WO2004077690A2

WO2004077690A2 - Noise cancellation system in an analog fm receiver

Info

Publication number: WO2004077690A2
Application number: PCT/IB2004/050121
Authority: WO
Inventors: Anthony Kerselaers; Felix Elsen
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2003-02-26
Filing date: 2004-02-18
Publication date: 2004-09-10
Also published as: WO2004077690A3

Abstract

The system for noise cancellation in analog FM radio reception, comprises a signal level measuring unit (113) for measuring a signal level (S) of a received intermediate frequency radio signal (IF), a control unit (122) arranged to evaluate a predetermined criterion based on the signal level (S), the evaluation yielding a control variable (C) capable of having a first value and a second value, and a noise cancellation unit (124), arranged to output a left output audio signal (LDN) and a right output audio signal (RDN), based on a left input audio signal (LP) and a right input audio signal (RP), comprising an equalization unit (125) arranged to make the left output audio signal (LDN) substantially equal to the right output audio signal (RDN) if the control variable (C) equals the first value.

Description

Noise cancellation system, analog FM receiver, circuit, wireless audio speaker, wireless headphone, method, computer program product

The invention relates to a system for noise cancellation in analog FM radio reception.

The invention also relates to an analog FM receiver.

The invention also relates to a circuit for a system for noise cancellation in analog FM radio reception.

The invention also relates to a wireless audio speaker.

The invention also relates to a wireless headphone.

The invention also relates to a method for noise cancellation in analog FM radio reception. The invention also relates to a computer program product enabling a processor to execute said method.

A problem with analog wireless radio transmission systems, e.g. for wireless headphones, is the noise in their output audio signals. For popular music there is often less of a problem, but during silent passages of classical or movie music the noise may be perceptibly objectionable. The problem is worsened the larger the distance between the receiver and the transmitter. In indoor applications there are a lot of reflections of transmitted signals on walls and furniture, and the problem becomes dynamical in case there are persons moving around in the room. This manifests itself in noise bursts. In the particular case of surround speakers, the amplifiers of those speakers are set almost to maximal amplification, to have a good balance between all channels. Any noise to these speakers will hence be severely amplified.

It is an object of the invention to provide a system of the kind mentioned in the opening paragraph which has relatively little noise on its output audio signals. This object is realized in that the system comprises: a signal level measuring unit for measuring a signal level of a received intermediate frequency radio signal; a control unit arranged to evaluate a predetermined criterion based on the signal level, the evaluation yielding a control variable capable of having a first value and a second value; and a noise cancellation unit, arranged to output a left output audio signal and a right output audio signal, based on a left input audio signal and a right input audio signal, comprising an equalization unit arranged to make the left output audio signal substantially equal to the right output audio signal if the control variable equals the first value. A property of the demodulation and matrixing in analogue FM is that the noise on the left and right audio signal is roughly 180 degrees different in phase. The noise of the two signals could hence be appreciably reduced by adding it together. This can be achieved by adding the two audio signals together. A disadvantage is that the output signal then becomes mono instead of stereo. But this need not be a problem. Typically people movements indoors happen in time intervals of the order of seconds. This leads to a few short stereo to mono switch time intervals, which is often less perceptible than noise.

In an embodiment, the noise cancellation system according to the invention comprises a level-scaling unit for scaling the value of the signal level with a predetermined scaling constant. It is important that the trade off between the amount of noise and the amount of time switched to mono is chosen intelligently, the more so because the switching to mono is a hard switch. From FM receivers in car radios a gradual switching from mono to stereo is known. However in this case the noise is not appreciably reduced, but only gradually. The purpose of those systems is different: they are designed to compensate for bad stereo reception rather than for noise. The physics of the transmitted waves are also entirely different, e.g. the field strengths are different than for a home or office system according to the invention -which could of course also be used in the vicinity of a home such as in the garden-, and the time intervals of perturbations are different. This leads to entirely different solutions comprising different features.

In an alternative or additional embodiment, the system comprises a reference voltage means arranged to supply a reference voltage. Existing frequency demodulation IC's typically provide output audio signals on a DC reference voltage. In a simple circuit or algorithm for switching to mono, this reference voltage is needed, e.g. for switching a FET on or for subtracting a DC from the digitized signals. In a further alternative or additional embodiment, the system comprises a muter, arranged to mute the left output audio signal and the right output audio signal if the signal level is below a predetermined mute value.

