JPS592233B2 - Multi-frequency signal reception method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a signal processing method for a digital multi-frequency signal receiver that employs a discrete Fourier transform method. The present invention relates to a multi-frequency signal receiving system that detects signals using two different types of fixed threshold values and protects against instantaneous interruptions that occur in input signals.

FIG. 1 shows a circuit block of a conventional digital multi-frequency signal receiver, and FIG. 2 is a waveform diagram for explaining the operation of the circuit.

In FIG. 1, 1 is an A/D converter that converts an input analog signal into a digital signal and inputs it to a multi-frequency signal receiver 2. The multi-frequency signal receiver 2 converts a digital input signal a and a time window function section 3. A multiplication unit 4 that multiplies the function value output b of
, a discrete Fourier transform calculation unit 5, a shift register 6 for threshold setting, and an output c(+) from the calculation unit 5,
It consists of an adder 7 for adding the threshold output dH from the shift register 6, and an output logic section 8 for gate-controlling the receiver output. The input/output waveforms of each part in Figure 1 are shown in Figure 2. Figure 2 a shows the steady state input signal waveform input to the Λ/D converter 1, and a' shows a momentary interruption in the input signal. This is a waveform when the following occurs, and b indicates a time window waveform.

As shown in the circuit of FIG. 1 and the waveform diagram of FIG. Signal detection, ie signal reception, was performed by comparing with a fixed threshold value.

In order to satisfy the impossibility level standard and the impossibility standard for out-of-band signals, the fixed threshold value is set 2 to 3 d below the lower limit of the sensation level range, as shown in Figure 2c.
Beside B, it must be set to a value. A in FIG. 2c indicates the range of the DFT output level when the input signal is normally input. However, when an instantaneous interruption occurs in the input signal as shown in Figure 2 a', the DFT
Since the multiplication value of is zero, the DFT output level range becomes smaller than when a normal signal is input, as shown in waveform diagram B of FIG. 2c. Therefore, for a low input level, the output level at the time of momentary interruption falls below the fixed threshold, making signal detection impossible. The receiver assumes that the signal input has ended at that point, and at that point the receiver immediately recovers, making it no longer possible to receive any further signals, making it impossible to protect against instantaneous interruptions in the input signal. SUMMARY OF THE INVENTION An object of the present invention is to eliminate the drawbacks of the prior art described above and to provide a multi-frequency signal receiving system that can continuously maintain the receiving function even if a momentary signal interruption occurs in the input signal.

In order to prevent malfunctions caused by noise and out-of-band signals and to satisfy the receiver's unimpressive level characteristics, the present invention sets the signal at a fixed threshold at a relatively high level before receiving the signal. Perform detection and signal detection (reception)
Thereafter, the threshold value is switched according to the received information, and subsequent signal detection is performed using a fixed threshold value of a relatively low level, thereby improving the instantaneous power failure protection characteristics.

When setting the fixed threshold after receiving the signal, the condition is that it is insensitive to the noise superimposed on the regular signal, and it is set to the lowest level within the range that satisfies this condition. It is something to do. An example of the digital multi-frequency signal receiver of the present invention will be described below with reference to a block diagram shown in FIG.

In the figure, IN is an input terminal for a PCM encoded multi-frequency signal, and 0UT is a signal output terminal. Further, 9 is an expander that converts the companded and encoded multi-frequency signal into a linear PCM code, and 10 is a window function generation circuit, and its output and the expander 9 are
The linear PCM code outputs are multiplied by a multiplier 11. 12 is a DFT calculation circuit; 13 and 14 are first and second threshold generation circuits; 15 is a selector circuit for selectively switching between the first and second threshold generation circuits;
16 is the output from the DFT calculation circuit 12 and the selector circuit 1
17 is a signal present circuit, and 18 is an output circuit.

To explain the circuit operation in Fig. 3, first, the receiver input terminal 1
A companded PCM encoded multi-frequency signal is input from N, and is converted into a linear PCM code through an expander 9.

Next, after being multiplied by a window function in a multiplier 11, calculations are performed in a DFT calculation circuit 12, where it is converted into a frequency spectrum and sent to an adder 16. The other input of the adder 16 comes from the output of the selector circuit 15, but until now, since no signal has been received, the output of the signal present circuit 17 has been a logic zero, which is Since it is input to the select terminal S of the selector and selects the first fixed threshold of the selector circuit 15, the adder 16
The first input from the threshold generation circuit 13 is connected to the negative input of
Threshold is sent out. Therefore, the output level of the DFT calculation circuit 12 is compared with the first fixed threshold value, and a signal is detected here. When the output level of the DFT arithmetic circuit 12 is sufficiently large and exceeds the first fixed threshold, a signal is detected and sent to the output circuit 18.The output circuit 18 detects, for example, two of the six frequencies. If it is determined to be correct, the result is written to the memory and output. On the other hand, this detection result is also sent to the signal present circuit 17, whose output becomes logic [1], and the selector circuit 15 is switched to a second fixed threshold, which is lower in level than the first fixed threshold. Select.

