JPS6333823B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION When one of a plurality of different and close frequencies arrives, the present invention determines its presence and at the same time clearly and reliably detects and distinguishes from the case where the frequency signal does not exist. The purpose of the present invention is to provide a circuit suitable for obtaining output for display.

Since the year before last, an audio multiplexing system has been developed and broadcast in our country for television broadcasting. In this broadcasting station, in addition to regular monaural broadcasting, three modes of broadcasting are performed based on the program: stereo broadcasting and heterogeneous broadcasting. When receiving this on a television receiver, etc., the transmitting side does not do anything special for monaural broadcasts and continues as usual, but for stereo broadcasts and different types of broadcasts, it is 55.125 so that they can be distinguished. It is designed to convert the stereo pilot signal (982.5Hz) amplitude-modulated to KHz and the different broadcast pilot signal (922.5Hz) into FM and transmit them.

Therefore, on the receiver side, after FM demodulating these signals through a normal television audio intermediate frequency amplification circuit, the receiver receives a stereo pilot signal of 982.5 Hz or a different broadcast pilot signal of 922.5 Hz modulated by a 55.125 KHz carrier wave. At the same time, this signal is used to automatically switch the necessary circuits for each type of broadcast.

As a circuit for determining whether it is a stereo broadcast or a different type of broadcast, a circuit system using two lead filters, which has been most commonly used in the past, will be described with reference to FIG.

The output of the AM demodulator 1 enters rectifier circuits 4 and 5 via 922.5 Hz and 982.5 Hz reed filters (tuning fork resonance filters) 2 and 3, respectively. Rectifier circuit 4,
The configuration is such that an OR circuit 6 receives the outputs of each of the rectifying circuits 4, 5, and the OR circuit 6 as inputs, and the three outputs of the rectifying circuits 4, 5 and the OR circuit 6 serve as circuit outputs. The operation of the circuit having such a configuration will be explained below. Amplitude modulated pilot signal
922.5Hz or 922.5Hz by AM detection with AM demodulator 1
Take out one of the 982.5Hz pilot signals,
This is added to the two lead filters 2 and 3. Since the characteristics of lead filters 2 and 3 have signal passbands of approximately 922.5Hz±2Hz and 982.5Hz±2Hz,
Only these two signals are allowed to pass. Therefore, the obtained output can be immediately determined to be one of the two signals, and by rectifying this signal through the rectifier circuits 4 and 5, it is possible to determine whether it is a stereo broadcast or a different type of broadcast. In addition, rectifier circuits 4 and 5
It can also be determined whether the broadcast is monaural by passing the output through the OR circuit 6.

However, these reed filters 2 and 3 are expensive compared to other components (usually small signal transistors).
In addition to being 10 times more expensive, it also has the disadvantage of causing howling when used with audio equipment (such as speakers) that generates large vibrations.

In order to improve the above drawbacks, a method was devised in which no reed filter was used at all.

Next, we will introduce a conventional circuit that does not use a lead filter.
This will be explained using figures.

The output of the AM demodulator 16 is output to the phase comparator 17,1
2, and the output of the phase comparator 17 is passed through the loop filter 18 to the voltage controlled oscillator 1.
Control 9. The output of the voltage controlled oscillator 19 is irregularly shaped via the waveform shaping circuit 10 and sent to the phase comparator 1.
7 and the input of the 90° phase shifter 11. The output of the 90° phase shifter 11 enters the other input of the phase comparator 12, the output of which enters the Schmitt circuit 15' via the rectifier circuit 13. On the other hand, from the connection point between the loop filter 8 and the voltage controlled oscillator 9, the Schmitt circuit 1 is connected via the amplitude limited amplifier 14.
Enter 5.

With this circuit configuration, the 922.5 Hz or 982.5 Hz pilot signal detected by the AM demodulator 16 is added as an input signal to one of the phase comparators 17 and 12. The oscillation frequency of the voltage-controlled oscillator 19 is voltage-controlled by the output obtained by converting the output of the phase comparator 17 into direct current using the loop filter 18 . The free running frequency of this voltage controlled oscillator 19 is as described above.
Any frequency between 922.5 Hz and 982.5 Hz should suffice, but considering the drift of the free-running frequency, it is preferable to set it near the center of the two frequencies, 952.5 Hz.

The waveform shaping circuit 10 is a circuit that shapes the output waveform of the voltage controlled oscillator 19 for the purpose of increasing the comparison sensitivity when applying it to the phase comparator 17, and may not be necessary in principle. Thus, the phase comparator 17, the loop filter 18, the voltage controlled oscillator 19, and the waveform shaping circuit 10 have a phase controlled by the pilot signal frequency (usually a phase lock loop).
The normally known capture range (pulling frequency range) and the lock range (holding frequency range) are input to the phase comparator 17.
The PLL is designed with a capture range that is sufficient to detect two different frequencies, 922.5Hz and 982.5Hz. If the bandwidth of this capture range is ±60Hz or more, the free-running frequency will shift due to some influence (drift, etc.), and even if one pilot signal is approached, it will always lock when the other pilot signal is input. become.
In addition, the PLL is designed so that the bandwidth of the capture range and the lock range are the same, so that it operates as stably as possible against noise. The input of this voltage controlled oscillator 19 is simultaneously connected to the amplitude limited amplifier 1.
4 into the Schmitt circuit 15, a small control signal of the voltage-controlled oscillator 19 is amplified by the amplitude-limited amplifier 14, and the level is limited, so that each pilot signal is converted into high/low level (1 or 0) logic. Take it out.

