. Since the propagation of AM signals at night improves as a result of weather conditions, it is possible for distant stations with the same transmission frequency to interfere with the local station. To overcome this, a detector and a receiver are proposed, whereby the detector detects interferences of two or more stations and supplies a detection signal to resume scanning for the next station.

Description

Amplitude Modulation Quality Detector

Such receivers and methods are known and used as techniques.

It is also known that receivers have a means to overcome the interference problem of adjacent channels / stations. International patent application WO-97 / 23959, for example, defines such a receiver.

During the automatic scanning of the AM frequency band, the receiver checks the (RF) signal level and the IF frequency of the received signal. Based on this, it is determined whether an available AM signal has been received. In the affirmative case this signal is supplied to the loudspeaker of the receiver and processed. In the negative case the frequency is changed to a predetermined value and the same checks are repeated for this new signal.

The transmitter frequencies of the AM stations are normally selected in such a way that they cause each other minimum interferences. However, this is only valid during the day. The ionization of the ionosphere, which causes a significant improvement in propagation at night, changes rapidly and temperature reversal occurs. This in turn results in strong reception at distant stations that could not be received during the daytime. Since these distant stations cannot be received under normal conditions, they can use the same transmitter frequency as nearby transmitters. This is not a problem during the day, but at night the far station can cause major interference with the local station. The frequency of this interference is called the bit frequency. These interferences may reach a level that renders the station unusable if such station is skipped during automatic discovery.

A disadvantage of known receivers is that after checking the signal levels and the IF frequencies and determining that they can be used, the signals are supplied and processed in the loudspeaker. In contrast, undetected interferences may be the source of the supplied signal which is not suitable for reproducing the audio signal.

The invention relates to a receiver as defined at the outset of claim 1.

The invention also relates to a detector for use in such a receiver.

The invention also relates to a method for scanning a frequency band as defined at the outset of claim 5.

1 is a block diagram schematically showing an example of a receiver according to the present invention;

2 is a block diagram schematically showing an example of an AM quality detector according to the present invention;

It is an object of the present invention to provide a receiver and a detector which does not have the disadvantages of the prior art, and to further improve the function of the receiver. A first aspect of this object of the invention for this purpose is to provide a receiver as claimed in claim 1. A second aspect of the invention is to provide a detector as claimed in claim 4. A third aspect of the invention is to provide a method for scanning a frequency band as claimed in claim 5.

During a (automatic) search to use AM radio stations, the receiver preferably stops searching only when an available station is found. It has a usable RF frequency, a signal level above a certain value, and no interference with other stations, for example, far away (especially at night). This is accomplished with the receiver in accordance with the present invention by using a detector that detects whether two (or more) signals having (almost) the same transmitter frequency are received. When the detector detects interference, the detector supplies a detection signal to the processor to resume scanning for the next station.

Since the current AM stations in the United States are all privately owned (as compared to state-owned (current) European stations using atomic clocks), they use crystal stabilized transmitters. As a result, simultaneous reception of two US AM stations (with the same transmission frequency) produces a bit frequency at a receiver of about 0 to 40 Hz with an amplitude about twice the normal modulation. This bit frequency has the advantage that it can be used by the detector to obtain the amount of multiple receptions.

In particular under US conditions (or in other countries with similar transmitter conditions) this system provides a significant improvement in AM auto search during evening hours.

Since AM quality is checked after the signal level and IF frequency are checked, there is no decrease in search speed.

Detectors can be easily added to existing AM receivers at minimal cost.

In European patent application EP-A-0410663 it was recognized that a receiver is known for receiving signals modulated with amplitude in the upper and lower bands. In demodulating the upper and lower band audio signals, the receiver is a quality detector for providing upper and lower quality signals representative of unwanted noise and the upper and lower band audio signals for providing an audio output signal with reduced noise. And a selector responsive to quality signals for processing the signals.

An embodiment of the receiver according to the invention comprises the features of claim 2.

