JPH0797780B2 - Repeated signal detection circuit - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a repetitive signal detection circuit, and more particularly to a repetitive signal for detecting a repetitive signal included in a handshake signal at the time of initial setting in communication between modems. Regarding the detection circuit.

[Conventional technology]

2. Description of the Related Art Conventionally, a communication method has been widely used in which a handshake signal including a data signal of a predetermined repeating pattern is exchanged between modems for data transmission at the time of initial setting. As a repetitive signal detection circuit for detecting a repetitive signal in a handshake signal, for example, there is a turn-on sequence detection circuit described in Japanese Patent Application No. 54-127614. This circuit utilizes the fact that the frequency spectrum of the repetitive signal after demodulation is limited to the DC and Nyquist frequencies, and for example, the demodulated signal of the modem is passed through the demodulated signal of the modem through a filter that blocks spectral components near the DC and Nyquist frequencies. Repetitive signal arrival is detected when the level exceeds the threshold value and the output level of the filter becomes zero.

[Problems to be solved by the invention]

The above-described conventional repetitive signal detection circuit has a problem that, for example, when line noise is large, the above-mentioned filter output level exceeds the threshold value for zero determination even when a repetitive signal arrives, resulting in erroneous detection. is there.

An object of the present invention is to provide a repetitive signal detection circuit which solves such a problem and prevents erroneous detection due to the influence of noise.

[Means for solving problems]

In order to achieve the above object, the present invention provides a first low-pass filter for extracting a DC component from a demodulated signal, a subtracter that subtracts the DC component from the demodulated signal and outputs the subtracted signal, A bandpass filter for extracting and outputting a Nyquist component from the output signal of the multiplier, a multiplier for multiplying the conjugate signal of the output signal of the subtractor and the output signal of the bandpass filter, and outputting only the real part thereof, A second low-pass filter that outputs a DC component from the output signal of the multiplier; and a pulse that outputs a pulse only when the output signal of the second low-pass filter exceeds a predetermined threshold value. 1 determination circuit, an absolute value circuit that calculates and outputs the absolute value of the output signal of the first low pass filter, and only when the output signal of the absolute value circuit exceeds a predetermined threshold value Pulse output first And determination circuit, wherein in response to the presence or absence of a pulse coming from the first judging circuit and the second judging circuit is obtained by providing a discriminating circuit which outputs a pulse indicating the arrival of repetitive signals.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a signal arrangement diagram for explaining its operation. In FIG. 1, the demodulation signal is a data time series obtained by demodulating a quadrature amplitude modulation signal in the demodulation unit of the modem,
It is treated as a complex signal with the in-phase component as the real part and the quadrature component as the imaginary part. FIG. 2 exemplifies the constellation of the standardization level of the complex signal that can be taken for each symbol period on the transmission side in the case of the 16-value quadrature amplitude modulation system. According to Recommendation V.32 of the International Telegraph and Telephone Consultative Committee (CCITT), the signals of points A and B are alternately repeated to transmit a predetermined number of symbols to start training of an echo canceller for a modem and an automatic equalizer. The S signal and the signal that repeatedly transmits the points C and D alternately, the AA signal (or CC signal) that transmits the point A (or C) by an even number of symbols, and the points A and C (or C and A). AC that alternately and repeatedly sends
Signals (or CA signals) are recommended as repetitive signals for handshaking. Note that training signals and data signals other than repetitive signals are scrambled on the transmitting side, and the points in FIG. 2 appear in random order for each symbol period.

The demodulated signal is sent to the first low-pass filter (LPF) 1 for extracting the DC component, and is also sent to the subtractor 2. The subtractor 2 subtracts the DC component from the demodulated signal and sends it to the bandpass filter (BPF) 3 and the multiplier 4. The band-pass filter 3 is a filter for extracting the Nyquist component, and the multiplier 4 multiplies the output signal of the subtractor 2 by the conjugate signal of the output signal of the band-pass filter 3 (a signal obtained by inverting the sign of the imaginary part), Only the real part is sent to the second low pass filter (LPF) 5. The LPF 5 is a filter for obtaining the time average of the output signal of the multiplier 4, that is, the DC component, and sends the output signal to the first determination circuit 6. The first judging circuit 6 generates a pulse only when the output signal of the LPF 5 exceeds a predetermined threshold value and sends it to the judging circuit 9. on the other hand,
The absolute value circuit 7 calculates the absolute value of the transmission signal of the LPF 1 and sends it to the second determination circuit 8. The second judging circuit 8 generates a pulse only when the absolute value given from the absolute value circuit 7 exceeds a predetermined threshold value and sends it to the judging circuit 9. The discriminating circuit 9 generates and outputs pulse signals a to c indicating the arrival of repetitive signals according to the presence or absence of the pulse arrival from the first determining circuit 6 and the second determining circuit 8.