Since for very low received field strengths, there is still an objectionable amount of noise compared to the useful audio signal, for such low field strengths the muter cuts the delivery of audio to the loudspeakers all together.

In a variant of the previous embodiment, the muter is arranged to mute the left output audio signal and the right output audio signal if a pilot carrier is not detected. The frequency of the pilot is so chosen that it identifies the presence of transmission of audio for the receiver according to the invention. Otherwise the receiver may respond to other fields present in the room, and the loudspeakers may boost these nonsense signals through the room.

An analogue FM receiver is described, comprising: a noise cancellation system according to the invention; and - an expander.

By means of an expander, even more dynamic range can be achieved.

An analogue FM receiver is described, comprising: a noise cancellation system according to the invention; and a de-emphasis unit for reducing the power of frequencies above a predetermined frequency threshold.

By pre-emphasizing and de-emphasizing higher audio frequencies, the noise in this frequency range can further be reduced.

A circuit realizing a noise cancellation unit according to the invention, arranged to make the left output audio signal substantially equal to the right output audio signal if the control variable equals the first value is also described,

The circuit comprising a field effect transistor arranged to provide a substantially short circuit, between a first connection arranged to carry the left input audio signal and a second connection arranged to carry the right input audio signal, if the control variable imposed on a gate of the field effect transistor equals the first value. Putting a FET between the left and right audio connections is a very simple means to realize a short circuit, hence a stereo to mono conversion. Furthermore, due to the linearity of the FET, hardly any distortion is introduced while switching from open to short circuit.

A method for noise cancellation in analog FM radio reception, comprising: measuring a signal level of a received intermediate frequency radio signal; evaluating a predetermined criterion based on the signal level, the evaluation yielding a control variable capable of having a first value and a second value; and generating a left output audio signal and a right output audio signal, based on a left input audio signal and a right input audio signal, the left output audio signal being substantially equal to the right output audio signal if the control variable equals the first value.

Instead of realizing the noise cancellation unit as a discrete or integrated circuit, it can also be realized as software running on a processor, e.g. a microcontroller or digital signal processor.

These and other aspects of the audio reproduction apparatus according to the invention will be apparent from and elucidated with reference to the implementations and embodiments described hereinafter, and with reference to the accompanying drawings, which serve merely as non-limiting illustrations.

In the drawings :

Fig. 1 schematically shows an analog FM receiver comprising the noise cancellation system; Fig. 2 schematically shows a received radio signal and noise for different field strengths;

Fig. 3 schematically shows a discrete circuit embodiment of the noise cancellation system according to the invention;

Fig. 4 schematically shows another discrete circuit embodiment of the noise cancellation system according to the invention; and

Fig. 5 schematically shows a frequency spectrum of an analogue frequency modulated stereo audio signal;

In these Figures elements drawn dashed are optional, depending on the desired embodiment.

Fig. 1 shows an analog FM receiver 100, typically a stereo receiver. A radio signal HF is picked up by an antenna 102. The radio signal may e.g. be transmitted in any of number of different channels in the 863-865 MHz frequency band for Europe and in the 902- 926 MHz band for the United States. The radio signal HF is downconverted, by any do nconverter 104 according to the state of the art, to an intermediate frequency radio signal IF of e.g. 10.7 MHz, after filtering by an intermediate frequency filter 106. The downconvertor 104 uses a synthesizer, and can hence tune automatically to a received channel. Hence, a user does not have to adjust speakers manually at each turning on. The intermediate frequency radio signal IF is subsequently frequency demodulated by a frequency demodulator 110, resulting in an analogue left input audio signal LP and right input audio signal RP for the noise cancellation unit 124. In a prior art receiver, these signals are often send directly to a first amplifier 130 and a second amplifier 132 of a first loudspeaker 140 and a second loudspeaker 142. The FM receiver 100 example is presently described in an application in which those loudspeakers are a left surround loudspeaker and a right surround speaker, of a home cinema system. As is well known, sending a different audio signal to those speakers may result in the perception of e.g. a bundle of keys being dropped to the right of a user or in the perception of a helicopter flying overhead. If both audio signals are substantially equal, only a non-localized surround is perceived. However, user listener tests show that if the switch from different, i.e. stereo, audio signals to substantially equal, i.e. mono, audio signals occurs at only a few short instances of time, the perception of surround effects is not affected appreciably. The frequency demodulator 110 can be a standard IC, like e.g. the LAI 836 from Sanyo. It typically comprises an intermediate frequency amplifier 112. A signal level measuring unit 113 is present for measuring a signal level S of the intermediate frequency radio signal IF received, which may be embodied as e.g. a circuit measuring a voltage in an input stage of the intermediate frequency amplifier 112. This signal level S is dependent among other things on the distance of the receiver 100 to the transmitter, i.e. e.g. the distance of the wireless speaker to the base station at the position of a television, since the electromagnetic field strength drops with the distance. However the local field strength at the position of the receiver may also drop due to movement of persons in a room, which typically occurs for a few seconds. This is due to the fact that the electromagnetic fields reflect, absorb and diffract differently in the room dependent on the presence of persons. The frequency demodulator 110 further comprises a demodulator 114 and a stereo decoder 118.

