RU2189058C1 - Digital communication channel of seismic recording equipment - Google Patents

Abstract

FIELD: organization of digital data exchange between measurement modules, central information acquisition unit and operator of field telemetering seismic recording equipment. SUBSTANCE: digital communication channel incorporates source of analog signal, source of coder, encoder of communication channel in which noise-immune coding takes place, pulse transformer of transmitter, communication line proper which can be double-wire or radio channel, pulse transformer of receiver, decoder of communication channel in which noise-immune code is decoded, for instance, to standard PCM code, decoder of source, receiver of analog signal, adaptive former of consistent binary signals placed between pulse transformer of receiver and decoder of communication channel. EFFECT: improved noise immunity of digital channel thanks to insertion of adaptive former of consistent binary signals which operation is based on use of criterion of sluggishness of change of amplitude of following information pulse with regard to previous information pulse. 3 dwg

Description

 The invention relates to the field of automation and can be used to organize digital data exchange between measuring modules, a central information collection unit and an operator in a field telemetric seismic recording equipment.

 When organizing a digital communication channel in telemetry seismic recording equipment, one of the main problems is the noise immunity of the channel from external electrical and electromagnetic influences that distort the transmitted information. Since the large volume, and therefore the high density of transmitted information in this kind of equipment is due to the number of seismic channels reaching several hundred, even short-term interruptions in data transmission can lead to the loss of entire blocks and, consequently, to repeated requests for transmitting lost information, etc. e. in general, to reduce the speed of data exchange.

 In this regard, in telemetry equipment, before the digital information is transmitted to the communication line (wired or radio channel), special noise-resistant data encoding is used, and on the receiving side, complementary decoding, i.e. conversion of data to its original form.

 The essence of error-correcting coding, the theoretical foundations of which were laid down by C. Shannon in the 40s, is as follows. The noise immunity of data transmission over a distance is always provided due to redundancy [1]. So, if you transfer the digital symbol M twice to the communication channel (M, M), then on the receiving side you can identify one error, but establish its place, i.e. cannot be fixed. In order to make it possible to correct the error, the symbol M must be transmitted at least three times (M, M, M). To correct two errors, it is necessary to transmit the symbol M at least five times, etc. In general, to correct N errors, the symbol must be repeated P = 2N + 1 times. The disadvantage of P-fold repetition is the large redundancy in data transmission. Therefore, the main way to increase the noise immunity is not in P-fold repetition, but in the additional introduction of control and test characters in the general data packet M1, M2, M3, ..., and any functional dependence associated with it [1]. In particular, systematic codes, in which the number and location of control characters are strictly defined, are widely used in television broadcast systems. For example, the Hamming code, which although corrects only single independent errors, but, on the other hand, is optimal in terms of redundancy and the probability of incorrect decoding [2].

 Similar and more advanced methods of interference protection of the communication channel are used, for example, in the XI Telemetry system for collecting and recording XZone seismic data from SI Technology [3], in the telemetry multichannel seismic systems TMSMS, TSM-24 jointly produced by SNIIGGiMS and AO SibOKB [4,5 ]. These techniques are implemented, as a rule, using hardware, hardware-software or microcontroller means that act as a communication channel encoder on the transmitting side and, accordingly, a communication channel decoder on the receiving side.

 Known communication systems for transmitting digital data in the form of serial binary signals over a two-wire line. In [6], a system is considered consisting of a series-connected source of an analog signal (IAS), a source encoder (CDI), a communication channel encoder (CDC), a digital communication line (DLC), a communication channel decoder (DC), a source decoder (DI) and an analog signal receiver (PAS). In this case, the source of the analog signal in the general case can be any analog signal, including the signal from the seismic sensor. An analog-to-digital conversion takes place in CDI, and in the CDC the received digital code is converted (taking into account some kind of noise immunity algorithm) into a form suitable for transmission in sequential form over the DSC. At the receiving side, in the DC from the obtained sequence, the original digital word is restored, which in the source decoder, which is a digital-to-analog converter, is converted into an analog signal sample. If there is no need for the last operation, then the data received and decoded in the recreation center can immediately arrive, for example, in a digital storage device.

