JPS6237855B2 - - Google Patents

Description

[Detailed Description of the Invention] There is AM stereo broadcasting in which stereo broadcasting is performed using one AM broadcast wave. As one method of AM stereo broadcasting, a sum signal L+ of a stereo left channel signal L and a right channel signal R is used.
There is a method of AM modulating the carrier with R and FM modulating the carrier with the difference signal L-R.

Therefore, such an AM stereo broadcast receiver is configured as shown in FIG. 1, for example.

That is, in FIG. 1, 1 is an antenna tuning circuit, 2 is a mixer circuit, 3 is a local oscillation circuit, 4 is its resonance circuit, 5 is an intermediate frequency amplifier, and 6 is an AM detection circuit. Sum signal L+
R is taken out, and this signal L+R is the difference signal L−R
The signal is supplied to the matrix circuit 8 through the delay circuit 7 for delay time correction.

Further, the intermediate frequency signal from the intermediate frequency amplifier 5 is set to a constant amplitude by a limiter 11, and then supplied to the FM demodulation circuit 12, where the difference signal LR is demodulated. In this case, the difference signal L-R is transmitted at, for example, 4000 Hz (time constant τ
= 400 μsec) is pre-emphasized at a rate of 6 dB/oct, this demodulated difference signal L-R is supplied to the de-emphasis circuit 13 to have a flat frequency characteristic, and this de-emphasized difference signal LR is supplied to the matrix circuit 8.

Therefore, in the matrix circuit 8, the signal L
Since +R and LR are matrixed, terminal 9
Signals L and R are taken out to L and 9R.

In this way, the receiver shown in FIG. 1 performs stereo reproduction.

By the way, in this case, a ratio detection circuit, a PLL, etc. are generally used as the FM demodulation circuit 12, but these FM demodulation circuits have a finite peak separation of S-shaped characteristics, and it is difficult to detect the carrier from the demodulated output. Due to the time constant of a low-pass filter for removing the signal, a time delay occurs in the signal L-R, and this time delay increases with the signal frequency.

For this reason, in the receiver shown in Fig. 1, the delay circuit 7 corrects the time difference between the signals L+R and LR, but this is not necessarily sufficient, and for this reason, the delay circuit 7 corrects the time difference between the signals L+R and L-R. A phase difference is created between -R and
Channel separation between signals L and R deteriorates.

Also, if the AM suppression degree of limiter 11 is small,
A phenomenon occurs in which the intermediate frequency signal is interrupted during overmodulation or a weak electric field, and at this time, limiter noise is generated and a large burst sound is generated. This burst sound also occurs when the VCO loses lock even when a PLL is used.

This invention attempts to solve these problems.

Therefore, in the present invention, for example, as shown in FIG. 3, the intermediate frequency signal from the limiter 11 is supplied to the PLL 20, and its phase comparison output is supplied to the matrix circuit 8. That is, limiter 1
1 is supplied to the phase comparator circuit 21, and the oscillation output of the VCO 22 is supplied to the comparator circuit 2.
1, and its comparison output Sc is supplied as a control signal to the VCO 22 through the low-pass filter 23 to configure the PLL 20.

In this case, the cutoff frequency of the filter 23 is made sufficiently low, and the damping factor of the PLL 20 (usually about 0.7) is made sufficiently large, for example, 10 or more, so that it approaches the primary loop characteristic, and the pole on the high frequency side Make it equal to the pre-emphasis time constant τ.

Then, the comparison output Sc of the comparison circuit 21 is supplied as a difference signal LR to the matrix circuit 8, and the sum signal L+R from the detection circuit 6 is supplied to the matrix circuit 8 as it is.

According to this configuration, the loop gain of the PLL 20 causes the VCO 22 to follow the intermediate frequency signal at frequencies below the time constant τ, but not at frequencies above the time constant τ, so the signal
The signal Sc is a difference signal LR and also has a de-emphasis characteristic corresponding to the pre-emphasis characteristic shown in FIG. 2. In other words, the signal Sc is a difference signal LR subjected to de-emphasis.

Therefore, signals L and R are taken out to terminals 9L and 9R.

In this case, since the difference signal L-R has not passed through a circuit having frequency characteristics, the difference signal L
Since there is no time delay in -R and therefore no phase difference between signal L+R, channel separation between signals L and R will not deteriorate. Further, the delay circuit 7 for the sum signal L+R is also unnecessary.

Furthermore, since the response frequency of the PLL 20 is low, even if the intermediate frequency signal is interrupted due to overmodulation or a weak electric field, the PLL 20 will not lose lock, and therefore no burst sound will occur. However, the configuration for this purpose is extremely simple and there is no cost increase.

[Brief explanation of the drawing]

1 and 2 are diagrams for explaining this invention, and FIG. 3 is a system diagram of an example of this invention. 6 is an AM detection circuit, 8 is a matrix circuit, 2
0 is PLL.

Claims (1)

[Claims] 1 The carrier is the sum signal L+R of the stereo left channel signal L and right channel signal R.
In an AM stereo receiver that receives a broadcast wave whose carrier is FM modulated by a difference signal L-R between the AM-modulated and pre-emphasized signals L and R, an intermediate frequency signal with a constant amplitude and a VCO are used. Compare the phase with the oscillation signal and send the phase comparison output to the above VCO through a low-pass filter.
is supplied as a control signal to form a PLL, and the high-frequency side pole of this PLL is made to correspond to the time constant of the pre-emphasis, and the sum signal L+R obtained by AM detection of the phase comparison output and the intermediate frequency signal is obtained. An AM stereo receiver that obtains the above signals L and R from.