FR3022083A1 - UPGRADED SELECTIVE FILTERING IN FREQUENCY MODULATION - Google Patents

Abstract

A method for dynamic selective filtering of the channels in the FM band, the method comprising the steps of: / a / selecting a first frequency channel (FO) carrying signals of a station of interest, / b1 / applying to the received signals a filter of input (11) centered on FO, perform a baseband transposition, and digitize (12) the filtered signal, / b2 / apply a Fourier transform (14), and search in the curve forming the result of the transform of Fourier, a crossing frequency FC, defined as the abscissa of the local minimum of the curve closest to the central carrier of interest, / c / applying to the digitized signals a selective digital centered filter having a width (W) deduced from the crossover frequency FC, / d / continuously repeat the steps / b1 / / b2 / and / c / up to a frequency channel change.

Description

The present invention relates to vehicle audio systems and particularly to filtering methods for dynamic selective filtering of frequency modulation (FM) broadcast channels. More specifically, we are interested in audio systems capable of receiving and demodulating electromagnetic waves broadcasting radio programs on the frequency modulation. In the frequency modulation band, useful channels separated by 200 kHz or 300 kHz can be commonly found in terms of carrier frequency, but it also happens that two adjacent channels have carrier frequencies separated only by 100 kHz. poses a problem of discrimination and selective filtering to focus the reception on one of the channels and reject the signals of the other channel. In known systems, a more or less narrow input filter is applied depending on the reception conditions which are estimated after demodulation, that is to say on the audio quality; the width of the filter is controlled by a feedback loop. But it happens that this feedback loop can introduce useless or even harmful instabilities into the system, especially in the case of changes in FM content and / or the presence of adjacent channels or various electromagnetic disturbances and / or changes in conditions. reception. It has therefore emerged a need to provide a more efficient solution for selectively filtering the input signals to provide optimized reception of the channel of interest even with a nearby adjacent channel. According to the invention, there is provided a method for dynamic selective filtering of electromagnetic waves broadcasting radio programs on the frequency modulation, the method comprising the steps of: 25 / a / selecting a first frequency channel (F0) carrying signal signals a station of interest intended to be listened to, / b1 / apply to the received signals an input filter centered on the frequency channel, perform a baseband transposition, and digitize the filtered signal, 30 / b2 / apply a transform Fourier, and search in the curve forming the result of the Fourier transform, a crossover frequency (FC), defined as the abscissa of the local minimum of the curve closest to the central carrier of interest, / c / applying to the digitized signals a selective digital centered filter having a width deduced from the crossover frequency, i.e., a width of 2 x (FO - FC) + FH, FH being a horn value between 0 kHz and 10 kHz, / d / continuously repeat steps / b1 / / b2 / and / c / up to a frequency channel change.

The detection of such a local minimum makes it possible to identify the passage of the channel of interest to another disturbing adjacent channel. By means of which it is possible to apply a filtering width adapted to eliminate this adjacent disturbing channel. Demodulated audio signals are thus obtained which reproduce the signals diffused by the first channel of interest by strongly canceling or limiting the disturbances induced by the presence of a second disturbing adjacent channel, or the presence of interference outside the audio content. . Note also that, thanks to the transposition to the baseband, all digital processing is advantageously in the baseband, quite easily. In various embodiments of the method according to the invention, one or more of the following may also be used: - The detection of such a local minimum may be symptomatic of the presence of a second disturbing channel having a carrier frequency offset by 100 kHz with respect to the carrier frequency of the channel of interest; whereby the proposed method makes it possible to discriminate two very close channels, more effectively than the systems of the prior art, in particular with feedback. The digitization and the Fourier transform are preferably repeated at least every 500 ps. This provides rapid decision means so as to adjust the decision very dynamically. The input filter has a width of about 300 kHz. This advantageously eliminates upstream all the channels whose carrier is separated by 200 kHz or more from the carrier of interest. The width of the digital filter namely 2 × (FO - FC) + FH can take values between 40 kHz (+/- 20 kHz) and 300 kHz (+/- 150 kHz), and can be readjusted to a rate identical to the recurrence frequency of steps / b / and / c /, preferably greater than 1 kHz. So that the filter can be tightened if the audio content has little excursion or if an adjacent channel is detected and the filter can be expanded if the audio content is more supplied and without adjacent channel potentially disruptive. The nearest local minimum is preferably determined by computing the derivative of the curve forming the Fourier transform and identifying the points where this derivative vanishes and retaining the nearest point of the carrier of the channel. interest. This represents a mathematical treatment well mastered and easy to implement. - The value FH can be chosen according to the value of the second derivative at the point of the local minimum. It is thus possible to adjust the cut-off frequency of the digital filter as a function of the power of the disturbing adjacent channel, which is all the greater as the second derivative is high. - A temporal smoothing and / or a digital low-pass filter can be provided for the filter width adjustment mechanism; whereby the adjustment, although fast, does not involve a jump.