 Such channelization systems provide noise immunity mainly due to the formation of the corresponding codes in the channel encoder, which is not always sufficient for the following reasons. With prolonged and intense exposure to electrical noise, for example, if sections of the communication channel are located near power plants, power lines, and this often happens during field seismic surveys, simple encoding algorithms do not provide reliable transmission of digital information, up to a complete loss of communication. The implementation of complex algorithms that could maintain the communication line in working condition, in turn, requires a significant expenditure of computer resources and time, which in turn leads to an increase in the cost of equipment and a decrease in the profitability of work.

 The closest organization to the proposed one is the digital communication channel described in [7] - the prototype. A channel is organized in the form of a system that operates as follows. On the transmitting side, several analog signals are fed to the inputs of the same amplitude-pulse modulators (AIM), the outputs of which are combined. All AIMs are controlled by transmitter generator equipment (GOper), which provides temporary multiplexing of analog signals. Next, each AIM sample using an analog-to-digital converter (ADC) is converted into a digital code by the method of pulse-code modulation (PCM), which is then combined in a linear signal shaper (FLS) with additional synchronization and service information pulses. In fact, the FLS block performs the function of a channel encoder from [6], since at its output the code is redundant due to the introduction of overhead and clock pulses. At the last stage, in the transmitter encoder, the pulse sequence is transformed into a bipolar signal, alternating the polarity of the pulses that are transmitted to the pulse transformer of the communication line. Through the communication channel, the sequence is transmitted in the form of bipolar pulses. On the receiving side, the input pulse transformer amplifies the digital sequence and transfers it to the regenerator, where the main parameters of the pulses are restored (amplitude, duration). Then, the reconstructed bipolar PCM signal in the receiver code converter is converted to unipolar and fed to a decoder, the function of which is performed by a digital-to-analog converter (DAC). The obtained analog samples are demultiplexed, and the synchronization of this operation is carried out using the receiver generator equipment (GPR). In turn, the Gopr is synchronized with the Gopper by additional sync pulses and service information present in the PCM signal. As a result of demultiplexing, the group AIM signal is temporarily divided between the corresponding analog channels, consisting of a low-pass filter and an amplifier, at the output of which the original analog signal is generated.

 Generalizing the structure of the digital channelization system described in [7], it can be represented in the following form. The transmitting side comprises an analog signal source connected in series, a signal source encoder, a communication channel encoder, which functionally corresponds to the FLS unit and the transmitter code converter, and a pulse transformer of the communication line. The receiving side also contains in series a pulse transformer of the communication line, a regenerator and a code converter of the receiver, which actually act as a communication channel decoder, as well as a signal source decoder and an analog signal receiver. It is easy to see that this scheme in general form corresponds to the scheme described in [6]. At the same time, the presence of a regeneration unit in the receiving part distinguishes these schemes somewhat. As already noted, in the regenerator, the main parameters of the pulses are restored, which underwent various kinds of distortions during transmission over the communication line, including under the influence of external noise. In this case, erroneous decisions can be made in the regenerator. So instead of character 0, character 1 can be formed and vice versa. Such errors can occur if the true value of the received digital signal changes by more than Uпop = Um / 2, where Uпop is the threshold voltage fixed in the regenerator; Um is the nominal amplitude of the pulses at the input of the regenerator. An error will not occur only if the current external interference does not exceed Uop [7]. Thus, the regenerator is actually a comparator with a set threshold. However, it is known that when a pulse passes through a single threshold element, its duration is distorted, since the fronts and decays of the pulse are bent (drawn out) in the communication line. Then the higher the threshold, the shorter the pulse duration at the output of the threshold element due to the fact that the pulse narrows to the top. It is possible to use a tracking threshold set in proportion to the amplitude of the signal, which will lead to an increase in the degree of suppression of interference. However, the threshold monitoring the total amplitude of the signal must spontaneously decrease with time in the interval between the signal pulses, moreover, faster than the decrease in the signal amplitude, otherwise the latter will be suppressed. But spontaneous reduction of the threshold between the elementary premises leads to the fact that the maximum amplitude of the suppressed noise depends on the location of the interference in the gap between the elementary premises. The total amplitude criterion is ineffective for distinguishing the pulses of the useful signal from interference.

