Digital tape recorder. Encyclopedia of radio electronics and electrical engineering

Encyclopedia of radio electronics and electrical engineering / Digital technology

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In the early eighties, when working through meteor showers, the transmission rate was 600-800 characters per minute, and it was still possible to use a tape recorder to record the received signals, slowing down the movement of the tape in it during decoding. Now the transmission speed has increased significantly, reaching 2000 characters per minute. And abroad, the issue of conducting meteor communications at a speed of up to 4000 characters per minute is already being considered.

The magnetic recording was replaced by a digital one with a "hard" and program-controlled logic. Increasingly, a computer is used in amateur radio practice. However, not every shortwave and ultrashortwave who wants to work through meteors has the opportunity to make a "hard" logical device proposed by V. Bagdyan and described in [1-3], and even more so to assemble or purchase a computer with the necessary software.

(click to enlarge)

The simple digital "tape recorder" offered to readers (hereinafter referred to as the device) allows meteor communications at a transmission rate of 420 to 2000 characters per minute. It combines many advantages of analog recording (such as the participation of a human auditory analyzer in the reception process, which is especially important in conditions of interference; the ability to estimate the transfer rate of a correspondent when working on a general call) with the advantages of digital (the ability to operate a device with a narrow-band filter; instant automatic transition to the playback mode after the end of the recording to the playback mode after the end of the recording of the burst with a slowdown by several times, and when finalizing the device - up to a complete "stop", without changing the tone, of the reproduced signal; logical protection against switching to the playback mode from signals, not meeting some given parameters).

The decrease in the reliability of signals recorded at a speed of more than 1500 characters per minute during playback is justified by the simplicity of the device. If you increase the amount of memory and increase the clock speed, the speed range can be expanded. The higher the clock frequency in the device, the greater the reliability can be achieved.

The schematic diagram of the device is shown in fig. 1. It consists of an analog-to-digital converter based on transistors VT1-VT3 and a Schmitt trigger DD1.1, a signal envelope “recovery” node (performed on a waiting multivibrator DD2.1), a controlled clock generator based on 2I-NOT elements of the DD3 microcircuit, memory nodes (on counters DD4-DD6 and RAM DS1) and control (on a waiting multivibrator DD2.2 and a DD8 chip) and a tone generator on the elements 2I-NOT DD7.1 - DD7.3.

Voltage diagrams at some points of the device are shown in fig. 2.

Filtered tones with an amplitude of 2...3 V, transmitted at a rate of 420-2000 characters per minute, are fed from the receiver output to the ADC, which is made according to a scheme similar to that described in [4] (the input part is somewhat changed). Here they are limited by diodes VD1, VD2 and amplified by a differential amplifier based on transistors VT1, VT2.

Amplifying stages on transistors VT2 and VT3, covered by positive feedback through resistor R9, form a node with trigger properties, which generates rectangular pulses coming to the input of the Schmitt trigger DD1.1. From its output, a tonal message in the form of a burst of rectangular pulses enters the input D of the waiting multivibrator DD2.1 The function of this node is to fill in the pauses in the incoming burst and thereby restore the original duration of the telegraph message (with a slight error that increases with increasing transmission speed). The condition for the normal operation of the "recovery" node: Ti,<Tfm,<Ti+ti, where Tfm, is the duration of the pulse generated by the waiting multivibrator DD2.1, Ti is the duration of the pulse in the pack, Ti is the period of the pulses in it. At a frequency of tone bursts of 1 kHz and a duration of Tjm equal to 1 ms, the duration of the "restored" message is 0,25 ms longer than that of the received one. From the output of the waiting multivibrator DD2.1, the telegraph message arrives at the input D of the RAM DS1.

Before writing information to the RAM, you must first "clear" all the memory cells in it, for which the SB2 button is held down until the HL2 "Record" LED goes out. At the same time, a low logic level appears at the RO inputs of the counters DD4-DD6, and they begin to count the pulses coming from the clock generator, thereby sequentially sorting through the RAM addresses from 0 to 1023. A logical 0 will be written to all RAM cells, since from output 13 of the waiting multivibrator DD2.1 until the end of holding the SB2 button, the input D of the RAM receives a low logic level. At the 1024th cycle, a low-level pulse from the output 2 of the counter DD6 will switch the RS-trigger (on the elements DD8.2, DD8.3), and the device will go into playback mode. The mode change can be judged by the extinction of the HL2 LED.

