JP3340302B2 - Modem - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a demodulator mainly used for facsimile communication in a telephone network,
International Telecommunication Union Telecommunication Standardization Sector (ITU-T
ITU to be officially issued by S) in 1996
The present invention relates to a modem device (hereinafter also referred to as a V34 modem) compliant with -T Recommendation V34, and more particularly to a technique for detecting a PPh signal from a received signal.

[0002]

2. Description of the Related Art As is well known, ITU
−T Recommendation V34 is positioned as a revision of the old CCITT Recommendation T30. In a half-duplex facsimile communication procedure based on this recommendation V34, various parameters of a transmitter and a receiver are exchanged using a new method called a control channel signal. More specifically, control channel data is sent before and after an image signal called primary channel data, and a signal corresponding to a header portion of the control channel data starts with the Sh / inverted Sh signal or starts with the PPh signal. The subsequent signal format changes depending on this. Therefore, in the receiving modem, the header part of the control channel data is Sh / Inverted Sh.
It is necessary to correctly recognize whether to start with a signal or a PPh signal.

FIG. 1A shows a procedure in the case where the header part of the control channel data starts with the PPh signal. This is the initial training and restart sequence of the control channel when the transmitting modem requests a change. FIG. 1B shows a procedure when the header of the control channel data starts with the Sh / inverted Sh signal. In this sequence, no setting or change of the modulation parameter is performed.

[0004] The PPh signal and the Sh / inverted Sh signal are:
1200Hz from calling side, 2400Hz from called side
Is transmitted by the QAM modulation method of the carrier No. 1, and the modulation speed is 600 baud rate. . When receiving and demodulating this QAM modulated signal and monitoring it in constellation coordinate format,
The PPh signal and the Sh / inverted Sh signal are shown in FIG.
(A), (b) and (c) are obtained. FIG. 2A shows S
h signal, and point A in the third quadrant and point B in the fourth quadrant appear alternately. FIG. 2B shows the inverted Sh signal, in which the point C in the first quadrant and the point D in the second quadrant appear alternately. FIG.
(C) is a PPh signal, in which the point A in the first quadrant, the point B in the second quadrant, and the point C in the third quadrant appear four times in an AABAACBC pattern.

[0005]

As described above, the ITU
In a modem conforming to -T Recommendation V34, the header of the received control channel data is a Sh / inverted Sh signal or a PP
It is necessary to correctly recognize which one of the h signals starts.
Receives and demodulates a 600 baud rate QAM modulated signal,
During a short signal period of 32 symbols, Sh / Inverted Sh
It is very difficult to distinguish between a signal and a PPh signal.

In the demodulation circuit of the V34 modem, an analog reception signal is A / D converted, the digitized reception signal is input to two demodulation multipliers, and a carrier signal having a phase difference of 90 degrees is applied to each of them. X data and Y data are respectively demodulated by multiplication. The X data and the Y data are input to the constellation coordinate monitor circuit, and it is determined which of the four quadrants of the constellation coordinates the received signal belongs to. Further, by monitoring the constellation point change pattern of the received signal, it is determined whether or not the PPh signal has been received.

Conventionally, in order to directly detect the PPh signal in such a basic demodulation circuit, it is monitored whether or not a repeated pattern of AABAACBC appears in the constellation coordinate monitor representation shown in FIG. Was. But only 3
AGC (automatic gain adjustment) operation is completed between two symbols, timing reproduction is adjusted to stop rotation of constellation coordinates,
In addition, it is very difficult to confirm the repeating pattern of AABAACBC. If the adjustment of the timing reproduction is insufficient, the sampling timing of the constellation coordinate monitor circuit and the basic timing of the QAM modulation signal are greatly shifted, and the constellation point determination result in that state has a high probability of error. . Therefore, often PPh
There is a drawback that a signal is missed (detection leakage occurs).