Bit frequencies of about 0-40 kHz can be detected using a low pass filter with a 40 kHz cutoff frequency and can be performed with an amplifier / rectifier that generates, as a detection signal, a DC voltage proportional to the amount of interference.

An embodiment of the method according to the invention comprises the features of claim 6.

The operation of the method is further improved by measuring more than once at predetermined intervals in between to prevent the detector from being triggered by one single false pulse.

The present invention and additional features that may optionally be used to practice the advantages of the invention will be apparent from and elucidated with reference to the embodiments described below.

1 shows a schematic block diagram of a radio receiver RR having an antenna input AI for receiving an RF signal, in accordance with the present invention. The mixer unit MIX mixes the RF signal and the local oscillator signal Slo from the oscillator unit VCO and supplies an IF signal. This IF signal is supplied to the AM unit to amplify the IF signal, detect the AM signal, and supply an (unfiltered) AM signal LF. The AM unit supplies the signal level V1 and the frequency signal FIF to the processor unit PROC, which supplies the signal to the oscillator VCO. The processor unit PROC supplies a (new) voltage signal to the oscillator unit to correct the local oscillator signal, for example during automatic search, when the signal level V1 and / or the IF signal are not within a predetermined range. The output of the AM unit is connected to the audio unit AU to process the unfiltered AM signal LF and to further supply the processed signal to the loudspeaker LS.

The processor unit further includes means for automatically scanning (searching) the frequency band. In this example, this can be achieved by supplying a higher (or lower) voltage every hour to the oscillator unit (here called a voltage controlled oscillator).

The radio receiver RR according to the invention further comprises an AM quality detector (AQD) (shown in detail in FIG. 2) to determine whether an available AM signal is received. So run an interference check. The AM quality detector receives the unfiltered AM signal LF. This is necessary because otherwise the interference can be filtered (for example after the audio unit AU).

During an (automatic) search for available AM radio stations, the receiver preferably stops searching only when an available station is found. It has usable RF frequency, good signal level, and no interference at distant stations (especially at night). This is accomplished with the receiver according to the invention by using an AM quality detector which detects whether two (or more) signals are received from stations with (almost) the same transmitter frequency. When the AM quality detector detects interference, the AM quality detector supplies a detection signal AMql to the processor unit PROC to resume scanning for the next station.

Since the current AM stations are all privately owned in the United States (as compared to state-owned (current) European stations using atomic clocks), they use crystal stabilized transmitters. This results in the reception of two US AM stations (with the same transmit frequency) simultaneously producing a bit frequency in a receiver of about 0-40 kHz with an amplitude almost twice that of normal modulation. This bit frequency has the advantage that it can be used by the detector to obtain the amount of multiple receptions.

2 shows an example of an AM quality detector AQD2 in more detail. The AM quality detector receives an unfiltered AM signal LF2 at its input, which is for example a low pass filter (LPF2) having a cutoff frequency of 40 Hz to obtain only signal components below 40 Hz (ie interference). Supplied. Filtering the signal above about 40 Hz prevents the AM quality detector from reacting to the (real) low frequency AM audio signals. This signal is amplified by the amplifier AMP2. The output of the amplifier is connected to the detection means DET2 for detecting a signal value below 40 Hz, for example generating a DC level and for supplying a detection signal AMql2 at the processor unit PROC (shown in FIG. 1). The processor unit determines whether an available AM signal is received based on the detection signal AMql2 value.

A cutoff frequency of about 40 Hz is chosen to allow the AM quality detector to prevent measurement of normal audio signals. Four-pole filters are preferred because a sharp reduction is needed to allow the AM quality detector to prevent detection of low frequency audio signals.

Measurement of the detection signal to further improve the operation of the AM quality detector and receiver is performed at least once at a predetermined interval, for example three times at an interval of 20 ms.

AM quality detector sensitivity is preferably reduced if the signals of the two transmitters have a difference in signal strength. The larger the difference in the received antenna signal, the more the sensitivity of the AM quality detector can be reduced. This is preferable to overcome the weakest since the two stations have a large difference in signal strength when received at approximately the same transmitter frequency. So the strongest station produces the audio with the least audio distortion by the weakest station. Therefore preferably such stations cannot be skipped during automatic search.

