CN107707321A - Stereo FM receiver and binaural separation method - Google Patents

Abstract

The invention relates to a stereo FM receiver and a two-channel sharing method. The stereo FM receiver comprises a FM demodulation unit for receiving a receiving signal and generating a demodulation signal according to the receiving signal; a frequency division demultiplexer for generating a sum signal, a difference signal and a pilot amplitude signal according to the demodulation signal; a binaural separation unit for generating a left channel output signal and a right channel output signal according to the sum signal and a weak difference signal; and a weakening unit for weakening the difference signal according to the pilot amplitude signal or a signal-to-noise ratio to generate the weakened difference signal.

Description

Stereo FM receiver and two-channel separation method

technical field

The invention relates to a stereo FM receiver and a dual-channel separation method, in particular to a stereo FM receiver and a dual-channel separation method capable of improving left and right channel isolation.

Background technique

Stereo-Phonic Frequency Modulation (Stereo-Phonic Frequency Modulation) system has been widely used in daily life. Generally, a stereo FM receiver uses a pilot signal to separate a left channel signal and a right channel signal from a demodulated signal. In fact, the signal quality of the pilot signal may appear weak due to external environmental factors, or the overall Signal to Noise Ratio (SNR) may appear poor due to external environmental factors, resulting in inaccurate stereo FM receivers. The stereo signal is shifted to the base frequency band accurately, but there is a frequency offset (Frequency Offset), so the stereo FM receiver cannot accurately separate the left channel signal and the right channel signal, thereby reducing the left and right channel isolation.

Therefore, how to reduce the impact caused by the frequency deviation and further improve the isolation between the left and right channels has become one of the goals of the industry.

Contents of the invention

Therefore, the main purpose of the present invention is to provide a stereo FM receiver capable of improving left and right channel isolation and a dual-channel separation method, so as to improve the shortcomings of the known technology.

The present invention discloses a stereo FM (Stereo-Phonic FM) receiver, comprising a FM demodulation unit for receiving a received signal and generating a demodulated signal according to the received signal; a frequency division demultiplexer for To generate a sum signal, a difference signal (Difference Signal) and a pilot amplitude signal according to the demodulation signal, wherein the sum signal is related to the sum of a left channel signal and a right channel signal, and the difference signal is related to The difference between the left channel signal and the right channel signal; a binaural separation unit for generating a left channel output signal and a right channel output signal according to the sum signal and a weakened difference signal (Weakened DifferenceSignal). and a weakening unit, coupled between the frequency division demultiplexer and the two-channel separation unit, used to weaken (Weaken) the difference signal to generate the weakened difference signal.

The present invention discloses another dual-channel separation method, which is applied to a stereo FM (Stereo-Phonic FM) receiver. The dual-channel separation method includes receiving a frequency division demultiplexer from the stereo FM receiver. sum signal, a difference signal (Difference Signal) and a pilot amplitude signal, wherein the sum signal is related to the sum of a left channel signal and a right channel signal, and the difference signal is related to the left channel signal and the right channel signal The difference between the channel signals; according to the pilot amplitude signal or a signal-to-noise ratio, the difference signal is weakened (Weaken) to generate a weakened difference signal (Weakened DifferenceSignal); and according to the sum signal and the weakened difference signal, a A left channel output signal and a right channel output signal.

Description of drawings

FIG. 1 is a block diagram of a stereo FM receiver according to an embodiment of the present invention.

FIG. 2 is a block diagram of a weakening unit according to an embodiment of the present invention.

FIG. 3 is a block diagram of a weakening unit according to an embodiment of the present invention.

FIG. 4 is a schematic diagram of a spectrum of a demodulated signal.

FIG. 5 is a block diagram of a frequency division demultiplexer according to an embodiment of the present invention.

FIG. 6 is a block diagram of a signal-to-noise ratio calculation unit according to an embodiment of the present invention.

