Manlfunction monitoring equipment in a time division multi plexed transmission system

(74) Agent: Cabinet Chereau & Cabinet Rodes, combined. Patent attorneys, 107, boulevard Péreire, Paris (17).

@ Malfunction control equipment for a time multiplex transmission system.

(72) Invention of: Haruo Kaneko and Kaoru Yano.

© (32) (31) Conventional priority: Patent application filed in Japan on December 16, 1969, n. 101.482 / 1969 in the name of the plaintiff.

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The present invention relates to control equipment for the analog circuit of a time-division multiplex communication system with pulse code modulation and, more particularly, it relates to a malfunction control system using a pilot signal.

Generally, when a failure occurs in a communication system, it is urgent to locate the location of the failure and restore the communication system by replacing the wrong panel or by finding the wrong part. A time division multiplex communication system with pulse code modulation is made up of terminal equipment or terminals and a repeated line, the first in turn being made up of a digital circuit and an analog circuit. Most faults or breakdowns in the repeated line and in the digital circuit can be detected by a synchronization of the control image. However, this method cannot detect faults in the analog circuit. A prior art method for controlling faults in the analog circuit is to transmit a pilot signal by inserting it into an inactive or empty channel at the transmitter, and to control the pilot signal at the receiver. In this method, the pilot signal is a direct current pilot signal. For example, the method described in US Patent No. 3,259,695 makes use of a pilot DC signal. The reason for this use of a direct current pilot signal is on the one hand that the codification system employed therein uses a combination of an instantaneous compressor-expander, a linear coder and a decoder; and on the other hand that the main purpose of the pilot control is to detect the deviations of direct current in the encoder and the decoder as well as the increase of nonlinear distortions caused by an abnormal temperature of the radiator containing the semiconductor diodes of the instantaneous expander compressor . When variations in the DC operating point of the encoder and decoder in response to variations in radiator temperature cause variation in the level of the pilot signal received, failures in the encoder, decoder and radiator as well as deviations level in a large range and out-of-range deviations in the analog circuit, can be detected by controlling said level of the pilot signal. However, some parts of the analog to digital and digital to analog converters of the encoder and the uécoBAD ORIGINAL

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This cannot be controlled by this process which uses this DC pilot signal. For more details, when the pilot signal is a DC signal, the coded pilot signal output provides a particular mode, namely 1010111. In the decoder, a weighting network operates according to this mode to produce an analog signal. Even if the switching circuit controlling the weighting network is damaged in such a way that it is fixed in either 1 or o state, a pilot fault alarm signal will not be produced until such a state is reached. contradictory to the pilot signal mode. Similar failures can occur in the encoder. However, considering the reliability of the components, it is likely that most failures in the encoder and decoder cooperating with the Instant Compressor-Expander occur in the main parts of the latter, the decoder and the encoder, while the ratio such failures which cannot be detected by means of the DC pilot signal, as described above, is small.

The segment type non-linear coding system has been recently proposed and tends to be adopted as a standard system.

In this system, the instantaneous compressor-expander is not used, and therefore an appropriate control system to control faults in the ordinary encoder and decoder could be used rather than the appropriate DC pilot system to check breakdowns specific to the encoder and decoder cooperating with the instantaneous compressor-expander. One of these methods is to use a sine wave of single frequency as a pilot signal. With this method, the circuit can be simplified if the pilot signal is derived by dividing the sampling frequency. However, when the ratio of the pilot signal frequency to the sampling frequency is always constant, the sampling point is fixed at a few points. Consequently, only a few variations of the coded output modes can be produced, since it becomes impossible to completely detect faults or failures in the coder and the decoder. This disadvantage can be eliminated by choosing the pilot signal frequency independent of the sampling frequency, which can be achieved by the use of a particular oscillator to produce the pilot signal, although the circuit becomes a little more complicated. The process by which a

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45190 sine wave is used as a pilot signal and is thus a good means of controlling the operations of the encoder and the decoder, but it has a fatal disadvantage of crosstalk between the time slots, which is inevitable because of the band limit of the amplifiers, etc ... when the pilot signal which must be transmitted via an inactive or empty channel is multiplexed with the signals of other channels. The crosstalk occurring in the stages before coding, however small it may be, has a chance of reaching what is called the level of crosstalk hase which is decided with the quantization step.

Even if the crosstalk is not increased or amplified, at the basic level, an unauthorized level can be achieved since a single tone in the audio tape has a remarkable interference effect on the hearing sense. The crosstalk will specifically increase when the amplitude of the sine wave is chosen to be as large as possible in order to scan all the coded modes in order to effectively control the malfunctions of the coder and the decoder.

