JPS6214975B2 - - Google Patents

Description

[Detailed description of the invention] The present invention relates to an AM stereo receiver.

In the AM stereo system, the phase modulation signal obtained by quadrature modulating the left signal L and right signal R is L + R.
The signal S _O of the AM stereo system, which is amplitude-modulated by the signal, is generally expressed as shown in equation (1).

S _O = (1 + L + R) cos (ωt + φ) ... (1) = cos φ { (1 + L + R) cos ωt - (L - R + P) sin ωt} ... (2) = cos φ・S _A ... (3) Here, ω :Carrier angular frequency P: Pilot signal (5~25Hz) φ=tan ^-1 L-R+P/1+L+R...(4) S _A = (1+L+R) cosωt-(L+R +P) sinωt...(5) That is, this AM The stereo signal S _O includes monaural information in the amplitude term and stereo information in the carrier phase term.

As a receiver for such an AM stereo signal S _O , the one shown in FIG. 1 has been proposed.

In the figure, 1 is a high frequency amplification circuit, 2 is an intermediate frequency amplification circuit, and this intermediate frequency output S _IF is passed through a narrow band band pass filter (IF filter) 20 whose bandwidth is selected to be approximately 15 to 20 KHz. After being band-limited, it is supplied to a divider 3, and the resulting audio signal component S _A is supplied to a pair of periodic detectors 4 and 5.

A signal including the left signal L is detected by one of the periodic detectors 4, and the left signal L is extracted by the low-pass filter 6. Similarly, the other synchronous detector 5 detects a signal including the right signal R, and the subsequent low-pass filter 7 extracts only the right signal R. 8 and 9 indicate amplifiers.

A correction signal cosφ having phase information serving as a divisor is supplied to the divider 3, and a pair of periodic detectors 4,
5 performs periodic detection, one periodic detector 4 is supplied with a reference signal cos (ωt+π/4) whose phase is advanced by π/4 with respect to the audio carrier, and the other synchronous detector 5 is supplied with a reference signal cos (ωt+π/4) whose phase is advanced by π/4. A reference signal cos(ωt-π/4) whose phase is delayed by /4 is supplied.

Therefore, part of the intermediate frequency signal _SIF is supplied to the limiter 11 and the PLL circuit 12. PLL circuit 1
2 has a variable oscillator, for example a VCO, as is well known, and the VCO is controlled by the phase comparison output of this VCO output and the intermediate frequency output _SIF .

The VCO output is supplied to a pair of phase shifters 14 and 15 to form the above-mentioned reference signal, and is further supplied to a phase shifter 16 to form a zero-phase reference signal cosω.
t is formed. This reference signal cosωt and the above-mentioned limiter output cos(ωt+φ) are supplied to a multiplier 17, and among the outputs, a low-pass filter 18
Only the phase information signal cosφ is extracted by. Therefore, in the divider 3, S _IF /cosφ=cosφ・S _A /cosφ=S _A
...(6) The following division processing will be performed.

Now, the frequency spectrum of the AM stereo signal S _O as expressed by equation (1) is distributed over a wide range as shown in Figure 2A, so if the signal as it is is frequency-converted to an intermediate frequency signal and demodulated, Channel interference occurs, and because of wideband demodulation, noise components increase and the S/N ratio deteriorates.

Therefore, normally this should not happen
Although the IF filter 20 is provided to limit the band to a narrow band, this naturally degrades the distortion rate and separation.

Therefore, the present invention is devised so that the distortion rate and separation will not deteriorate even if the signal band is limited in order to prevent interference in adjacent channels and improve the S/N ratio.

Therefore, in the present invention, the AM stereo signal, in this example, the intermediate frequency signal _SIF , is first converted into a signal with a narrow frequency spectrum band distribution, and then the band is limited by the IF filter 20.