FIG. 7 is a block diagram of an FM demodulation unit according to an embodiment of the present invention.

FIG. 8 is a flow chart of a binaural separation method according to an embodiment of the present invention.

Symbol Description

10 Stereo FM Receiver

100 FM demodulation unit

102 frequency division demultiplexer

104 two-channel separation unit

106, 206, 306 weakening unit

108 SNR calculation unit

260, 360 decision-making units

500 low pass filter

502 bandpass filter

504 Mixer

506 low pass filter

508 Pilot Filter

510 Multiplier

512 amplitude unit

600, 608 filters

602 Subtraction unit

604 SNR unit

606 units of magnitude

610 ratio unit

700 RF filter and amplifier unit

702 RF Mixer

704 IF filter and amplification unit

706 IF Mixer

708 demodulation unit

80 Two-channel separation method

800～808 steps

A Amplitude signal

A _pilot pilot amplitude signal

A _est amplitude estimate

c_wkn Weakening coefficient

c_sw switch control signal

m demodulated signal

n _est noise estimate

MP multiplier

LO_1, LO_2 oscillator signal

LUT Correspondence Table

L_out left channel output signal

R_BPF bandpass filter result

R_MX muxing result

R_out right channel output signal

r receive signal

SNR signal to noise ratio

SW switch

y _IF intermediate frequency signal

y _RF radio frequency signal

(L+R) and signal

(L－R) difference signal

(L－R)’ weakens the difference signal

|n _est | noise magnitude value

|n _ave | noise average

detailed description

Please refer to FIG. 1 , which is a block diagram of a stereo FM receiver 10 according to an embodiment of the present invention. The stereo FM receiver 10 includes a FM demodulation unit 100 , a frequency division demultiplexer 102 , a binaural separation unit 104 , a weakening unit 106 and a SNR calculation unit 108 . The FM demodulation unit 100 receives a received signal r, and generates a demodulated signal m and an amplitude signal A according to the received signal r; the frequency division demultiplexer 102 receives the demodulated signal m, and generates a sum and signal (L+R), a difference signal (Difference Signal) (L−R), and a pilot amplitude signal A _pilot . The signal-to-noise ratio calculation unit 108 receives the amplitude signal A, and generates a signal-to-noise ratio SNR according to the amplitude signal A. Wherein, the sum signal (L+R) is received by the stereo FM receiver 10 and is related to the sum of a left channel signal L and a right channel signal R, and the difference signal (L-R) is the sum of the stereo FM receiver 10 The difference between the left channel signal L and the right channel signal R is received and correlated. In addition, the amplitude signal A corresponds to an amplitude of the received signal r, and the pilot amplitude signal A _pilot corresponds to an amplitude of a pilot signal (PilotSignal) x _p in the demodulated signal m.

The weakening unit 106 is coupled between the frequency division demultiplexer 102 and the two-channel separation unit 104, and is used to receive the difference signal (L-R) and weaken (Weaken) the energy/amplitude of the difference signal (L-R) , and then generate a weakened difference signal (WeakenedDifference Signal) (L-R)'. The two-channel separation unit 104 generates a left-channel output signal L_out and a right-channel output signal R_out according to the sum signal (L+R) and the weakened difference signal (L-R)', wherein the left-channel output signal L_out It can be expressed as L_out=(L+R)+(L-R)', and the right channel output signal R_out can be expressed as R_out=(L+R)-(L-R)'.