The invention also relates to an audio system capable of receiving and demodulating electromagnetic waves broadcasting radio programs on the frequency modulation, remarkable in that it is configured to implement the method according to one of the preceding characteristics; such a system has the advantages mentioned above.

Other aspects, objects and advantages of the invention will appear on reading the following description of one of its embodiments, given by way of non-limiting example. The invention will also be better understood with reference to the accompanying drawings, in which: FIG. 1 schematically represents a partial electromagnetic spectrum in the FM band and its evolution over time, with a channel of interest and a potentially disruptive adjacent channel, FIG. 2 represents a block diagram of an audio system and according to the invention; FIG. 3 schematically represents an electromagnetic spectrum, transposed from that of FIG. 1 to the baseband; FIG. FIG. 5 schematically represents the respective audio contents of the channel of interest and the potentially disruptive adjacent channel over time. FIG. In the different figures, the same references designate identical or similar elements. Figures 1 and 2 illustrate an audio system 1 for a vehicle, also called car radio, capable of receiving and demodulating electromagnetic waves broadcasting radio programs on the FM band. With regard to radio programs, as known per se, the principle of frequency modulation is based on the use of a frequency deviation with respect to a carrier frequency (F0) for encoding the audio signals to be broadcast. The frequency deviation is proportional to the encoder audio signal. A plurality of channels may be broadcast over the entire FM band (88 MHz - 108 MHz in many countries, 76 MHz - 90 MHz in Japan).

Each channel typically has a useful bandwidth of about 240 kHz, i.e. +/- 120 kHz around the carrier; this corresponds to a starting composite audio spectrum (Mono + Stereo + RDS baseband) bounded by 60 kHz, modulating the carrier according to its amplitude and its spectral content to give a channel of at most 240 kHz wide.

It is therefore understood that when two channels are separated, in terms of carrier frequency F0, by 100 kHz, there may be a slight overlap of signals on the spectrum, a problem to which the present invention seeks to provide a solution. . In the remainder of this document, the frequency FO will designate the carrier frequency of a first channel, also called channel of interest, that the user of the audio system has selected, this user wishing to listen to the radio program broadcast by this first channel. carrier frequency F0. We note that the spectral content of this first channel evolves over time, mainly because of the modulating audio content that also evolves over time.

The spectrum is not always centered on F0, and in particular the location of the maximum 71 moves around FO continuously according to the audio content. FIG. 1 illustrates the evolution of the spectral content of the channel of interest of carrier FO as well as the spectral content of a second channel F2, which will be called here disturbing channel. The second channel has a carrier frequency F2 not far from the carrier frequency F0, for example the difference can be 100 kHz. The disruptive channel may be located either above (as illustrated) or below the frequency of the first channel F0. The spectral content of the second channel also evolves with time independently of the spectral content of the first channel. In FIG. 2, the audio system 1 comprises an analog front part 2, a digital central part 4 with a logic control unit 3 (digital core), and an analog downstream part with an amplifier 19 and 9 loudspeakers. elsewhere, the audio system includes a user interface 6 with a display screen and a touchpad or physical buttons (not shown); This allows the user to choose the station / channel of interest.

