JPS6159592B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION In particular, when one of a plurality of different and close frequencies arrives, its presence can be determined, and at the same time, the difference between the frequency signal and the absence of the frequency signal can be clearly and reliably detected. The present invention also relates to a signal discrimination circuit for display.

Since the year before last, an audio multiplex system has been developed and broadcast in television broadcasting in our country.
In this broadcasting station, broadcasting is carried out in three modes: regular monaural broadcasting, stereo broadcasting, and heterogeneous broadcasting, based on the program.

When receiving this on a TV receiver, etc., the transmitting side does not do anything special for monaural broadcasts, as usual, but for stereo broadcasts and different types of broadcasts, the frequency is set to 55.125KHz so that they can be distinguished. Amplitude modulated stereo pilot signal (982.5Hz) and heterogeneous broadcast pilot signal (922.5Hz)
Hz) signal is converted into FM and transmitted.

Therefore, on the receiver side, after FM demodulating these signals through a normal television audio intermediate frequency amplification circuit, the signals are converted into a 982.5Hz pilot signal for stereo broadcasting modulated by a 55.125KHz carrier wave or a pilot signal for a different type of broadcasting. It detects 922.5Hz and knows whether it is a stereo broadcast or a different type of broadcast, and at the same time automatically switches the necessary circuits for each type of broadcast based on this signal.

A circuit system using two lead filters, which is most commonly used in the past, will be explained with reference to FIG.

The output of the AM demodulator 1 enters respective rectifier circuits 4 and 5 via 922.5 Hz and 982.5 Hz reed filters (tuning fork resonance filters) 2 and 3. Each output of the rectifier circuits 4 and 5 is input to an OR circuit 6, and the outputs of the rectifier circuits 4 and 5 and the OR circuit 6 are configured as circuit outputs. The circuit operation of such a configuration will be explained. The amplitude modulated pilot signal is subjected to AM detection by an AM demodulator 1 to extract a pilot signal of either 922.5 Hz or 982.5 Hz, which is applied to two read filters 2 and 3. Since the reed filter has signal passbands of approximately 922.5Hz±2Hz and 982.5Hz, it only passes the above two signals. Therefore, the obtained output can be immediately determined to be one of the above 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. Furthermore, by passing the outputs of the rectifier circuits 4 and 5 through an OR circuit 6, it can be determined whether the broadcast is monaural.

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 creating a housing when used with audio equipment (such as speakers) that generates large vibrations.

In order to improve the above drawbacks, a method was devised that does not use this reed filter at all. The circuit system will be explained using FIG. 2.

The output of the AM demodulator 1 is input to one of the phase comparators 7 and 12, and the output of the phase comparator 7 controls a voltage controlled oscillator 9 via a loop filter 8. The output of the voltage controlled oscillator 9 is sent to the waveform shaping circuit 1
0 to the other input of the phase comparator 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 16 via the rectifier circuit 13. On the other hand, the signal enters the Schmitt circuit 15 from the connection point between the loop filter 8 and the voltage controlled oscillator 9 via the amplitude limited amplifier 14.

With this circuit configuration, the signal detected by AM demodulator 1
A pilot signal of 922.5 Hz or 982.5 Hz is applied as an input signal to one of the phase comparators 7 and 12. The oscillation frequency of the voltage-controlled oscillator 9 is voltage-controlled by the output obtained by converting the output of the phase comparator 7 into direct current using the loop filter 8. The free-running frequency of this voltage-controlled oscillator 9 should be arbitrary as long as it is between the above 922.5Hz and 982.5Hz, but considering the drift of the free-running frequency, it is preferably around 952.5Hz, the center of the two frequencies. .

The waveform shaping circuit 10 is a circuit that shapes the waveform for the purpose of increasing the comparison sensitivity when applying the output waveform of the voltage controlled oscillator 9 to the phase comparator 7, and in principle, it may be omitted.