In detail, the weakening unit 106 can generate a weakening coefficient c_wkn according to the pilot amplitude signal A _pilot- or the signal-to-noise ratio SNR, and multiply the difference signal (L-R) by the weakening coefficient c_wkn to generate a weakened difference signal (L- R)'. Please refer to FIG. 2 , which is a block diagram of a weakening unit 206 according to an embodiment of the present invention. The weakening unit 206 is used to realize the weakening unit 106, which includes a decision-making unit 260 and a multiplier MP. The decision-making unit 260 can receive the pilot amplitude signal A _pilot- and the signal-to-noise ratio SNR, which can be based on the pilot amplitude signal A _pilot- or signal-to-noise ratio SNR to generate a weakening coefficient c_wkn, and the multiplier MP is used to multiply the difference signal (L-R) by the weakening coefficient c_wkn to generate a weakening difference signal (L-R)', where the weakening difference signal (L-R )' can be expressed as (L-R)'=c_wkn×(L-R), and the value of the weakening coefficient c_wkn can be between 0 and 1. It should be noted that the method for the decision-making unit 260 to generate the weakening coefficient c_wkn according to the pilot amplitude signal A _pilot- or the signal-to-noise ratio SNR is not limited. In one embodiment, when the pilot amplitude signal A _pilot- is less than a threshold When the limit value TH is reached, the decision unit 260 can generate the weakening coefficient c_wkn to be 0, and the left channel output signal L_out and the right channel output signal R_out are both sum signals (L+R). In another embodiment, the decision-making unit 260 can generate the weakening coefficient c_wkn according to the signal-to-noise ratio SNR. Specifically, the decision-making unit 260 can store a correspondence table LUT (Look Up Table), and the correspondence table LUT stores the signal-to-noise ratio SNR The corresponding relationship with the weakening coefficient c_wkn, when the decision-making unit 260 receives the signal-to-noise ratio SNR, the decision-making unit 260 can find out the weakening coefficient c_wkn corresponding to the signal-to-noise ratio SNR from the corresponding table LUT, and output the weakening coefficient c_wkn to the multiplication Device MP. The corresponding relationship between the signal-to-noise ratio SNR and the weakening coefficient c_wkn may be adjusted depending on system requirements or actual conditions, and is not limited to a specific corresponding relationship. For example, please refer to Table I. When the signal-to-noise ratio SNR is greater than 30dB, the decision-making unit 260 can output a weakening coefficient c_wkn of 0.707; when the signal-to-noise ratio SNR is less than 18dB, the decision-making unit 260 can output a weakening coefficient c_wkn of 0.054; when the signal-to-noise ratio SNR is between 30dB and 18dB, the weakening coefficient output by the decision-making unit 260 The coefficient c_wkn may decrease as the signal-to-noise ratio SNR decreases.

Table I

It should be noted that the weakening unit is not limited to the way shown in FIG. 2 , please refer to FIG. 3 , which is a block diagram of a weakening unit 306 according to an embodiment of the present invention. The weakening unit 306 is similar to the weakening unit 206, so the same elements continue to use the same symbols. The difference from the weakening unit 206 is that the weakening unit 306 further includes a switch SW. When the pilot amplitude signal A _pilot- is less than the threshold value TH, the weakening unit A decision unit 360 included in 306 generates a switch control signal c_sw so that the switch SW is cut off (Cutoff), and the difference signal (L-R) at this time cannot be transmitted to the two-channel separation unit 104, so the left channel output signal L_out and The right channel output signal R_out is both a sum signal (L+R); when the pilot amplitude signal A _pilot- is greater than the threshold value TH, the decision-making unit 360 generates a switch control signal c_sw so that the switch SW is conducted (Conducted), and the decision-making The unit 360 can generate the weakening coefficient c_wkn according to the signal-to-noise ratio SNR, and its operation method is similar to that of the decision-making unit 260 , which will not be repeated here.

In addition, the signal components and spectrum of the demodulated signal m will be described in detail below. For a stereo FM system, (ignoring the channel response and noise) the time function of the demodulated signal m can be expressed as Among them, L(t) and R(t) respectively represent the time function of the left channel signal L and the right channel signal R, x _p (t) is the time function of the pilot signal x _p , and the pilot signal x _p (t) can be expressed as A _p , f _p and Represent the amplitude, frequency and phase of the pilot signal x _p (t), L(t)+R(t) represents the time function of the sum signal (L+R), and L(t)-R(t) represents the difference signal (L-R) as a function of time. In addition, the spectrum of the demodulated signal m(t) can be shown in Figure 4. Generally speaking, the spectrum of L(t)+R(t) is roughly located at 0-15KHz, and the spectrum of the pilot signal x _p (t) is roughly at 19KHz, The frequency spectrum is roughly located at 23KHz ~ 53KHz.