The electromagnetic signals are received by an antenna device 10, the signals are then filtered by an input filter 11. Advantageously, the input filter can be completed by a frequency change operation (offset F0) to transpose baseband signals. The signals from the filtering are then digitized by a digital analog converter 12. The further processing of the signal is performed by digital operations, including storage in a data buffer 13. From the buffered data a Fourier transform analysis 14 is carried out, which gives a power spectral density curve output which will be discussed in detail below. In addition, the buffered data is digitally filtered by a digital filter of programmable width. A filter width control block 16 controls the width of the filter W as a function of the content analysis of the power spectral density curve. All or part of the operations of buffering, Fourier transform, digital filtering can be performed by a dedicated circuit of DSP type 5. The term "DSP" comes from the English "Digital Signal Processor", a signal processor 15 digital. Note that the input filtering 11 has a width of the order of 200 to 300 kHz centered on F0, which makes it possible to eliminate the signals coming from the channels distant from F0, but does not allow the signals to be correctly eliminated. a nearby adjacent channel whose spectral density may overflow within the input filter. As illustrated in FIG. 3, in the case where the second potentially interfering channel F2 has a carrier shifted at 100 kHz from F0, there is a crossover point denoted FC at which the power spectral density of each channel is is at the same level, where the curves intersect. The signals downstream of the input filtering, after digitization and spectral transposition, then Fourier transform processing, are illustrated in FIG. 4. It should be understood that with reference to FIGS. 3 and 4, they illustrate the baseband processing. i.e. after frequency transposition of -F0. The effect of the presence of the second disturbing channel F2 is manifested by a local rise in the spectral density to the right of the point marked "FC +" reference 92b. The analysis of the spectral density curve at the output of block 14 makes it possible to identify local minima, and in particular to identify the local minimum closest to the central frequency of interest. By determining FC, the filter width control block 16 applies a FO-centered selective digital filter with a parameterized width derived from the crossover frequency FC. For example, a width equal to 2 x (FO - FC) can be chosen. In a general manner, the value W will be taken as the value of filter width 16 such that: W = 2 × (FO - FC) + FH, FH being a correction value between 0 kHz and 40 kHz, FH possibly being an offset parameterized being the subject of a learning mechanism, and / or correlation with respect to the value of the second derivative of the spectral curve derived from the Fourier transform. If no minimum is identified, as for example in the case of the curve 93 5 in Figure 4, the width of the digital filter can then be deduced from the spectral content of the channel of interest, as will be detailed below. In FIG. 4, in phantom, in the case of the curve 91, the local minimum closest to the central frequency denoted FC-referenced 91b is to the left of the latter. In this case, the curve also has a local maximum, that is, a hump 91a symptomatic of the presence of an adjacent channel on the left. Similarly, in the case of curve 92, there is a local maximum 92a to the right of the local minimum 92b; we can even find a second local minimum 92c, but that does not play role because it is further away from the center frequency 0 / F0. The detection of a local minimum is obtained by calculating the derivative of the spectral curve and by searching for the roots, that is to say the places where this derivative vanishes. In addition, the second derivative is calculated, a minimum being characterized by a positive value of the second derivative at the point where the primary derivative is canceled. Note that generally the height of the lateral boss will be related to the value of the second derivative, and the correction value FH can be obtained as a function of the value of the second derivative at the nearest point of the frequency . In the case where the channel of interest is framed by two potentially disruptive channels, the analysis can lead to finding 2 minimums, one on each side, and we will retain the one that is closest to the central frequency (0 in band of based). The mechanism described above is constantly repeated which leads to dynamic decision making. The frequency of recurrence is preferably at least 1 kHz. Typically, the digitization and the Fourier transform are repeated at least every 500 ps. As is illustrated in FIG. 5, the spectral content 70 of the channel 30 of interest FO evolves with time, as well as the spectral content 72 of a possible interfering channel F2 represented in dashed lines. The references 71 and 73 respectively denote the location of the maximum spectral density frequencies which also change with time. During the first illustrated period of time 81, the spectral density curve resulting from the Fourier transform does not reveal a minimum and consequently the digital filtering is operated with a width W of default value, for example 100 kHz wide.

