Sound transmission by amplitude modulation of ultrasound

A simple sound transmission system with amplitude modulation of ultrasound consists of a transmitter and a receiver. In the transmitter, the Arduino Pro Micro emulates a sound card with a USB interface and generates two signals: output 5 is an audio signal, which is then cleared from the PWM RC filter, and output 9 is a carrier frequency of 40 kHz. Both signals are fed to a single-transistor cascade, parallel to the load resistor of which an ultrasonic transducer is connected from a range finder.


In the receiver, the same ultrasonic transducer converts the acoustic signal back into an electric one, then everything is done in the same way as in a direct-gain radio receiver: UHF, detector and preliminary ULF. The power amplifier is a standard PAM8403 module.



In order to emulate a USB sound card, the LuFA library was required, so instead of the Arduino IDE, avr-gcc was used. To fill the firmware (lying here ) you need to reset the Arduino by hardware, which will cause it to go into USB bootloader mode for a few seconds. During this time, you need to have time to start pouring with avrdude.
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To set up a device, you must first use an oscilloscope to ensure that there is no overmodulation and no limitation:



Eliminating both by reducing the volume and selecting the resistors in the transmitter, you need to connect the oscilloscope to the output of the UHF receiver, and in the transmitter adjust the PWM frequency (software) in the range from 38 to 42 kHz so that it coincides with the resonant frequency of the converters. The author recommends experimenting with the introduction of an AGC system into the receiver, similar to that used in conventional AM radio receivers.

So everything works after adjustment:


You can program the Arduino in the usual way by placing only carrier generation on the firmware, and you can do without it at all - by implementing a 40-kilohertz generator by hardware. At the same time, an audio signal can be fed directly to the modulator, and the RC filter becomes unnecessary.