FIG. 3 is a system diagram of an example of the present invention, in which a divider 3 is provided between the intermediate frequency amplification circuit 2 and the IF filter 20, and signal processing as shown in equation (6) is performed. Correction signal S _D (=cosφ) serving as divisor
is formed based on the output S _B of the IF filter 20. The means for forming the correction signal S _D is exactly the same as that shown in FIG. 1, except that the output of the IF filter 20 is used to form the correction signal S D. Therefore, the same parts are given the same reference numerals and their explanation will be omitted.

Now, when the AM stereo receiver is configured like this, the intermediate frequency output S _IF is S _IF = (1 + L + R) cos (ωt + φ) ... (7) = cos φ {(1 + L + R) cos ωt - (L - R) sin ωt} ...(8) However, φ=tan ^-1 L-R/1+L+R, and if this output S _IF is supplied to the divider 3 for division processing, the output S _A is S _A = (1+L+R )cosωt−(L−R)sinωt
...(9) becomes.

The output S _A expressed as equation (9) is a carrier (cosω
This is nothing but the output obtained by adding the signals after balanced modulation of the signals (t, sinωt), that is, the orthogonal modulation signal. Since quadrature modulation is phase modulation, the frequency spectrum of quadrature modulation signal S _A is as shown in FIG. 2B.

In this way, if the intermediate frequency output S _IF is once converted into the orthogonal modulation signal S _A and then supplied to the IF filter 20, even if the band is limited to about 15 KHz,
There is no sideband component of the orthogonal modulation signal S _A in the frequency band around 15 KHz, or even if it exists, its level is extremely small, so it is not affected by the band limitation.

Therefore, the output S _B that has passed through the IF filter 20 has almost no change, and S _B = (1+L+R) cosωt-(L-R) sinωt=√(1++) ² + (-) ²・cos(ωt +φ' ) ...(10) However, φ'=tan ^-1 LR/1+L+R Therefore, φ'=φ.

Therefore, the limiter output obtained from the limiter 11 is cos (ωt + φ') = cos (ωt + φ), and the reference signal obtained from the PLL circuit 12 is
Since cosωt, the output S _C of the multiplier 17 is S _C = cos (ωt + φ)・cos ωt = 1/2 {cos (2ωt + φ) + cosφ} ...(11) This output S _C is filtered by the low-pass filter 18 by 2ω
Since the frequencies above are cut, the output S _D
As follows, S _D =cosφ...(12) is obtained. That is, even if the correction signal S _D is formed based on the output S _B that has passed through the IF filter 20,
A correction signal cosφ having phase information is obtained as in the conventional case.

As explained above, according to the present invention, even if the IF filter 20 is inserted to limit the signal band in order to prevent adjacent channel interference and improve S/N, the frequency distribution of the divided output S _{A is} Since it is narrow, most of the sideband components of the division output S _A can be passed through without being omitted, so there is almost no deterioration in distortion rate or separation.

Note that in the embodiment described above, the divider 3 may be provided before the intermediate frequency amplification circuit 2. The synchronous detection method is not limited to the example shown in the figure.

[Brief explanation of the drawing]

FIG. 1 is a system diagram showing an example of a conventional AM stereo receiver, FIG. 2 is a frequency spectrum diagram, and FIG. 3 is a system diagram showing an example of the AM stereo receiver according to the present invention. 2 is an intermediate frequency amplification circuit, 3 is a divider, 20 is
IF filter, 12 is a PLL circuit, and 11 is a limiter.

Claims (1)

[Claims] 1. When the left side signal is L and the right side signal is R, (1+L+R)cos(ωt+φ), where φ=tan ^-1 L-R/1+L+R. In the machine, instead of installing an intermediate frequency filter immediately after the intermediate frequency amplification circuit, a dividing circuit is provided, and an IF filter is provided to receive the output of the dividing circuit, and an IF filter is provided to limit the band of the output. An AM stereo receiver comprising: a circuit for generating a correction signal cosφ based on the output of the AM stereo receiver; and adding the correction signal cosφ to the division circuit to divide the AM stereo signal.