Next, the operation of this embodiment will be described with respect to an example in which the S signal is received. As described above, the S signal is a signal in which the signals at points A and B in the point arrangement shown in FIG. 2 repeatedly appear alternately, and its DC component is before quadrature amplitude modulation. (However, i is an imaginary unit e is the base of the natural logarithm, φ = tan ^-1 2) a and, also Nyquist frequency component (However, m is the Nyquist angular frequency, θ = (π / 2) −
φ). When a signal quadrature amplitude modulated by this S signal arrives through the transmission line is demodulated, the DC node is
^{iφ is} the Nyquist frequency component Becomes a demodulated signal, and noise R such as line noise
If (t) is superposed (B ₁ , B ₂ > 0) and there is no line distortion, that is, amplitude distortion or phase distortion of the transmission line characteristics, B ₁ = B ₂ and θ ₁ = θ ₂ holds). In the circuit of FIG. 1 which receives this demodulated signal, the signal sent by the subtractor 2 is become. The output signal of the bandpass filter 3 is , The multiplier 4 multiplies the signal represented by the equation (1) by the signal represented by the conjugate of the equation (2), and the real part of the multiplication result, that is, − (B ₁ ² + B ₁ ² ) + 2B ₁ B ₂ cos (2mt + Δθ) + R (t) R ^{′ *} (t) −R (t) C (t) + R ^{′ *} (t) D (t) …… (3) Signal is sent. Pass this through LPF5,
The DC component of the first term in the equation (3) is extracted, and the AC component of the second term and the random components of the third and subsequent terms are substantially removed. The first determination circuit 6 generates a pulse according to this DC component. On the other hand, the absolute value circuit 7 receiving the DC component (Ae ^iφ ) of the demodulated signal extracted by the ^{LPF 1} generates a signal indicating the absolute value (A) of the DC component and sends it to the second determination circuit 8. The second determination circuit 8 generates a pulse in response to this. Since the S signal has both the DC component and the Nyquist frequency component as described above, pulses are sent from both the first determination circuit 6 and the second determination circuit 8 to the determination circuit 9.

At the time of receiving the AA signal, the AA signal has a DC component but does not have a Nyquist frequency component, so only the second determination circuit 8 sends a pulse to the determination circuit 9. When the AC signal is received, the AC signal has a Nyquist frequency component but does not have a DC component, so only the first determination circuit 6 sends a pulse to the determination circuit 9.

Therefore, for example, by using a decoder as the discrimination circuit 9,
A signal a in which a pulse appears when the S signal is received, a signal b in which a pulse appears when the AA signal is received, and a signal c in which a pulse appears when the AC signal is received can be individually obtained. Alternatively, if only the signal a needs to be obtained, an AND gate may be used as the determination circuit 9.

In this embodiment, the thresholds for level identification in the first decision circuit 6 and the second decision circuit 8 are set to appropriate values, so that when a random signal such as noise or a data signal is received, Since the autocorrelation is small and the DC component is substantially zero, it is possible to prevent both the first decision circuit 6 and the second decision circuit 8 from outputting a pulse.
Therefore, it is possible to obtain a circuit that prevents erroneous detection due to the influence of noise.

〔The invention's effect〕

As described above, in the repetitive signal detection circuit of the present invention,
At the time of noise reception, the autocorrelation of the signal is small and the DC component is substantially zero, so the influence is eliminated by the threshold value set in the determination circuit.

That is, the present invention can realize a repetitive signal detection circuit that prevents erroneous detection due to the influence of noise.

[Brief description of drawings]

FIG. 1 is a block diagram showing an embodiment of the present invention, and FIG. 2 is a signal arrangement diagram for explaining the operation of the embodiment. 1 ... First low-pass filter 2 ... Subtractor 3 ... Band-pass filter 4 ... Multiplier 5 ... Second low-pass filter 6 ... First determination circuit 7 ... Absolute value circuit 8: second determination circuit 9: determination circuit

Claims (1)

[Claims] 1. A first low-pass filter for extracting a DC component from a demodulated signal, a subtracter for subtracting and outputting the DC component from the demodulated signal, and a Nyquist component from the output signal of the subtractor. From the output signal of the multiplier, the output signal of the subtracter and the output signal of the subtractor and the multiplier of the output signal of the band pass filter A second low-pass filter that outputs a DC component; a first determination circuit that outputs a pulse only when the output signal of the second low-pass filter exceeds a predetermined threshold; An absolute value circuit that calculates and outputs the absolute value of the output signal of the first low-pass filter, and a second output circuit that outputs a pulse only when the output signal of the absolute value circuit exceeds a predetermined threshold value. A determination circuit and the first Depending on the presence or absence of a pulse coming from the determination circuit and the second judging circuit, repeat the signal detection circuit, characterized in that is composed of a discrimination circuit for outputting a pulse indicating the arrival of repetitive signals.