Design of high loudness speech circuit based on ISD1420

Design of a High Volume Voice Circuit Based on ISD1420

LI Ping, LI Yarong, GUAN TIanmin?

(Dalian Jiaotong University, Dalian 116028, China)?

Key words: voice record / playback chip; BTL circuit; single? Chip computer

1 system constitutes the block diagram of voice circuit composition shown in Figure 1. The RS232 level signal is issued by the PLC, and after level conversion, it is received by the single-chip AT89C51. On the one hand, it controls the playback of the voice chip ISD1420, and at the same time, it controls the high-brightness digital display card to give a countdown signal to provide reminders for the trainees. The watchdog guarantees the normal operation of the circuit.

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2.1 Voice circuit composition According to the training requirements, the voice circuit emits a "long reminder tone" 10 seconds before the start of the run, and then emits a "short reminder tone" every second, and sends a "starting gun sound" when the timer returns to zero (the sounding rule is adjustable) ).
The voice circuit design is shown in Figure 2. The "beep sound" and "starting gun sound" are stored in sections by the voice storage / regeneration 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. MAX232 converts the RS? 232 level signal sent from PLC to TTL level and sends it to the single-chip AT89C51. AT89C51 controls the playback of ISD1420 through P3.6 according to the instructions of PLC, and P3.2 ～ P3.5 are used to adjust the playback Tone address. MAX813L is used as a watchdog to provide power-on reset and operation monitoring for the microcontroller AT89C51, and the dog feeding signal is provided by P3.7.

Select voice storage / regeneration chip ISD1420. This circuit uses the EEPROM storage method to write the analog voice data directly into the semiconductor storage unit, which has many advantages such as natural sound quality, repeatable recording and playback, anti-interference, low power consumption and so on. The playback time of the ISD1420 is 20 seconds; it can be divided into a maximum of 160 segments, each segment is at least 125ms long; input sampling is 6.4kHz; 100,000 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, the 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 ~ A7 are interpreted as address bits and used as the starting address of the current recording and playback operation. The circuit we designed uses the address bit function of A0 ～ A7. The address space allocation of ISD1420 is shown in Table 1. ?

As shown in Figure 2, the voice circuit is only controlled to play when used in the field,

AT89C51's P3.6 control, the playback time is determined by the P3.6 low-level hold time. The playback address is determined by P3.2 ～ P3.5. P3.5 = 0 is the address of “prompt tone”; P3.5 = 1 is the address of “starting gunshot”, and P3.2 ～ P3.4 are the address fine-tuning. The PLC issues commands when to play, which sound to play, and how long to play.
2.3 The starting device of the BTL power amplifier circuit requires that the battery can be used to supply power when there is no AC power supply, so the power supply is set to 12V. Since it is used outdoors, it requires that the sound emitted has a certain loudness, that is, the voice circuit is required to have a larger power output. The internal output stage of ISD1420 has an amplifier, and its direct speaker drive power is 12.2mW (16Ω load), which is very different from our actual needs, so the post-stage amplifier must ensure that it can output high-power signals at low voltage to promote 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 (that is, a BTL circuit), and its basic working principle is illustrated by the discrete component BTL circuit diagram shown in FIG. 3. There are four output tubes VT1 ～ VT4 in the picture, and the speaker is connected between the two output ends. In the positive half of the input signal u, VT1 and VT4 are turned on, VT2 and VT3 are turned off, and the output current is shown as I1 in the figure. In the negative half of the input signal u, VT2 and VT3 are turned on, VT1 and VT4 are turned off, and the output current is shown as I2 in the figure. In the positive and negative half cycles, the potential difference on the speaker is in the opposite direction, and the magnitude is equal, both are close to the power supply voltage, so the utilization rate of the power supply is very high. The theoretical BTL circuit efficiency 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, which is directly coupled to the speaker through a capacitor. Using the principle of "push" and "pull" at the same time, the peak-to-peak voltage of the sine wave on the speaker is approximately the power supply. Twice the voltage. The output can basically meet the requirements.

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The software design of AT89C51 is relatively simple. Mainly include 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 repeated here. ?

The voice link of the starting training device has a simple structure, low cost, convenient adjustment, and high reliability, and the actual use proves that it can meet the requirements for on-site bicycle training. Through appropriate adjustments, this link can be used for training in other venues. At the same time, the design also provides a useful attempt for the intelligent instrument to provide a larger power voice signal output. ?