The present invention has been made in view of the above technical background, and an object of the present invention is to enable a modem conforming to ITU-T recommendation V34 to reliably detect a PPh signal by a signal processing method which can be easily implemented. It is to make.

[0009]

Therefore, according to the present invention, two constellation coordinate monitor circuits are provided in parallel to determine which of the four quadrants of the constellation coordinates the received and demodulated QAM modulated signal belongs to. The sampling timing of the constellation coordinate monitor circuits of the two systems is shifted by 1/2 of the time corresponding to the reciprocal of the baud rate of the QAM modulation signal, and the constellation point change patterns determined by the two system of constellation coordinate monitor circuits are used to determine PPh and The presence or absence of a signal is determined, and when the PPh signal is detected in one of the systems, P
It was made to recognize that the Ph signal was received.

According to this configuration, when the sampling timing of one of the two constellation coordinate monitor circuits greatly deviates from the basic timing of the QAM modulation signal, the sampling timing of the other system deviates from the basic timing. It will be very small. Therefore, one of the constellation coordinate monitor circuits determines the coordinate point at a relatively correct timing.

[0011]

FIG. 3 shows a schematic circuit configuration of a demodulator according to an embodiment of the present invention. The received analog QAM modulated signal is converted into digital data by the A / D converter 1. The received data is input to the two demodulation multipliers 2 and 3, respectively, and X data (real component) and Y data (imaginary component) of constellation coordinates are obtained by multiplying the cosine ωt and the sine ωt of the carrier frequency ω, respectively. . The X data and the Y data are passed through roll-off filters 4 and 5, respectively, from which high-frequency noise is removed, and are input to two constellation coordinate monitor circuits provided in parallel.

One constellation coordinate monitor circuit includes an equalizer 6a, an automatic phase shift control circuit (CAPC) 7a, and a coordinate point determination circuit 8a. The other constellation coordinate monitor circuit includes an equalizer 6b and an automatic phase shift control circuit (CAPC) 7.
b and a coordinate point determination circuit 8b. As will be described in detail below, the operation of the two constellation coordinate monitor circuits is exactly the same except for the sampling timing. X data and Y data are input to the equalizer 6a (6b), their amplitude components are controlled and normalized by a feedback signal from the determination circuit 8a (8b), and the processed X data and Y data are automatically phase-shifted. The phase components are input to the control circuit 7a (7b), and each phase component is controlled and normalized by a feedback signal from the determination circuit 8a (8b). The X data and Y data thus normalized are input to the determination circuit 8a (8b), and the constellation point of the received signal is determined.

The judgment outputs of the two constellation point judgment circuits 8a and 8b are input to a reception processor 9. The receiving processor 9 monitors the change pattern of the judgment output of each of the constellation point judgment circuits 8a and 8b, and judges whether or not the change pattern of the coordinate point of the PPh signal (repetition pattern of AAABAACBC) appears. If a change pattern of the PPh signal is detected from either the output of the constellation point determination circuit 8a or the output of the constellation point determination circuit 8b, it is recognized that the PPh signal has been received.

A feature of the present invention is that two constellation coordinate monitor circuits are provided in parallel as described above, and the sampling timing of both systems is shifted as follows.

The modulation speed of the V34 QAM modulation signal is 60
0 baud rate. Therefore, 1 of the QAM modulated signal
The symbol time is 1/600 second. The constellation coordinate monitor circuit described above samples input data (X data and Y data) at a period of 1/600 second to determine a coordinate point. In the present invention, the first constellation coordinate monitor circuit (6a ·
7a and 8a) and the sampling timing of the second constellation coordinate monitor circuit (6b, 7b and 8b).
There was a time difference of 1/1200 seconds between the timings. 1/1200 seconds means QA of V34
1/2 of the time corresponding to the reciprocal of the baud rate of the M modulation signal
It is.