At the beginning of the second illustrated time period 82, the analysis of the spectral curve makes it possible to identify a local minimum induced by the presence of the adjacent channel F2 whose spectral content has approached the central frequency of interest F0. The width W of the digital filter 15 is then gradually reduced as the interference between the two channels increases. At the beginning of the third illustrated time period 83, the width of the digital filter 15 is increased again as the interference between the two channels decreases. At the beginning of the fourth illustrated time period 84, in the absence of detecting a local minimum in the spectral curve, the width of the digital filter 15 is again increased to the default value. During the next time period 85, there is a situation with no local minimum detection in the spectrum similar to the first time period 81. In the next time period 86, a new overlap sequence caused by a Negative excursion on the second channel F2 results in a reduction of the width W of the digital filter. At the beginning of the illustrated time period 87, the width of the digital filter 15 is decreased due to a narrower spectral content. During the next time period 88, the spectrum from the Fourier transform has a narrower profile, and the width W of the digital filter can be reduced accordingly.

Claims (8)

REVENDICATIONS1. A method for dynamic selective filtering of electromagnetic waves broadcasting radio programs on frequency modulation (FM), the method comprising the steps of: / a / selecting a first frequency channel (F0) carrying signals of a station of interest for to be listened to, / b1 / to apply to the received signals an input filter (11) centered on F0, to perform a baseband transposition, and to digitize (12) the filtered signal, / b2 / to apply a Fourier transform (14) ), and search in the curve forming the result of the Fourier transform, a crossover frequency FC, defined as the abscissa of the local minimum of the curve closest to the central carrier of interest, / c / apply to the signals digitized a selective digital centered filter having a width (W) deduced from the crossover frequency FC, namely a width of 2 x (FO - FC) + FH, FH being a correction value between 0 kHz and 10 kHz, / d / continuously repeat the steps / b1 / / b2 / and / c /, up to a frequency channel change, so that the demodulated audio signals reproduce the signals diffused by the first channel by strongly canceling or limiting the disturbances induced by the presence of a second adjacent channel, or interference outside the audio content. 2. Method according to claim 1, wherein the detection of a local minimum is symptomatic of the presence of a second disturbing channel having a carrier frequency (F2) shifted by 100 kHz with respect to the carrier frequency (F0) of the channel interest. 25 3. Method according to one of claims 1 or 2, wherein the digitization and the Fourier transform are repeated at least every 500 ps. 4. Method according to any one of claims 1 to 3, wherein the input filter (11) has a width of about 300 kHz. 5. A method according to any one of claims 1 to 4, wherein the width (W) of the 2 x (FO-FC) + FH digital filter takes values between 40 kHz and 300 kHz, and is readjusted rate identical to the recurrence frequency of steps / b1 /, / b2 / and / c /, preferably greater than 1 kHz. The method according to any one of claims 1 to 5, wherein the nearest local minimum is determined by computing the derivative of the curve forming the Fourier transform and identifying the points where this derivative vanishes and retaining the point closest to the carrier of the channel of interest. The method according to claim 5 and claim 6, wherein the correction value FH is chosen according to the value of the second derivative at the point of the local minimum. 8. An audio system capable of receiving and demodulating electromagnetic waves broadcasting radio programs on the frequency modulation, characterized in that it is configured to implement the method according to one of the preceding claims.