At the same time, the pilot signal detected by the AM demodulator and the output of the voltage controlled oscillator 19 are passed through the waveform shaping circuit 10 and the 90° phase shifter 11 to output the voltage controlled oscillation waveform with a 90° phase shift. Synchronous detection is performed by a phase comparator 12, and the signal is input to a Schmitt circuit 15' via a rectifier circuit 13 to determine whether it is a monaural broadcast, a stereo broadcast, or a different type of broadcast based on the difference in logic between high and low levels (1 or 0).

In the circuit system shown in Figure 2 that does not require a lead filter,
Although the disadvantages of the lead filter circuit system have been eliminated, when the electric field of broadcast radio waves becomes weak in some areas, the tuner reaches its maximum gain due to AGC (automatic gain adjustment circuit) and noise components increase. In order to make it difficult to distinguish the pilot signal, a circuit having a wider frequency bandwidth than the reed filter becomes unable to distinguish the pilot signal earlier than the reed filter.

Currently, the circuit system shown in FIG. 2, which uses a lead filter, is worse than the circuit system shown in FIG. 2, which has an electric field of 2 to 3 d _B μ weaker at the input of a television antenna.

According to the present invention, it is possible to distinguish between stereo broadcasts and different types of broadcasts without using a lead filter at all, and at the same time, the discrimination ability is improved using a weak electric field.If it is incorporated into an integrated circuit, the cost of the integrated circuit itself is only slightly increased and reliability is improved. A highly efficient and inexpensive audio multiplexing circuit can be obtained.

Next, the present invention will be explained in more detail using FIG. 3 showing one embodiment of the present invention.

The output of the AM demodulator 26 is output to the phase comparators 27 and 2.
2, and the output of the phase comparator 27 is passed through a loop filter 28 to the voltage controlled oscillator 2.
Control 9.

The output of the voltage controlled oscillator 29 is sent to the waveform shaping circuit 2
0, and enters the other input of the phase comparator 27 and the input of the 90° phase shifter 21.

The output of the 90° phase shifter 21 enters the other input of the phase comparator 22, the output of which enters the Schmitt circuit 25' via the rectifier circuit 23.

On the other hand, the signal enters the Schmitt circuit 25 from the connection point between the loop filter 28 and the voltage controlled oscillator 29 via the amplitude limited amplifier 24. The above configuration is the same as that shown in FIG. 2, but the following circuits are added in this embodiment. This circuit system performs switching control of the conversion gain determined by the phase comparator 27 and loop filter 28 from the connection point between the shaping circuit 23 and the Schmitt circuit 25' via the loop gain switching circuit 30. The output of the controlled oscillator 29 is passed through the waveform shaping circuit 20 and the 90° phase shifter 21, and the output of the voltage controlled oscillation waveform is shifted by 90° to the phase comparator 2.
2 performs synchronous detection, and when the pilot signal is input, the rectifier circuit 23 converts the output into a direct current to operate the loop gain switching circuit 30, and the conversion gain determined by the phase comparator 27 and loop filter 28 is set. lower the overall loop gain of the PLL. or,
When no pilot signal is input, the loop gain switching circuit 30 does not work.

Lowering the loop gain narrows the bandwidth of the capture range and reduces the noise bandwidth of the PLL (closed loop determined by the bandpass characteristics of the loop filter 28, the integral characteristics of the voltage controlled oscillator 29, and the loop gain). (narrowband characteristics), reduce the influence of input noise, and operate stably.

The relationship between the capture range and the lock range is that when there is no pilot signal, that is, the PLL is not locked, the capture range is ±60Hz or more relative to the free-running frequency, and when the pilot signal is input, that is, the PLL is not locked. When locked, the lock range should be at least ±60Hz with respect to the free-running frequency, and the capture range should be within that band. With such a capture range and lock range, each problem of the conventional circuit system can be solved.

[Brief explanation of the drawing]

FIG. 1 is a circuit diagram showing a conventional circuit system using two lead filters, which is most commonly used. FIG. 2 is a circuit diagram showing a conventional circuit system that does not use any lead filter. FIG. 3 is a circuit diagram showing one embodiment of the present invention. 1, 16, 26...AM demodulator, 2, 3...Lead filter (922.5Hz, 982.5Hz), 4, 5, 1
3, 23... Rectifier circuit, 17, 12, 27, 22
... Phase comparator, 18, 28 ... Loop filter, 19, 29 ... Voltage controlled oscillator, 10, 2
0...Waveform shaping circuit, 11, 21...90 phase shifter,
15, 15, 25, 25... Schmitt circuit, 3
0...Loop gain switching circuit.

Claims (1)

[Claims] 1 One input of the phase comparator is supplied with input signals of relatively close different frequencies, and the other input is supplied with the output of a voltage-controlled oscillator circuit whose free-running frequency is set between the above frequencies. A signal determining circuit comprising a phase lock loop circuit and circuit means for determining the presence of the input signal using the output of the voltage controlled oscillator circuit, the phase lock loop circuit detecting the phase lock in response to the output of the circuit means. A signal discriminator circuit characterized in that it is provided with means for switching the loop gain of a loop.