As a specific circuit configuration, a difference in sampling timing is provided as follows. In this example, a clock signal CK1 of 9.6 KHz is generated by the clock generator 10, and this clock signal CK1 is set to A / A
The sampling clock of the D converter 1 is used. Therefore, 16 clocks of the clock signal CK1 are generated in 1/600 second of one symbol time of the QAM modulated signal, and the received signal of one symbol is sampled 16 times and A
/ D conversion.

As shown in FIG. 4, the frequency of the clock signal CK1 of 9.6 KHz is divided into 1/16 by the frequency dividing circuit 11, and
A 00 Hz clock signal CK2 and an inverted clock signal CK3 are generated. And the clock signal C
K2 is the first constellation coordinate monitor circuit (6a, 7a, 8a)
And the clock signal CK3 is used as the sampling clock of the second constellation coordinate monitor circuit (6b, 7b, 8b). Since the clock signals CK2 and CK3 have a phase difference of 180 degrees, CK2 and CK3
There is a time difference of 1/1200 seconds between the rising timings of No.3 and No.3. Thus, 1/1200 seconds between the sampling timing of the first constellation coordinate monitor circuit (6a, 7a, 8a) and the sampling timing of the second constellation coordinate monitor circuit (6b, 7b, 8b). There is a time difference.

[0018]

In the demodulator of the V34 modem according to the present invention, the sampling timing of the two constellation coordinate monitor circuits provided in parallel is set to 1/1200 seconds (1/1 of the time corresponding to the reciprocal of the baud rate of the QAM modulated signal). 2) The operation is shifted by only 2), and the presence or absence of the PPh signal is determined based on the constellation point change pattern determined by the two constellation coordinate monitor circuits, and the PPh signal is received when the PPh signal is detected in one of the systems. Recognize that you have done it. According to this configuration, when the sampling timing of one of the two constellation coordinate monitor circuits is significantly different from the basic timing of the QAM modulation signal, the sampling and sampling of the other system is performed.
As for the timing, the deviation from the basic timing is very small. Therefore, one of the constellation coordinate monitor circuits determines the coordinate point at relatively correct timing, and can reliably determine and detect the PPh signal having only 32 symbols.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a header part of control channel data of facsimile communication according to ITU-T recommendation V34.

FIG. 2 is a pattern diagram when the Sh signal, the inverted Sh signal, and the PPh signal in the header portion of the control channel data are monitored in constellation coordinates;

FIG. 3 is a schematic configuration diagram of a demodulation device according to an embodiment of the present invention;

FIG. 4 is a timing chart showing the operation of the demodulation device;

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 A / D converter 2, 3 Demodulation multiplier 4, 5 Roll-off filter 6a, 6b Equalizer 7a, 7b Automatic phase control circuit 8a, 8b Constellation point judgment circuit 9 Receiving processor 10 Clock generator 11 1/16 frequency dividing circuit

──────────────────────────────────────────────────続 き Continuation of front page (72) Inventor Yoshiyuki Okubo 4-3-1 Tsunashima Higashi, Kohoku-ku, Yokohama-shi, Kanagawa Prefecture Matsushita Communication Industrial Co., Ltd. (56) References JP-A-8-223246 (JP, A) JP-A-6-37815 (JP, A) JP-A-5-114982 (JP, A) JP-A-3-127550 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) H04N 1/32-1/36 H04N 1/42-1/44

Claims (1)

(57) [Claims] 1. A demodulation device conforming to ITU-T Recommendation V34, comprising two constellation coordinate monitor circuits for judging to which of four quadrants of a constellation coordinate a received and demodulated QAM modulated signal belongs in parallel. The sampling timing of the two constellation coordinate monitor circuits is shifted by 1/2 of the time corresponding to the reciprocal of the baud rate of the QAM modulated signal, and the constellation points determined by the two constellation coordinate monitoring circuits are operated. A demodulation device characterized by determining the presence or absence of a PPh signal from each change pattern, and recognizing that a PPh signal has been received when a PPh signal is detected in one of the systems.