 Therefore, in order to improve the noise immunity of a digital communication channel at the stage of reception and primary processing of a digital sequence, one should use the inertia criterion for changing the amplitude of the useful signal, the essence of which is that each subsequent pulse is considered to be a signal if it differs in amplitude by no more than established tolerance. This tolerance is determined by the value of the threshold relative to the amplitude of the last transmitted signal pulse (when using the criterion of the total amplitude, the threshold is set proportionally to the total amplitude level of the useful signal). The use of the inertia criterion in comparison with the general amplitude criterion leads to a higher degree of interference suppression, since the threshold between the useful pulses is practically unchanged and, in addition, it is forcibly corrected by each signal pulse. All pulses that are less in amplitude (subject to tolerance) than the signal that appeared before them are considered interference and are suppressed. Thus, in order to improve the noise immunity of the communication channel, it is proposed to supplement it with an adaptive shaper of serial binary signals that implements the inertia criterion described above.

 Figure 1 shows the structural diagram of a digital communication channel, consisting of a series-connected source 1 of an analog signal, encoder 2 source, encoder 3 communication channel, pulse transformer 4 transmitter, communication line 5, pulse transformer 6 receiver, adaptive shaper 7 serial binary signals, decoder 8 of the communication channel, decoder 9 of the source, receiver 10 of the analog signal. The principle of operation of the entire system and the individual blocks included in its composition is similar to that described in [6, 7]. An exception is the additionally introduced adaptive shaper 7 serial binary signals. Let us dwell in more detail on the structure and operation of this unit.

 In FIG. 2 shows a block diagram of an adaptive driver. It consists of two identical blocks A1 and A2, the outputs of which are connected to a common RS trigger 20. Common for both channels is the input inverter 11, which provides inversion of the same type of pulses at the inputs of the blocks A1 and A2 relative to each other. Each of the blocks consists of a preliminary amplifier 12, a differentiating circuit 13, a first key 14, a first integrator 15, a second integrator 16, a second key 17, a divider made on resistors R1 and R2, a phase inverter 18 and an emitter follower 19. Moreover, for block A1 the input of the pre-amplifier 12 is connected to the output of the inverter 11, and for block A2, the input of a similar pre-amplifier is connected to the input of the inverter 11, i.e. directly with the output of the pulse transformer 6 of the receiver (figure 1). Further, for blocks A1 and A2, everything is identical. The output of the pre-amplifier 12 is connected to the inputs of the differentiating circuit 13, the first integrator 15, the second integrator 16 and the control input of the phase inverter 18, the output of which is connected to the input of the emitter follower 19. The output of the latter is the output of block A1 (similarly for A2). The output of the differentiating chain 13 is connected to the input of the first key 14, the output of which is connected to the shunt input of the first integrator 15. The output of the first integrator 15 is connected to the input of the second key 17, the output of which is combined with the output of the second integrator 16 through a divider on resistors R1, R2 and connected to the supply bus of the bass reflex 18. Fig. 3 shows voltage plots illustrating the operation of the device. FIGS. 3, 21 show the shape of the original signal transmitted to the communication line 5 (FIG. 1), FIG. 3.22 - waveform distorted by interference after the communication line 5 (Fig. 1), received at the input of the adaptive driver 7 (Fig. 1), i.e. the input of the pre-amplifier 12 (figure 2). The adaptive driver operates as follows. The signal pulse is fed to the input of the pre-amplifier 12, the operating point of which is set in such a way that it limits the signal from above. The waveform at the output of the pre-amplifier 12 is shown in FIG. 3.23 for block A1 and Fig. 3.24 for block A2. The limitation of the signal by the preamplifier 12 increases the signal to noise ratio. The degree of interference suppression is the higher, the greater the level of restriction. However, it should be small enough so that the useful signal, changing with time in amplitude, could not settle below its level. From the output of the pre-amplifier 12, the signal pulse enters the first integrator 15, charging its capacitor to an amplitude close to the amplitude of the pulse, as a result of which the second key 17 is unlocked. At the same time, the signal pulse arriving at the input of the second integrator 16 charges its capacitor to a level of at least 70% of the voltage across the capacitor of the first integrator 15. This level is selected using the resistor divider R1 and R2. To the point of connection of the resistors of the divider is connected the supply circuit of the phase inverter 18, which opens only when the amplitude of the pulse received at its control input (i.e., directly from the output of the preliminary amplifier 12) exceeds the voltage (threshold) of its supply circuit, the level of which, as noted above, is formed by the voltage across the capacitor of the second integrator 16 and the state of the second switch 17, which determine the potential at the junction point of the resistors of the divider Rl and R2. The signal pulse in the differentiating circuit 13 is converted into a short one, which, acting on the first key 14, momentarily unlocks it. The first key 14, in turn, briefly shunts the capacitor of the first integrator 15, partially discharging it. In the interval between the locking of the first key 15 and the end of the signal pulse, the capacitor of the first integrator 15 is recharged to a voltage close to the amplitude of the pulse. Thus, each signal pulse adjusts the threshold in proportion to its amplitude. The interference, the amplitude of which is less than the threshold, does not pass through the bass reflex 18 and the threshold is not adjusted. Therefore, changing in amplitude from pulse to pulse by less than 30%, the signal through the phase inverter 18 and the emitter follower 19 is fed to the RS trigger and switches it.