Therefore, the subsequent output level of the DFT calculation circuit 12 is compared with this second threshold value to detect a signal. next,
When a momentary interruption occurs in the input signal, the output level of the DFT calculation circuit 12 in that section becomes small as shown in Fig. 2c, but if the time width of the momentary interruption is within the permissible range, this output level becomes , will never fall below the second threshold, so one or more of the six frequencies will always be detected.

Then, in the output circuit, the number of frequencies is determined, and if the last two frequencies of the six frequencies are detected, the result is written to the memory and output, but if a number other than the second frequency is detected, The result is not written to memory, but the previous memory contents are output. Therefore, in this case, the receiver output does not recover and the receiving operation continues as in the normal state. Next, when the signal completely disappears, apart from the instantaneous interruption, the output level of the DFT calculation circuit 12 at that time falls below the second threshold, and the detection result is sent to the output circuit 18.

Then, in the output circuit 18, in order to determine the number of frequencies when there is no signal (that is, O frequency), the result is written to the memory and output. Therefore, based on this output, the receiver determines that there is no received signal and recovers. At this time, the detection result is also sent to the signal present circuit 17, whose output becomes logic zero again, switches the fixed threshold again, and selects the first threshold (threshold generation circuit 13). . Therefore, for subsequent signal inputs, the signal will be detected again using the first threshold value.

As described above, since the signal is detected using the second threshold while receiving the signal, the receiver will not be able to recover from momentary interruptions at low input levels that could not be protected by conventional circuits. It becomes possible to protect against instantaneous power outages. fruit. According to our experience, the emotional level range of the receiver is from -35 dBm/1 wave to -5 dBm.
/1 wave, whereas in the conventional circuit, the input level range is -23 dBm/1 wave to -5 dBm/wave, which is completely protected against instantaneous power interruption under the worst-case conditions.
1 wave, -35 dBm/1 wave to -23 dBm/1
In the range up to the wave, the receiver will recover as soon as the instantaneous interruption occurs, and especially at -30 dBm/1 wave or less, there is no protection against instantaneous interruption.

On the other hand, according to the instantaneous interruption protection method for the multi-frequency signal reception system in the embodiment of the present invention, it was possible to completely protect against instantaneous interruption under the same conditions and obtain satisfactory receiver characteristics. In addition,
The worst condition when the above instantaneous interruption occurs is shown in Figure 2 a'.
As shown in FIG. 2, this is a case where an instantaneous interruption of a predetermined time width occurs in the center of the time window of the DFT calculation circuit (FIG. 2b). As is clear from the embodiments described above, the present invention provides two types of fixed thresholds with different levels, performs discrimination control using the first threshold before receiving an input signal, and performs discrimination control using the first threshold after receiving the signal. Since the first threshold value is switched to the second threshold value based on the received information and discrimination control is performed, it is possible that a momentary interruption occurs in the signal input and the signal becomes low level, as in the conventional circuit. This solves the problem of the signal becoming lower than the threshold value and preventing normal signal reception, and allows highly reliable multi-frequency signal reception.

[Brief explanation of drawings]

Figure 1 is a receiver circuit plotter diagram to explain the conventional multi-frequency signal reception system, and Figure 2 shows the signal waveform input to the receiver and the output characteristics to explain the problems that occur at that time. FIG. 3 is a circuit block diagram of a multi-frequency signal receiver showing an embodiment of the present invention. 9...Extender, 10...Window function generation circuit, 11...Multiplier, 12...DFT
Arithmetic unit, 13...First threshold generation circuit, 14
...Second threshold generation circuit, 15...
Selector circuit, 16... Adder, 17...
...Signal blending circuit, 18...Output circuit.

Claims (1)

[Claims] 1. Signal reception of a digital multi-frequency receiver that receives an encoded multi-frequency signal, separates the selected signal from the received input signal, discriminates a frequency that is more dominant than each separated frequency, and receives the signal. In the method, the first,
A second fixed threshold is set, and before the signal is received, discrimination from the input signal is performed using the first fixed threshold, and after the signal is received, the first fixed threshold is determined based on the reception information. A multi-frequency signal receiving system characterized in that even if a momentary interruption occurs in an input signal and a level change occurs, it can be protected by switching to a second fixed threshold value and performing discrimination control from the input signal.