For details of the frequency division demultiplexer 102 , please refer to FIG. 5 , which is a block diagram of the frequency division demultiplexer 102 according to an embodiment of the present invention. The frequency demultiplexer 102 includes a low-pass filter 500, a band-pass filter 502, a mixer 504, a low-pass filter 506, a pilot filter 508, a frequency multiplier 510 and an amplitude unit 512. The low-pass filter 500 is used to filter out the sum signal L(t)+R(t) at 0KHz˜15KHz from the signal m(t) to obtain the sum signal (L+R). In addition, the pilot filter 508 is used to filter out the pilot signal x _p (t) at 19 KHz from the signal m(t) to obtain the pilot signal x _p , the amplitude unit 512 receives the pilot signal x _p , and Generate a pilot amplitude signal A _pilot corresponding to the pilot signal _xp , wherein the amplitude unit 512 can use a coordinate rotation digital computer (Coordinate Rotation Digital Computer, _CORDIC ) algorithm to obtain the pilot amplitude signal A according to the pilot signal xp _pilot , but not limited to this. In addition, the band-pass filter 502 is used to convert the 23KHz～53KHz signal Filter out from the signal m(t) and generate the bandpass filtered result R_BPF. In addition, the frequency division demultiplexer 102 uses the frequency multiplier 510 to generate a double frequency signal (approximately at 38KHz) of the oscillating pilot signal x _p ; multiplied to generate a mixing result R_MX; the high-frequency components in the mixing result R_MX are filtered out by the low-pass filter 506 to obtain a difference signal (L−R). In other words, mixer 504 and low-pass filter 506 will Move to a fundamental frequency band to obtain the difference signal (L-R).

In addition, for details of the SNR calculation unit 108 , please refer to FIG. 6 , which is a block diagram of the SNR calculation unit 108 according to an embodiment of the present invention. The SNR calculation unit 108 includes a filter 600 , a subtraction unit 602 and a SNR unit 604 . The filter 600 is a low-pass filter for receiving the amplitude signal A, and performing a low-pass filtering/averaging operation on the amplitude signal A, which can filter out high-frequency noise components in the amplitude signal A to generate an amplitude estimation Measured value A _est . The subtraction unit 602 is used for subtracting the estimated amplitude value A _est from the amplitude signal A to generate an estimated noise value n _est . The signal-to-noise energy ratio unit 604 may include a magnitude unit 606, a filter 608, and a ratio unit 610. The magnitude unit 606 is used to perform a magnitude operation on the noise estimation value n _est to obtain a The noise level value |n _est |, the filter 608 is also a low-pass filter for performing a low-pass filtering/averaging operation on the noise level value |n _est | to obtain a noise average value |n _ave | , the ratio unit 610 receives the estimated amplitude value A _est and the noise average value | _nave | to calculate the signal-to-noise ratio SNR, wherein the signal-to-noise ratio SNR can be expressed as SNR=|A _est | ² /| _nave | ² . In this way, the SNR calculation unit 108 outputs the calculated SNR to the weakening unit 106, and the weakening unit 106 can generate the weakening coefficient c_wkn according to the SNR.