An object of the present invention is to provide pilot malfunction control equipment which eliminates the disadvantages of prior art pilot malfunction control equipment, and is suitable for monitoring the malfunction of multiplex communication systems in the pulse-modulated time using the non-linear coding method.

The principle of the present invention is that a noise signal such as white noise is inserted into an empty or inactive channel as a pilot signal, sampled and coded, and then transmitted via a line transmission signal to the receiving terminal, where it is decoded and demodulated to control the noise signal or distortion noise such as quantization noise at the pilot channel output. In contrast to the other channel single tone crosstalk which is, as described above, a disadvantage of the pilot control system using a single frequency sine wave, interference with other channels caused by crosstalk when noise used as a pilot signal can be remarkably reduced as compared with the noise produced in these channels from other causes, thus preventing the degradation of the characteristics of these channels. Consequently, we can say that a noise is approBAD ORIGINAL COï'Vj

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It is required for a pilot signal for pulse-modulated time-division multiplex systems. Furthermore, when a noise of large crest factor is used as the pilot signal, a small power is sufficient to wobble or scan many coded modes, and thus the effective verification of the operation of the coder and of the decoder is carried out with less interference with other channels. With noise as the pilot signal, severe faults in the encoder and decoder can be detected by monitoring only the pilot signal output level at the receiving terminal, and even slight faults can be detected by using band-limited noise as the pilot and control signal, at the reception terminal, the distortion noise, such as the quantization noise, falling outside the frequency band of the pilot signal.

The present invention will now be described in relation to the attached drawings showing some examples of embodiment thereof, and in which:

Figure 1 is a block diagram of an exemplary embodiment of the present invention.

Figure 2 is a graph for the explanation of another exemplary embodiment of the present invention.

FIG. 3 is a block diagram of this other exemplary embodiment of the present invention, and

Figure 4 is a block diagram of another exemplary embodiment of the present invention.

In FIG. 1, which represents a first exemplary embodiment of the present invention, a signal sampled and modulated in pulse and amplitude by a modulator 11 is time-multiplexed with the signals of other channels and a pilot signal, the latter being obtained by sampling and modulating the output noise in pulse and amplitude from a white noise generator 16 by a pilot modulator 17. These multiplexed signals are coded by an encoder 12, the output of which is transmitted via a repeated transmission line 13 to a reception terminal, at which it is decoded into analog signals by a decoder 14, and then the demodulators of channels 15 perform channel separation and demodulation. The pilot signal is demodulated by a pilot demodulator 13. When the level of the demodulated output is abnormal, it is detected by an alarm circuit 19 to give an alarm indication. This

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45190 The basic structure of the control system itself is similar to that of the prior art systems, but the essential feature of the present invention is the use of noise as a pilot signal for a multiplex communication system. over time with pulse code modulation.

Referring now to Figure 2, which explains a more detailed exemplary embodiment in which white noise which is band limited to 2 ~ 4 KHz is used as the pilot signal.

If the spectrum of the pilot signal is uniformly distributed from 2 to 4 KHz, as shown in 21, the spectrum of the demodulated output passing through the pulse code modulation system is given by the superimposition of the spectrum of the signal represented in 21 on that of the distortion noise component, such as quantization noise, shown at 22. When the spectrum of the quantization noise component is between 0 and 4 KHz, the power of the noise component falling in the range 0 ~ 2 KHz occupies about half of the total distortion noise power. By controlling the power of the noise component below 2 KHz, which can be extracted by a low-pass filter having a cut-off frequency of 2 KHz, failures in the encoder and the decoder which can appear as an increase in noise can be easily detected. Since quantization noise currently determines the quality of communication, it is apparent that the advantage of the control system is excellent.

A detailed structure of the present system is shown in FIG. 3, of which (a) is the transmission terminal and (b) is the reception terminal. White noise produced by a noise generator 30 is limited in band to 2 ~ 4 KHz by a bandpass filter

31, the output noise power of which is kept constant by an automatic gain control circuit (AGC) 32. The output of the AGC circuit 32 is then sampled by a modulator 33 and supplied via an output terminal 3²¹ to a coding device. For more details, the noise generator

30 consists of a noise production circuit comprising a Zener diode 301, a resistor 302 and a power supply 303, and an amplification circuit comprising a capacitor 304, resistors 303, 307, 308 and 309 and an operational amplifier 306. Power supply ^z03 in com40 tnun with resistor 302 supplies an original bad bias current