An example of a typical modulated stereo signal is shown in Fig. 5. The sum of the left input audio signal LP and right input audio signal RP is frequency modulated between 0 kHz and e.g. 20 kHz. The difference signal is modulated on a subcarrier SC. A pilot tone PIL of half the subcarrier SC frequency is also transmitted, which can be used to lock the oscillator of the stereo decoder 118, but also for identifying the presence of a radio signal for the surround speakers, since it is typically chosen different from e.g. the 19 kHz pilot prescribed by the FCC standards organization.

Returning to Fig. 1, a reference voltage means 116 may also be present, e.g. yielding a stable DC reference voltage VREF of 3.6 Volt. An IC like the LAI 36M yields the left input audio signal LP and right input audio signal RP on a DC reference voltage VREF of 3.6 Volt. This reference voltage VREF is separately available on a pin of the IC, and can hence be used later on. A level scaling unit 120 may also be present, e.g. a potentiometer. It can be used to finetune during manufacture of the receiver 100 the switching voltage ST (see Fig. 2) below which the stereo to mono switching occurs. This is useful because characteristics tend to change from IC to IC. The behavior of the level scaling unit 120 can be formulated with the following transfer function in which k is a scaling constant predetermined during manufacture:

S'=kS [Eq. 1] The left input audio signal LP and right input audio signal RP are processed by a noise cancellation unit 124, yielding a left output audio signal LDN and a right output audio signal RDN, based upon the left input audio signal LP and right input audio signal RP. E.g., if a control variable C has a second value, the left output audio signal LDN is substantially equal to the left input audio signal LP and the right output audio signal RDN is substantially equal to the right input audio signal RP. If the control variable C has a first value, the left output audio signal LDN and the right output audio signal RDN are substantially equal to each other and substantially equal to half the sum of the left input audio signal LP and right input audio signal RP. It is important to note that this stereo to mono conversion is performed after stereo decoding, hence the noise cancellation unit can be embodied as a circuit connectable to the left and right audio pins of any commercially available frequency demodulator 110 IC, as well as being integrated in such an IC.

A control unit 122 determines when the stereo to mono conversion should occur. A control criterion may e.g. be: C = V1 if S' < VREF C = V2 if S' >= VREF [Eq. 2], in which VI is a first value and V2 a second value of the control variable C.

The left and right output audio signals LDN resp. RDN may be further processed. An expander 128, which can be any expander according to the state of the art, can be incorporated if the sent signal was first compressed, to achieve a higher dynamic range. A de-emphasis unit 160 may also be present to lower the high frequency noise. At the sender side, a pre-emphasis unit boosts frequencies above a predetermined frequency threshold, e.g. by means of a filter with a predetermined transfer function. The de-emphasis unit 160 reduces the power of these frequencies in the received signal by a corresponding amount, thereby reducing any noise accumulated in the chain from receiver up to the de-emphasis unit 160. In order to make the expander 128 work correctly, a multinotch filter 126 is present to remove the pilot PIL frequency and the double of the pilot frequency, being the subcarrier SC frequency. For very low signal levels S resp. S', a muter 150 provides hard muting. In these cases there is probably not much relevant audio signal and mostly noise, which is very audible because there is hardly any masking by the audio signal, hence the audio to the loudspeakers 140, 142 is switched off entirely.

The muter may also be arranged to mute if no pilot is detected. This avoids that the loudspeakers respond to and start reproducing other FM signals in the room, having another pilot frequency. Removal of the pilot can also be used to mute the speakers right before the signal of the sender is being turned off.