In addition, for details of the FM demodulation unit 100 , please refer to FIG. 7 , which is a block diagram of the FM demodulation unit 100 according to an embodiment of the present invention. The FM demodulation unit 100 can be a superheterodyne receiver (Superheterodyne Receiver), which includes a radio frequency filter and amplifying unit 700, a radio frequency mixer 702, an intermediate frequency filtering and amplifying unit 704, an intermediate frequency mixer 706 and A demodulation unit 708 . The radio frequency filtering and amplifying unit 700 is used to filter the signal component of the received signal r in a radio frequency (Radio Frequency) band from the received signal r, and amplify the signal component of the radio frequency band to generate a radio frequency signal y _RF . The RF mixer 702 and the IF filtering and amplifying unit 704 use a first oscillating signal LO_1 to shift the RF signal y _RF in the RF band to an Intermediate Frequency (Intermediate Frequency) band to generate an Intermediate Frequency signal y _IF . In addition, the IF mixer 706 uses a second oscillating signal LO_2 to shift the IF signal y _IF in the IF band to a base frequency band, and the demodulation unit 708 can generate the demodulated signal m and the amplitude signal A accordingly.

It should be noted that when the signal quality of the pilot signal x _p is poor (that is, the pilot amplitude signal A _pilot- is less than the threshold value TH) or the signal-to-noise ratio SNR is poor, the signal It cannot be accurately shifted to the base frequency band, but there is a frequency deviation. In this case, the present invention uses the weakening unit 106 to timely weaken (or even block) the difference signal (L-R) when the signal quality of the pilot signal x _p is poor or the signal-to-noise ratio (SNR) is poor, so as to reduce ( The influence of the difference signal (L-R) with frequency deviation on the sum signal (L+R) further improves the left and right channel isolation. It has been proved by experiments that the stereo FM receiver 10 can achieve a left and right channel isolation of 40.6 dB, while the known stereo FM receiver can only achieve a left and right channel isolation of 20 dB. In other words, the stereo FM receiver 10 can achieve better left and right channel isolation.

Those skilled in the art should know that the functional units/circuits in FIGS. 1 to 3 and FIGS. 5 to 7 can be realized or implemented by digital circuits (such as RTL circuits) or a digital signal processor (Digital Signal Processor, DSP), No more details here.

The operation details of the stereo FM receiver 10 can be further summarized as a two-channel separation process. Please refer to FIG. 8 , which is a flowchart of a method 80 for separating two channels according to an embodiment of the present invention. The two-channel separation method 80 can be performed by the stereo FM receiver 10, and the two-channel separation method 80 includes the following steps:

Step 800: The FM demodulation unit 100 generates a demodulated signal m according to the received signal r.

Step 802: The frequency division demultiplexer 102 generates a sum signal (L+R), a difference signal (L−R) and a pilot amplitude signal A _pilot according to the demodulated signal m.

Step 804: The weakening unit 106 generates a weakening coefficient c_wkn according to the pilot amplitude signal A _pilot- or the signal-to-noise ratio SNR.

Step 806: The weakening unit 106 multiplies the difference signal (L-R) by the weakening coefficient c_wkn to generate a weakened difference signal (L-R)'.

Step 808: The two-channel separation unit 104 generates a left-channel output signal L_out and a right-channel output signal R_out according to the sum signal (L+R) and the weakened difference signal (L-R)'.

For the details of the operation of the binaural separation method 80 , please refer to the relevant paragraphs above, which will not be repeated here.

From the above, it can be seen that the present invention can timely weaken the difference signal when the signal quality of the pilot signal is poor or the signal-to-noise ratio is poor, reduce the influence of the difference signal with frequency deviation, and further improve the left and right channel isolation.