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Zener diode 301, which produces broadband noise. This noise is amplified by the operational amplifier 306 provided with a serial reaction device (for example of the μΑ-709 IC type manufactured by the company known as: Fairchild). The output noise from the noise generator is applied to the bandpass filter 31 whose output noise is limited in band to 2 ^ 4 KHz. The output noise from the bandpass filter 31 is applied to the automatic gain control circuit 32, which consists of an AGC amplifier comprising resistors 321, 323 and 324, an operational amplifier 322 (of the jiA-709 type). IC, for example, mentioned above), a capacitor 325 and a diode 326, and a control circuit comprising resistors 327, 3211, 3213 and 3215, an operational amplifier 329, a capacitor 328, diodes 3210 and 3214 and a power supply 3212. The control circuit converts the output noise from the AGC amplifier into a rectified direct current, which is kept equal to the reference current supplied from the power supply 3212 by l via resistor 3215 by controlling the gain of the AGC amplifier. The operational amplifier 329 amplifies the error signal resulting from the difference between the reference current and the rectified current to control the bias current flowing through the diode 326, thereby adjusting the gain of the AGC amplifier. The output noise from the AGC circuit 32 is converted into a PAM signal (with pulse and amplitude modulation) by the modulator 33 and then is supplied to the encoder. The modulator 33 consists of a diode gate circuit comprising diodes 331, 332, 333 and 334, and of a gate drive circuit comprising resistors 335 and 336, a capacitor 337 and a transformer 338. The transformer 338 is

3 ° powered by modulation pulses with a repetition rate of 8 kHz at its terminals 339 and 3310.

In the reception terminal, a terminal 35 receives the output of the decoder, which is separated from the pulse modulated and amplitude signals of other channels by a channel separation gate 3-6 to be applied to a low-pass filter. 37 whose cut-off frequency is chosen at 2 KHz so that the noise component below 2 KHz is obtained at its output. This output is amplified to a desired level by an amplifier ÿ8, and subsequently is rectified by a conversion circuit 39 θη a direct current to drive a relay of measuring device

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40. For further details, the channel separation gate 36 consists of diodes 361, 362, 363 and 364, resistors 365 and 366, a capacitor 367 and a pulse transformer 368. The demodulation pulses are applied to a pair of terminals 369 and 370. The PAM signal is supplied via terminal 35 at gate 36, the output signal of which is applied to the low-pass filter 37 · The output signal from the low-pass filter 37 is supplied to an amplifier 38 and amplified by the latter, which consists of an operational amplifier 382 (of the type mentioned above pA-709 IC, for example), two resistors 381, 383 and 384 and a capacitor 385. The output signal from the amplifier 38 is supplied to the conversion circuit 39 which consists of resistors 391, 394 and 397, of a capacitor 398, of diodes 392 and 393, of an operational amplifier 396 (of the type mentioned above). (above pA-709 IC, for example), and a power supply 395, and is converted into a DC signal. This DC signal is fed to the measuring device relay 40 to indicate noise power.

Another embodiment of the present invention is shown in Figure 4 in which only the receiving terminal is shown, the transmitting terminal being similar to that of Figure 3 (a). In FIG. 4, the decoded signal applied to an input terminal 41 is demodulated by a demodulator 42 into the superimposed signal of the pilot signal and the distortion noise such as the quantization noise. Since this demodulated signal is extracted by a high-pass filter 43 the component above 2 KHz (referenced as main band power) to be converted into a DC signal by a converter 44. On the other hand, from the demodulated output is extracted by the low-pass filter 45 the component below 2 KHz (brought as reference as distortion waveband power), which is amplified to a desired level by an amplifier 46 to be converted into a direct current signal by a converter 47. The outputs from converters 44 and 47 are applied to a comparator 48, which obtains the ratio of the main band power (which is substantially equal to the signal power) to the distortion waveband power (which is substantially half the total distortion noise power), the output being supplied to a 4g indication device. With this structure, it is possible to always control the signal /

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45190 ⁸ 2073797 noise which is the problem present in the use of channels, although the circuit becomes somewhat complicated.

It should be understood that all that has been described here is simply taken as illustration of examples of the present invention, and that the scope thereof is not limited to these examples. For example, while the white noise used as a pilot signal has been limited in band representative to 2 ^ / 4 KHz, any other frequency band can be adopted, where an accentuated noise can be used, to be properly chosen in consideration difficulties with band limitation means, detection in the reception terminal, etc. It is also possible to control, each independently, the signal component power and the noise component power in the demodulated pilot signal output.

The present invention is not limited to the embodiments which have just been described, it is on the contrary liable to variants and modifications which will appear to those skilled in the art.

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