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1 system composition
The block diagram of the voice circuit is shown in Figure 1. The RS232 level signal is released by the PLC and is received by the single chip AT89C51 after level conversion. On one hand, the voice chip ISD1420 is controlled to play, and at the same time, the high-brightness digital display card is controlled to provide a countdown signal to provide prompts for the trained personnel. The watchdog ensures that the circuit is operating normally. ?
2 voice signal control
2.1 voice circuit composition
According to the training requirements, the voice circuit will make a "long beep" 10 seconds before the start, and then send a "short beep" every second, and when the time is zero, the "fire" will be issued (the sound law is adjustable).
The voice part circuit design is shown in Figure 2. The "beep" and "starting gun" are stored in segments by the voice storage/reproduction chip ISD1420. The audio signal output by the ISD1420 is capacitively coupled to the BTL power amplifier circuit composed of two integrated power amplifiers TDA2003, and finally output by the speaker. The MAX232 converts the RS?232 level signal sent by the PLC into a TTL level and sends it to the AT89C51 microcontroller. The AT89C51 controls the playback of the ISD1420 according to the PLC command, and the P3.2~P3.5 is used to adjust the release. Sound address. The MAX813L is used as a watchdog to provide power-on reset and operation monitoring for the AT89C51 microcontroller. The dog signal is provided by P3.7.
2.2 voice circuit characteristics
The voice storage/reproduction chip ISD1420 is selected. The circuit uses the EEPROM storage method to directly write the analog voice data into the semiconductor storage unit, and has many advantages such as natural sound quality, reproducible recording and playback, anti-interference, low power consumption and the like. ISD1420 playback time is 20 seconds; up to 160 segments, each segment length is at least 125ms; input sampling 6.4kHz; 100000 recording cycles; 5V single power supply, playback current 15mA, maintenance current 0.5μA. Fully meet the design needs. Use a 28-pin DIP package chip, pin function see reference [5]. The address pins (A0 to A7) have two functions, depending on the state of A7 and A6. When one of A7 or A6 is "0", A0 to A7 are interpreted as address bits as the starting address of the current recording and playback operation. The circuit we designed uses the address bits of A0~A7. The address space allocation of ISD1420 is shown in Table 1. ?
According to the needs, the voice signal is divided into two parts: “prompt tone†and “starting gunshotâ€. The “long tone†or “short tone†is distinguished by the playback time. The internal memory space of the ISD1420 is divided into two parts by using the address function of the A0 to A7 pins. The 20-second storage space is divided into the first 8 seconds to store the “prompt tone†and the last 12 seconds to store the “starting gunshotsâ€. This design does not require high sound quality for voice signals, so voice data is previously entered in analog mode. Since the actual playback duration is less than the stored time (8 seconds, 12 seconds), the appropriate data segment playback can be selected by adjusting the lower address A0, A1, A2 (P3.2 to P3.4 control) of the voice chip.
As shown in Figure 2, the voice circuit is only controlled for playback when used in the field.
The P3.6 control of the AT89C51, the playback time is determined by the P3.6 low hold time. The playback address is determined by P3.2 to P3.5. P3.5=0 is the “prompt tone†address; P3.5=1 is the “starting gunshot†address, and P3.2~P3.4 is the address fine-tuning. When the sound is played, which sound is placed, and how long it takes to be released by the PLC.
2.3BTL power amplifier circuit
The starting device requires that the battery be powered when there is no AC power, so the power supply is set to 12V. Since it is used outdoors, the sound required to be emitted has a certain loudness, that is, the voice circuit is required to have a large power output. The ISD1420's internal output stage has an amplifier with a direct speaker drive power of 12.2mW (16Ω load), which is quite different from our actual needs, so the final stage amplifier must ensure that high-power signals can be output at low voltages to drive The speaker sounds.
To output a high-power signal under a single-supply low-voltage condition, the power amplifier circuit selects a bridge-type transformerless output circuit (ie, a BTL circuit), and its basic working principle is illustrated by the schematic diagram of the discrete component BTL circuit shown in FIG. There are four output tubes VT1 ~ VT4 in the figure, and the speaker is connected between the two outputs. In the positive half cycle of the input signal u, VT1 and VT4 are turned on, and VT2 and VT3 are turned off, and the output current is as shown by I1 in the figure. During the negative half cycle of the input signal u, VT2 and VT3 are turned on, and VT1 and VT4 are turned off, and the output current is as shown by I2 in the figure. Positive and negative half cycles, the potential difference on the speaker is opposite, equal in size, and close to the power supply voltage, so the utilization of the power supply is very high. In theory, the efficiency of the BTL circuit can reach 78.5%. The actual circuit used is shown in Figure 2. Two 10W integrated power amplifiers, TDA2003, are connected in the form of a BTL circuit. The capacitor is directly coupled to the speaker. By using the principle of simultaneous “push†and “pullâ€, the peak-to-peak voltage of the sine wave on the speaker is approximately Double the power supply voltage. The output can basically meet the requirements.
3 software design
The software design of the AT89C51 is relatively simple. It mainly includes serial communication program with PLC (12MHz crystal oscillator, 4800 baud rate, eight-bit asynchronous mode), playback control program, display control program and dog feeding program. Since they are all conventional methods, they are not described here.
4 Conclusion
The voice link of the starting training device has a simple structure, low cost, convenient adjustment and high reliability, and the actual use certificate can meet the requirements of the field bicycle training. Through appropriate adjustments, this link can be used for other venue competition training. At the same time, the design also provides a useful attempt to provide a large power speech signal output for smart instruments. ?
references
[1] Zhu Minxiong, et al. Computer speech technology [M]. Beijing: Beijing University of Aeronautics and Astronautics Press, 2002.
[2]STMICRO DataSheet. TDA2003?10W Car Radio Audio Amplifier[EB/OL].
[3]ISD DataSheet. ISD1400 Series Single?Chip Voice Record/Playback Devices[EB/OL].
[4] Deng Kuanlin. How to use the set to successfully form a BTL circuit [J]. Modern Electronic Technology, 2003, (14).
[5] Zhang Shoubo, Guo Songlin. High-fidelity voice recording and playback circuit and its application in instruments [J]. Instrumentation Technology, 2004, (3).
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