The wide application of wireless sound reinforcement systems solves the problems of wiring and mobile use in practical engineering. The wireless transmission method has also evolved from the traditional U-segment and V-segment wireless amplification to today's infrared, Bluetooth and 2.4 GHz band wireless digital transmission. The use of traditional analog signal wireless sound amplification equipment transmitters will be interfered by the same frequency, adjacent frequency or external radio waves, the sound reproduction will be large, and the high frequency radio wave radiation will be large. The sound amplification will cause a certain amount of human eardrum. s damage. Audio is less likely to be interfered with during digital signal transmission and has strong anti-interference ability. The digital wireless sound reinforcement system can be widely used in teaching, conference venues, modern office, home life and other fields.
Works at 2.4 GHz ISM. There are 400 million available address codes in the frequency band, which can be used simultaneously in the same place without frequency crossover by frequency hopping address technology. The bandwidth of the transmitted signal is greater than the minimum bandwidth necessary for the transmitted information, and the broadening of the frequency band is achieved by the extended function, independent of the transmitted information data, and only the transmitter and the receiver know that the same spreading code is used at the receiving end. Perform correlation demodulation to despread and recover the transmitted information data. The data is carried by all the hopping points. If the noise does not affect all the hopping points, the information can be repaired. Under certain conditions, multiple systems can coexist in the same frequency range. This paper introduces the design of wireless intelligent frequency hopping digital amplifier developed by using ATmega8 MCU and nRF24L01 RF transceiver. With intelligent frequency hopping enquiry technology, the transmitter can be automatically recognized by the receiver more quickly, any transmitter can match any receiver, and automatically lock after matching until the transmitter is turned off or leaves the radio coverage. The output power is 5 W and the effective distance of transmission and reception is ≤ 60 m under the condition of linear transmission of obstacle-free objects.
1 System Analysis and DesignThe system consists of an MCU, a transmitting and receiving system. The audio signal is amplified by the front end signal processing circuit of the transmitting end and sent to the internal A/D of the MCU for sampling, and the MCU packs the sampled data and sends it out through the RF module. The receiving MCU reads the data packet from the RF module and sends it to the TIBR1 inside the MCU for PWM modulation, and then outputs it to the external low-pass filter, and finally restores the corresponding audio signal. The system principle is shown in Figure 1.
1.1 Master MCU Module
The MCU selects the ATmega8 of the AVR family, which is a low-power 8-bit CMOS microcontroller based on the enhanced AVR RISC architecture. Thanks to its advanced instruction set and single clock cycle instruction execution time, ATmega8's data throughput rate is 1 MIPS/MHz and 16 MIPS performance at 16 MHz, thus reducing the system's contradiction between power consumption and processing speed. Operating voltage 2.7 ~ 5.5 V, internal integration of 8 10-bit ADC, SPI serial interface, 16-bit timer with PWM modulation output, 512 Byte EEPROM. Its internal resources can meet the requirements of the transmitting and receiving MCUs.
1.2 RF module
The nRF24L01 is a new single-chip RF transceiver that operates in the 2.4~2.5 cHz ISM band. Built-in frequency synthesizer, power amplifier, crystal oscillator, modulator and other functional modules, combined with enhanced ShockBurst technology, where the output power and communication channel can be configured through the program. Address and CRC check functions are available. The nRF24L01 has low power consumption. When operating at -6 dBm, the operating current is 9 mA. When receiving, the operating current is 12.3 mA. A variety of low-power modes of operation make energy-saving design more convenient. The carrier transmitting and transmitting signals of the transmitting and receiving parties are discretely changed according to a predetermined rule to avoid interference and complete transmission. In summary, the frequency hopping technique FHSS does not suppress interference but tolerate interference. Since the carrier frequency is a hopping, it has the ability to resist high frequency and partial bandwidth interference. When the number of hopping frequencies is sufficient and the hopping bandwidth is wide enough, the anti-interference ability is strong. The use of fast hopping of the carrier frequency has the effect of frequency diversity, so that the system has the ability to resist multipath fading. The orthogonality of the frequency hopping pattern can be used to construct a frequency hopping code division multiple access system, share spectrum resources, and have the ability to withstand overload.
1.3 Audio amplification
As shown in FIG. 2, the circuits U5A, R8, C17, R7, R14, R9, R16, and R13 are responsible for the amplification of the microphone input signal, and the magnification is 10 times. The R8 provides a DC bias to the microphone and is coupled to the op amp U5A via C17. R7, R14, and R9 are used to provide a virtual ground for the op amp. If a 3.5 mm audio signal connector is plugged into J5, the subsequent circuit will disconnect from the preamplifier to switch between the MIC sound and the external audio input. U5B, R11, R15, R17, R19, C21 are responsible for the amplification of the input MIC and external audio signals, the magnification is 5 times, the principle is similar to the preamplification. The LMV358 is used in the op amp. The LMV358 is a Rail to Rail dual op amp with an operating voltage of 2.7~5 V, a gain bandwidth product of 1 MHz, and an operating current of 140μA, suitable for battery power.
1.4 Power supply voltage regulator
The LDO uses the PAM3101, a family of forward linear regulators featuring low quiescent current and low dropout, making them ideal for battery-powered applications. The small-volume SOT--23 and SOT-89 packages are attractive for portable and launch devices. Thermal shutdown and current limiting prevent the device from failing under extreme operating conditions.
2 system interface design2.1 Transmitter system interface
As shown in Figure 3, the ATmega8 is connected to the NRF24L01 via SPI. Before initializing the NRF24L01, the IO port must be initialized. The direction register DDR is set as indicated by the arrow on the figure. The ATmega8 operates at 16 MHz. Therefore, by setting the SPCR and SPSR registers to allow the SPI to operate in clock double mode, the SPI clock frequency can be up to 8 MHz. The internal A/D operating clock is 250 kHz after being divided by 64; the single conversion period is 52 μs; in continuous conversion mode, the sampling frequency is approximately 20 kHz, 8 bit precision. The ADC interrupt is triggered each time the conversion is completed. The power supply part is an important part of the circuit, and its performance directly affects the output sound quality. Since the transient current is large when the transmitting RF module is operating in the transmitting state, if the analog device and the RF module use the same LDO, the output audio will be seriously interfered, so the analog device and the digital device each use an independent LDO to minimize the impact. .
Product Name: Car Charger
Place of Origin: Guangdong, China (Mainland)
Brand Name: OEM
Output Type: DC
Connection: Other
Rated Voltage: 12V-24V
Working Temp: 0-55℃
Weight: 36g
Materials: PC+ABS
Color: White Black
Warranty: 1 year
Suitable for:Most digital devices
SMART PROTECTION & ATTRACTIVE DESIGN ------ Intelligent circuit design protects against short circuiting,over-heating,over-current,and over-charging. Charging stops when battery is full. Car charger with blue LED indicator,which makes it convenient to find exactly where the connection should go; And the light is soft enough not to distract at night.
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