In addition, when the receiver tunes to the station, the AM quality detector is able to supply a detection signal as a result of the tuning step. The detection signal is proportional to the signal level of the received signal. The result is an error indicating that the AM quality detector receives a disturbed, unavailable signal and restarts scanning for the (next) station.

To overcome this it is possible to obtain a hardware solution, but this requires a connection between the so-called front-end and the so-called IF / AM detector (AM unit). This connection is undesirable due to the risk of cross interference between other elements of the receiver, resulting in sound, destabilization or deterioration.

Thus, using a software algorithm, it is desirable to obtain a software solution. The first signal level V1 is checked when the station is found during the automatic search and the IF frequency FIF is checked if the level is found above a predetermined value. Also, when the IF frequency is found in the required IF window, the AM quality (AMql) is measured. Since the signal level of the tuned station is known at that point, there is a possibility of creating a delay proportional to this level. For example, three measurements following this delay are carried out at intervals of 20 ms, for example. This measurement mean is not greater than or equal to the given level. This proportional delay allows sufficient time to reduce unwanted pulses (because they tune the received station) before the AM quality measurement (interference check) is performed. The short delay for small antenna signals and the delay becomes longer as the antenna signal increases. The delay can be determined using shape measurement of unwanted pulses at several antenna inputs.

Since AM quality is read after the signal level and IF frequency are checked, there is no decrease in search speed.

In addition, when one of the two signals is significantly less than the other, the operating sensitivity of the AM quality detector is reduced because the received signal with the strongest signal level dominates.

The concept of the invention described above has been described on the basis of detailed examples. Those skilled in the art are familiar with many other solutions that fall within the scope of the invention.

Several blocks in the figures may be executed separately or in integrated circuits but may be digitally executed, for example, by a digital signal processor (eg, processing unit PROC in FIG. 1).

The present invention provides a receiver, detector and method that uses an AM quality detector to overcome the problem of stopping automatic scanning (searching) at unavailable (radio) stations, especially at night.

After determining that the AM signal level is above the minimum level and that the IF frequency is within a predetermined frequency window, the AM quality detector detects whether interference is present. This interference results in a detection signal in the low frequency range (eg 0-40 kHz) which is used in the processing unit to restart the search for the next station.

Claims (8)

A mixer unit for receiving RF signals, an oscillator unit connected to the mixer unit, an AM unit for receiving IF signals from the mixer unit, and for amplifying and detecting AM signals, processing AM signals and supplying output signals to loudspeakers A receiver comprising: an audio unit for processing, and a processing unit further connected to the AM unit and the oscillator unit for controlling the oscillator unit and for auto scanning the frequency bands, the receiver comprising: And a detector coupled to the output of the AM unit for detecting interference between at least two received input signals having approximately the same frequency and for supplying a detection signal to the processing unit. The method of claim 1, A receiver comprising a detector comprising a low pass filter to filter the AM signal. The method of claim 2, The detector comprises rectifying means for rectifying the filtered signal and supplying a detection signal. A detector for use in the receiver as claimed in claim 1. A method for scanning an AM frequency band at a receiver, the method comprising: changing a frequency every hour to a predetermined value, detecting whether a signal has been received, checking a signal level, examining an IF frequency, and scanning A method of injection, having a step of determining to stop or resume injection, After checking the signal level and the IF frequency, an interference check is performed, including low pass filtering of the unfiltered AM signal and determining whether to obtain a detection signal and to stop scanning for the station. The method of claim 5, And after the interference check is performed, a delay is determined depending on the received signal value and determines whether to stop scanning. The method of claim 5, A scanning method in which an interference check is performed several times at predetermined intervals therebetween. The method of claim 5, A scanning method in which the sensitivity of the interference check decreases according to a difference in signal values of signals simultaneously received.