 The shape of the pulses transmitted through the adaptive driver is shown in FIG. 3.25 (at the output of block A1) and Fig. 3.26 (at the output of block A2). At the RS output of the trigger 20, a signal is generated (Fig. 3.27), similar in form to the original one.

 Since the thresholds remain unchanged between pulses, adaptive driver units A1 and A2, which separately distort the duration of the chips, working together, compensate for mutual distortions (if one channel reduces the duration of the chip due to the curvature of its front, then the other by as much it also increases the duration of the same elementary premise due to the same curvature of its decline). Therefore, the transmitted binary signal is restored in the device without distortion, only the chips are delayed for a time equal to the rise time of the pulse fronts until a threshold level is reached.

 The adaptive shaper was made on silicon low-power transistors of pnp and npn structures, such as KT315G, KT361 G and a CMOS chip of the K561 series. The device reliably works with signals with an amplitude of at least 20 mV and a repetition rate of 20 to 200,000 baud. The speed can be increased to several million bauds if the appropriate element base is used (microwave transistors, high-speed microcircuits, for example, the KR1554 series).

 In conclusion, it should be said that the combined use of an adaptive shaper of serial binary signals and a fairly simple noise-resistant coding, which allows correcting only single errors, significantly increases the noise immunity of a digital communication channel compared to using only one noise-resistant coding, even if more complex algorithms are used for such coding , allowing to correct double and triple errors.

Sources of information:
1. V. M. Mutter. Fundamentals of noise-resistant television information transmission. L., Energoatomizdat, 1990, p. 5, 6.

 2. B.V. Shevkoplyas. Microprocessor structures. Engineering solutions. M., Radio and Communications, 1990, p. 289, 290.

 3. XZone. Radio telemetry system for collecting and recording seismic data. Advertising booklet of SI Technology Company, Russia, Gelendzhik.

 4.S.A. Fedotov. About telemetric multichannel seismic systems. Geophysics 1, 1996, p. 62 - 64.

 5. Telemetry seismic station. Technical description. Album of electrical concepts. AO SibOKB, Novosibirsk, 1995.

 6. I. M. Dvoretsky, I.N. Driatsky. Digital audio broadcasting. M., Radio and Communications, 1987, p. 36 - 38.

 7.A.S. Ajemov, A.I. Koblenz, V.N. Gordienko. Multichannel telecommunication and channel-forming telegraph equipment. M., Radio and Communications, 1989, p. 111 - 115. PROTOTYPE.

Claims (1)

 A digital communication channel of telemetry seismic recording equipment containing a series-connected analog signal source, a signal source encoder, a communication channel encoder, a pulse transformer of a transmitter, a communication line, a pulse transformer of a receiver, as well as a series-connected communication channel decoder, a source decoder and an analog signal receiver, characterized in that it additionally introduced an adaptive shaper of sequential binary signals that implements the inertia criterion the amplitude of the useful signal, the input of which is connected to the output of the pulse transformer of the receiver, and the output is connected to the input of the decoder of the communication channel, and consisting of two identical units, common to which are the input inverter and output RS trigger, while the input of the inverter connected to the input of the second block is the input of the adaptive driver, and the inverter output is connected to the input of the first block, each block consists of a series-connected input pre-amplifier, a differentiating chain and the first to yuch, as well as from the first integrator, second integrator, second switch, divider and series-connected phase inverter and emitter follower, while the output of the first key is connected to the shunt input of the first integrator, the output of the input pre-amplifier is connected to the inputs of the first integrator, second integrator and phase inverter, the output of the first integrator is connected to the input of the second key, the output of which is connected to one of the inputs of the divider, the output of the second integrator is connected to its second input, the output of the divider it is single with the busbar of the bass reflex, the output of each of two identical blocks is the output of the emitter follower, which is connected to one of the inputs of the output RS of the trigger, the output of the emitter follower of the second unit is connected to the other input, the output of the RS trigger is the output of the adaptive shaper.

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