The system is typically comprised e.g. in a wireless audio speaker or speaker set, e.g. for surround signals. A receiver unit with amplifiers may be mounted in the back of the room, and from this receiver unit two cables may be attached to the surround speakers, carrying the left resp. right surround signal. Alternatively, in a more expensive variant each surround speaker may comprise its own receiver, and both the left and right surround speaker may receive the same wireless stereo channel, from which each uses the appropriate left resp. right audio signal. The proposed system may also be used for sending mono signals. In this case there is only audio information in the L+R band of Fig. 5, and not in the L-R bands. But there is still noise in the L-R bands, so the same system can be used to reduce the noise in case of mono transmission. Also the muting upon the detection of the absence of the stereo pilot PIL can be used in such a mono application. In this case there is only one loudspeaker connected, e.g. the first loudspeaker 140.

The system may also be comprised in a wireless headphone, e.g. for home cinema or guided tour applications. A single receiver 100 unit in the headphones is then arranged to send the left resp. right audio signals to the corresponding headphone speakers. The noise cancellation system is advantageous in such an application, since for headphones in addition to field perturbations due to people walking in the room, there are perturbations due to head movement of the person wearing the headphones. The receiver 100 can also be used in a wireless audio link for portable players -e.g. an MP3 player- or a mobile phone with audio capabilities. These can be used indoors or outdoors, e.g. for sports purposes.

Fig. 2 schematically shows a received radio signal and noise for different field strengths. The coordinate system shows on the x-axis a voltage as it could be measured under operation of the system on the antenna 102 input. During fine-tuning of the system, this would be a voltage imposed on the antenna input by a calibrated signal generator. On the y- axis are signals measured at the output of the receiver, e.g. before the amplifiers 130 and 132. The audio signal 202 is always maximal due to the high gain setting of the amplifiers in the receiver chain. However, in low field conditions, there is a lot of amplified noise, as can be seen in a typical original noise curve 204, as observed without the noise cancellation system. The antenna voltage U (HF in Fig. 1) and switching voltage ST can with the appropriate scaling by equated with the signal level S and reference voltage VREF. Each time low field is detected, i.e. an antenna voltage U below the switching voltage ST, or a signal level S resp. S' below the reference voltage VREF, the noise cancellation unit 124 switches from stereo to mono, thereby reducing the noise in its audio output signals LDN and RDN. An improved noise curve 206 is obtained with the noise cancellation system. Because the noise on the left and right input audio signals LP resp. RP is roughly 180 degrees out of phase, the noise in a mono signal is considerably reduced up to low antenna voltages equaling a muting voltage MT. The system is so fine-tuned by means of the level scaling unit 120 that an acceptable trade-off between the amount of remaining noise and the number of times and amount of time the system switches to mono is obtained. This can be seen by the bump 210. The switching voltage ST is so chosen that the noise can not rise above a predetermined level on the original noise curve 204. Below the muting voltage MT there is too much noise even with the noise cancellation system operable, hence the muter switches off the audio to the loudspeakers 140 and 142 entirely. For a fixed desired signal to noise level, or dynamic range of the audio, with the noise cancellation system a marked increase in useful distance of the wireless receiver can be achieved.

Fig. 3 shows a very simple circuit to realize the noise cancellation system. The level scaling unit is embodied in the voltage divider comprising a first resistor Rl and a second resistor R2. The signal level S' is imposed on the basis of a bipolar transistor 304, which has the reference voltage VREF on its emitter. Hence when

S'+ VT < VREF [Eq. 3], The bipolar conducts and hence there is a voltage equal to VREF on the gate of a FET 302, which conducts and hence provides a short circuit.

If S'+ VT > VREF [Eq. 4], transistor 304 is off and a zero ground voltage is imposed on the gate of the FET 302, in which case the input audio signals LP and RP are transmitted to the output of the noise cancellation unit 124, in which case stereo comes out. Hence the bipolar transistor 304 realizes the function of the control unit 122 and the FET 302 realizes the function of the noise cancellation unit 124.

VT is a bias voltage which is temperature dependent and e.g. 0.5 to 0.6 Volt. Since the switching point should be fixed, the temperature dependency is corrected for by a diode 306 or even better by a transistor with substantially identical characteristics to the transistor 304.

Eqs. 3 and 4 in the presence of a diode become:

S'+VT-Vdiode < VREF and S'+VT-Vdiode > VREF.

A fourth resistor R4 is present to minimalize distortion. Fig. 4 shows an alternative circuit in which an operational amplifier (opamp)

402 provides the same functionality as transistor 304. The positive input of the opamp is connected to the reference voltage VREF, and its negative input to the signal level S. Its positive supply voltage pin is connected to a supply voltage VCC, and its negative supply voltage pin to ground. At the output of the opamp there is a voltage equal to VREF or to 0 Volt. Resistor R44 is again to minimize distortion.