Thus, the phase comparator 7, the loop filter 8,
The voltage controlled oscillator 9 and the waveform shaping circuit 10 are phase-controlled by the pilot signal frequency (usually referred to as a phase lock loop or PLL), and the constants of the phase comparator 7 and loop filter 8 are The commonly known capture range (pulling frequency range) and locking range (holding frequency range) are determined by the conversion gain determined by the voltage controlled oscillator 9 and the voltage controlled oscillator conversion gain of the voltage controlled oscillator 9. The PLL is designed with a capture range that is sufficient to cover two different frequencies, 922.5Hz and 982.5Hz.

If the bandwidth of this capture range is ±60Hz or more, the free-running frequency will deviate due to some influence (drift, etc.) and it will always lock when one pilot signal approaches the other pilot signal and is input. become. In addition, the PLL is designed to have the same bandwidth as the capture range and lock range so that it operates as stably as possible against noise. The input of this voltage controlled oscillator 9 is simultaneously inputted to the Schmitt circuit 15 via an amplitude limited amplifier 14, so that a small control signal of the voltage controlled oscillator 9 is amplified by the amplitude limited amplifier 14, and the level is limited. Each pilot signal is extracted as a logic high or low level (1 or 0).

At the same time, the pilot signal detected by the AM demodulator and the output of the voltage controlled oscillator 9 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. Phase comparator 1
2 performs synchronous detection and inputs it into a Schmitt circuit 16 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).

Although the circuit system shown in Figure 2 that does not require a reed filter eliminates the disadvantages of the circuit system that uses a reed filter, when the electric field of broadcast radio waves becomes weak in some areas, the tuner is As the gain reaches its maximum, the noise component increases, making it difficult to distinguish the pilot signal. Therefore, in a circuit having a wider frequency bandwidth than the reed filter, it becomes impossible to distinguish the pilot signal earlier than the reed filter.

Currently, the circuit system shown in Figure 2 is worse than the circuit system shown in Figure 1, which uses a reed filter, at a TV antenna input to a weaker electric field of 2 to 3 dBμ.

The purpose of the present invention is to distinguish between stereo broadcasts and different types of broadcasts without using any lead filter, and at the same time, to improve the discrimination ability by using a weak electric field, and if it is incorporated into an integrated circuit, the cost of the integrated circuit itself will increase only slightly. DESCRIPTION OF THE PREFERRED EMBODIMENTS An object of the present invention is to provide a discrimination circuit that can provide a highly reliable and inexpensive audio multiplexing circuit.Examples of the present invention will be described in detail below with reference to the drawings.

FIG. 3 is a block diagram showing an embodiment of the present invention, and this configuration is substantially the same as that of FIG. 2, with the following circuits added. That is, the rectifier circuit 18
The signal enters the frequency shift circuit 13 and the reset circuit 21 from the connection point between the and Schmitt circuit 16, and the two outputs of the frequency shift circuit 18 are connected one to the loop gain switching circuit 17 and the other to the voltage controlled oscillator 9, respectively. The output of the loop gain switching circuit 17 is connected to both or one of the phase comparator 7 and the loop filter 8. The output of the amplitude-limited amplifier 14 enters a stereo broadcast latch-up circuit 19 and a different type broadcast latch-up circuit 20, and from the outputs of both latch-up circuits 19 and 20, a signal for determining whether the broadcast is a stereo broadcast or a different type broadcast is extracted. Further, the output of the reset circuit 21 is connected to the stereo broadcast latch up circuit 19.
and the different broadcast latch-up circuit 20 for control.

With the above circuit configuration, during monaural broadcasting when no pilot signal is input, the frequency shift circuit 18 and the loop gain switching circuit do not operate, and the reset circuit operates to reset the stereo broadcast latch-up circuit 19 and the dissimilar broadcast latch-up circuit 20. Control both circuits so that they do not operate. Therefore, no discrimination signal appears at the two outputs of the stereo broadcast latch-up circuit 19 and the different broadcast latch-up circuit 20.