The control node works as follows. With a short press of the SB2 button, a differentiated low-level pulse will transfer the RS flip-flop on the elements DD8.2, DD8.3 to a state in which the output of the DD8.2 element will be a low logic level, and the output of DD8.3 will be high. The device will enter recording mode. In this case, the HL2 LED will light up, the current will stop flowing through the winding of the relay K1, the RAM is ready to record information from the air. The waiting multivibrator DD2.2 is used to start the node when tone bursts appear at the input of the device. In addition, it is a selective element that allows you to increase the noise immunity of the device. Starting with the edge of the pulse of the first telegraph message from the output of the waiting multivibrator DD2.1, the waiting multivibrator DD2.2 allows the counters DD4-DD6 to work with a signal passing through the elements DD8.1 and DD3.4. If a pause in a series of telegraph messages or the duration of a message during the recording process exceeds the duration of the pulse generated by the waiting multivibrator DD2.2 (Tjm2 = 100 ms), the device will return to its original state - to the information standby mode. The same will happen when the duration of a series of bursts does not satisfy the condition Тс>tз/2-ТЖм2, where Тс is the duration of a series of bursts, tз is the recording time, depending on the position of switch SA1 (in position "600" tз==2 s, " 1200 "-tz \u1d 2 s), Tzhm100 \uXNUMXd XNUMX ms.

If an incoming series of telegraph packets satisfies the conditions listed above, it will be written to RAM. The pulse from the second bit of the counter DD6, differentiated by the C9R26 circuit, will change the state of the RS flip-flop, and the device will go into playback mode. In this case, relay K1 will work and with its contacts K 1.1 it will connect capacitor C5 in parallel with capacitor C6 in the clock generator, which will lead to a decrease in the clock frequency by approximately 8 times. The EWR RAM input from the RS flip-flop (with DD8.2) will receive a high logic level that allows reading. A low logic level from the output of the element DD8.3, passing through the elements DD8.1, DD3.4, will allow the counters DD4 - DD6, cyclically changing the addresses of the RAM. Thus, the recorded information will be reproduced at the output of the RAM, which is fed to the lower input of the DD7.4 element according to the circuit, which plays the role of a logical adder. Its second input receives a signal from a tone generator. From the output of the DD7.4 element through the emitter follower (VT4), the tone signal is fed to low-ohm headphones BF1.

Device parameters depending on switch position "600"/"1200"

Digital "tape recorder" assembled on a double-sided printed circuit board (Fig. 3), (Fig. 4), (Fig. 5) . The device uses fixed resistors MLT-0,125 and MLT-1 (R21), trimmer SP4-1V (R13). Capacitors KM-5B, KM-bB. Decoupling capacitors Cp - KM-5B, Cp "- K53-1. Relay K1-RES55 (passport RS4.569.603).

Establishing the device is reduced to the selection of resistors R4, R15, R21 and the resistance of the resistor R13.

A sinusoidal signal with a frequency of 1 kHz and an amplitude of 300 mV is fed to the input of the device, and by selecting the resistor R4, the maximum sensitivity of the ADC is achieved by monitoring the signal at the collector of the transistor VT3. Then, instead of the selected resistor, a new one with a slightly higher resistance is soldered, so that in the absence of an input signal, the VT3 transistor is securely closed. In this case, the trigger hysteresis in the ADC is about 100 mV.

By adjusting the resistor R13 at an average frequency of a narrow band filter of 1 kHz, the duration of the pulse generated by the first waiting multivibrator is set to 1,25 ms. For other values ​​of the input frequency, the pulse duration must be corrected according to the equation Tfm = Ti + ti / 2, where Ti is the period of the pulse train, ti is the duration of the pulses in the series.

By selecting the resistor R15, it is achieved that the pulse duration of the second waiting multivibrator becomes equal to 100 ms. Resistor R21 is selected so that the input signal level is independent of the position of switch SB2.

Finally, some practical advice.

If the receiver used has a 3-hour gain control, then in the digital "tape recorder" the resistor R1 can be eliminated, and the input signal can be fed to the resistor R2 (point 1 on the board).

To get maximum sensitivity, the 3H gain control is set to near maximum volume. Which one exactly can be specified as follows. A signal is applied to the input of the receiver so that it exceeds the noise by 2-3 points (on the S scale). The SA2 switch is moved to the "Play" position, while holding the SB1 button pressed, the gain controls ensure that a clear tone signal is heard in the headphones. If you stop supplying a useful input signal, the HL1 LED should light up only at peaks of noise, but not glow constantly, since the device, having recorded noise interference, must switch to playback mode.

To increase the noise immunity from short impulse noise, the interface described in [1.1] can be built into the device. It is connected between the output of the Schmitt trigger DD2.1 and the input D of the DDXNUMX chip.

Literature

1. Bagdyan V. Amateur display. - Radio, 1982, No. 5, pp. 19-24.
2. Bagdyan V. Block for processing CW and RTTY signals. - Radio, 1982, No. 8, p. 17-20.
3. Bagdyan V. CW interface to an amateur display. - Radio, 1983, No. 6, p. 19-20.
4. Biryukov S. Digital frequency counter. - Radio, 1981, No. 10, p. 44-47.

Author:I. Nikiforov, (UB5WBL), Stary Lviv region; Publication: N. Bolshakov, rf.atnn.ru

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