The noise cancellation system can also be realized digitally e.g. on a processor. In this case the input audio signals LP and RP would be digitized and e.g. equalized by a digital addition operation. Other units could also run on the processor, e.g. the expander 128. Under computer program product should be understood any physical realization of a collection of commands enabling a processor -generic or special purpose-, after a series of loading steps to get the commands into the processor, to execute any of the characteristic functions disclosed in the present invention. In particular the computer program product may be realized as data on a carrier such as e.g. a disk or tape, data present in a memory, data traveling over a network connection -wired or wireless- , or program code on paper.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention and that those skilled in the art are able to design alternatives, without departing from the scope of the claims. Apart from combinations of elements of the invention as combined in the claims, other combinations of the elements within the scope of the invention as perceived by one skilled in the art are covered by the invention.

In particular, a good embodiment of the noise cancellation system comprises the level scaling unit, reference voltage means and the muter with pilot tone detection, but the subelements may also be present in any other combination.

In the analogue FM receiver, the expander and/or the de-emphasis means may be present with any of the possible noise cancellation system configurations.

Any combination of elements can be realized in a single dedicated element. Any reference sign between parentheses in the claim is not intended for limiting the claim. The word "comprising" does not exclude the presence of elements or aspects not listed in a claim. The word "a" or "an" preceding an element does not exclude the presence of a plurality of such elements.

The invention can be implemented by means of hardware or by means of software running on a computer, and previously stored on a data carrier or transmitted over a signal transmission system.

Claims

CLAIMS:

1. A system for noise cancellation in analog FM radio reception, comprising: a signal level measuring unit (113) for measuring a signal level (S) of a received intermediate frequency radio signal (IF); a control unit (122) arranged to evaluate a predetermined criterion based on the signal level (S), the evaluation yielding a control variable (C) capable of having a first value and a second value; and a noise cancellation unit (124), arranged to output a left output audio signal (LDN) and a right output audio signal (RDN), based on a left input audio signal (LP) and a right input audio signal (RP), comprising an equalization unit (125) arranged to make the left output audio signal (LDN) substantially equal to the right output audio signal (RDN) if the control variable (C) equals the first value.

2. A system as claimed in claim 1, comprising a level scaling unit (120) for scaling the value of the signal level (S) with a predetermined scaling constant (k).

3. A system as claimed in claim 1, comprising a reference voltage means (116) arranged to supply a reference voltage (VREF).

4. A system as claimed in claim 1, comprising a muter (150), arranged to mute the left output audio signal (LDN) and the right output audio signal (RDN) if the signal level

(S) is below a predetermined mute value.

5. A system as claimed in claim 4, in which the muter (150) is arranged to mute the left output audio signal (LDN) and the right output audio signal (RDN) if a pilot carrier (PIL) is not detected.

6. An analogue FM receiver (100) comprising: a system as claimed in claim 1; and an expander (128).

7. An analogue FM receiver (100) comprising: a system as claimed in claim 1; and a de-emphasis unit (160) for reducing the power of frequencies above a predetermined frequency threshold.

8. A wireless audio speaker or audio speaker set comprising: an analogue FM receiver as claimed in claim 6 or 7; and an electro-acoustical transducer.

9. A wireless headphone comprising: an analogue FM receiver as claimed in claim 6 or 7; and a left and right electro-acoustical transducer.

10. A circuit realizing a noise cancellation unit (124) as claimed in claim 1, arranged to make the left output audio signal (LDN) substantially equal to the right output audio signal (RDN) if the control variable (C) equals the first value, by comprising a field effect transistor (302) arranged to provide a substantially short circuit, between a first connection (310) arranged to carry the left input audio signal (LP) and a second connection (312) arranged to carry the right input audio signal (RP), if the control variable (C) imposed on a gate of the field effect transistor (302) equals the first value.

11. A method for noise cancellation in analog FM radio reception, comprising: measuring a signal level (S) of a received intermediate frequency radio signal (IF); evaluating a predetermined criterion based on the signal level (S), the evaluation yielding a control variable (C) capable of having a first value and a second value; and generating a left output audio signal (LDN) and a right output audio signal (RDN), based on a left input audio signal (LP) and a right input audio signal (RP), the left output audio signal (LDN) being substantially equal to the right output audio signal (RDN) if the control variable (C) equals the first value.

12. A computer program product comprising commands enabling a processor to execute the method of claim 11.