The other circuits at this time operate in the same way as the circuit in FIG. 2, and the voltage controlled oscillator 9 oscillates at a free running frequency.
Capture range is ±60 around the free running frequency
It is higher than Hz.

When the pilot signal is input, the oscillation frequency of the voltage controlled oscillator 9 is controlled and matches the frequency of the input pilot signal, resulting in a stable state (abbreviated as a lock state), and the phase comparator 12 and The frequency shifting circuit 18 is activated by the output synchronously detected by the rectifier circuit 13 to shift the free-running frequency of the voltage controlled oscillator 9 from the pilot signal frequency, and the loop gain switching circuit 17 is activated to shift the frequency between the phase comparator 7 and The loop filter 8 is operated to lower the conversion gain determined by the loop filter 8.

The reset circuit does not operate during this operation.

This series of operations prevents the output of a signal for determining whether a stereo broadcast or a different type of broadcast occurs when no pilot signal is input, and reduces the frequency difference between the free-running frequency of the voltage-controlled oscillator 9 and the pilot signal frequency when a pilot signal is input. However, by making the lock more stable and lowering the conversion gain determined by the phase comparator 7 and loop filter 8, the bandwidth of the capture range can be increased from ±60Hz or more when no pilot signal is input to when a pilot signal is input. The input noise is narrowed to ±30Hz or more to reduce the noise bandwidth (closed-loop narrowband characteristic determined by the bandpass characteristic of the loop filter 8, the integral characteristic of the voltage-controlled oscillator 9, and the loop gain). Minimize the impact and operate stably.

In order to achieve such an operation, a stereo broadcast latch-up circuit 19 and a different broadcast latch-up circuit 20 are used instead of the Schmitt circuit 15 at the next stage of the amplitude-limited amplifier 14, as in the circuit configuration shown in FIG.

Moreover, both latch-up circuits 19, 2
By using 0, the discrimination circuit can operate stably even if the PLL becomes unstable due to input noise and jitter occurs.

As described above, by using the present invention, each of the problems of the conventional circuit system can be solved.

[Brief explanation of the drawing]

FIG. 1 is a block diagram showing a conventional circuit system using two lead filters, which is most commonly used. FIG. 2 is a block diagram showing a conventional circuit system that does not use any lead filter. Third
The figure is a block diagram showing one embodiment of the present invention. 1...AM demodulator, 2, 3... Lead filter (922.5Hz, 982.5Hz), 4, 5, 13... Rectifier circuit, 7, 12... Phase comparator, 8... Loop filter, 9... Voltage controlled oscillator, 10... Waveform shaping circuit, 11... 90° phase shifter, 15, 16... Schmitt circuit, 17... Loop gain switching circuit, 1
8... Frequency transfer circuit, 19... Stereo broadcast latch-up circuit, 20... Different type of broadcast latch-up circuit, 21... Reset circuit.

Claims (1)

[Claims] 1 a phase comparator having a first and a second input terminal and an input signal being supplied to the first input terminal; means for converting the output of the phase comparator into a direct current signal;
and a controlled oscillator that oscillates at a frequency controlled by the DC signal and feeds back the oscillation signal to the second input terminal of the phase comparator, the control oscillator controlling the phase of the pilot signal included in the input signal. a phase lock loop for synchronizing the phases of the oscillation signals of the oscillator; a determining means for determining the presence or absence of the pilot signal by synchronously detecting the input signal and the signal obtained from the controlled oscillator; and an output of the determining means. and when the output has information indicating that the pilot signal is included in the input signal, the capture range of the phase lock loop is greater than if the pilot signal was not included in the input signal. 1. A signal discriminating circuit comprising: means for changing at least the conversion gain determined by the phase comparator and the conversion means so that the conversion gain becomes narrower.