The above descriptions are only preferred embodiments of the present invention, and all equivalent changes and modifications made according to the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims (13)

1. A stereo FM receiver comprising: A FM demodulation unit, used to receive a received signal, and generate a demodulated signal according to the received signal; A frequency division demultiplexer is used to generate a sum signal, a difference signal and a pilot amplitude signal according to the demodulation signal, wherein the sum signal is related to the sum of a left channel signal and a right channel signal, The difference signal is related to the difference between the left channel signal and the right channel signal; A two-channel separation unit is used to generate a left channel output signal and a right channel output signal according to the sum signal and a weakened difference signal; and A weakening unit, coupled between the frequency division demultiplexer and the two-channel separation unit, used to weaken (Weaken) the difference signal according to the pilot amplitude signal or a signal-to-noise ratio, so as to generate the weakening bad signal. 2. The stereo FM receiver according to claim 1, wherein the weakening unit comprises a switch coupled between the frequency division demultiplexer and the two-channel separation unit, wherein when the pilot When the amplitude signal is smaller than a threshold value, the weakening unit controls the switch to be turned off. 3. The stereo FM receiver as claimed in claim 1, wherein the weakening unit comprises: a decision unit, used to generate a weakening coefficient according to the pilot amplitude signal or the signal-to-noise ratio; and A multiplier is used for multiplying the difference signal by the weakening coefficient to generate the weakened difference signal. 4. The stereo FM receiver as claimed in claim 3, wherein when the pilot amplitude signal is smaller than a threshold value, the weakening unit generates the weakening coefficient as 0. 5. The stereo FM receiver as claimed in claim 3, wherein the attenuation factor decreases as the signal-to-noise ratio decreases. 6 . The stereo FM receiver as claimed in claim 3 , wherein a correspondence table is stored in the decision-making unit, and the correspondence table stores a correspondence relationship between the signal-to-noise ratio and the attenuation coefficient. 7. The stereo FM receiver according to claim 1, further comprising a signal-to-noise ratio calculation unit coupled to the FM demodulation unit for generating an amplitude signal according to the FM demodulation unit , to calculate the signal-to-noise ratio, the signal-to-noise ratio calculation unit includes: a filter for receiving the amplitude signal and generating an amplitude estimate; a subtraction unit for subtracting the amplitude estimate from the amplitude signal to generate a noise estimate; and A signal-to-noise energy ratio unit is used for calculating the signal-to-noise ratio according to the estimated amplitude value and the estimated noise value. 8. The stereo FM receiver as claimed in claim 1, wherein the FM demodulation unit comprises a superheterodyne receiver. 9. The stereo FM receiver as claimed in claim 1, wherein the frequency division demultiplexer comprises: a first low-pass filter for receiving the demodulated signal and generating the sum signal; A frequency multiplier, used to generate a frequency multiplied signal according to an oscillating signal; a band-pass filter for receiving the demodulated signal and generating a band-pass filtering result; a mixer, used to generate a mixing result according to the multiplied signal and the bandpass filtering result; a second low-pass filter for receiving the mixing result and generating the difference signal; and An amplitude unit is used to generate the pilot amplitude signal according to the pilot signal. 10. A two-channel separation method applied to a stereo FM receiver, the two-channel separation method comprising: A sum signal, a difference signal and a pilot amplitude signal are received from a frequency division demultiplexer of the stereo FM receiver, wherein the sum signal is related to the sum of a left channel signal and a right channel signal, the The difference signal is related to the difference between the left channel signal and the right channel signal; weakening the difference signal based on the pilot amplitude signal or a signal-to-noise ratio to generate a weakened difference signal; and According to the sum signal and the weakened difference signal, a left channel output signal and a right channel output signal are generated. 11. The two-channel separation method according to claim 10, wherein, according to the pilot amplitude signal or the signal-to-noise ratio, weakening the difference signal to generate the weakened difference signal comprises: generate a weakening coefficient according to the pilot amplitude signal or the signal-to-noise ratio; and The difference signal is multiplied by the weakening coefficient to generate the weakened difference signal. 12. two-channel separation method as claimed in claim 11, is characterized in that, also comprises: When the pilot amplitude signal is smaller than a threshold value, the weakening coefficient is 0. 13. The binaural separation method as claimed in claim 11, wherein the attenuation coefficient decreases as the signal-to-noise ratio decreases.