0:00 Introduction: The AudioSpotlight is a highly directional speaker system that creates a focused beam of sound using ultrasonic transducers.
0:38 Principle of Operation: It amplitude-modulates ultrasound with the audio signal. A rectification effect, similar to that used in ultrasonic acoustic levitation, converts the modulated ultrasound into audible sound at the listener's location.
1:08 Applications: This technology is used for applications like advertising displays, where a focused beam of sound is necessary.
1:43 Cost: The AudioSpotlight system is relatively expensive, with prices ranging from $1,200 to $1,500.
2:22 Transducer Disassembly: The speaker array is taken apart, revealing numerous small transducers.
4:39 Transducer Design: Contrary to expectations, the transducers are not piezoelectric. They are electrostatic, using a thin, tensioned, metallized plastic film.
6:47 Electrostatic Principle: A changing voltage between the film and a metal disc creates a force that moves the film, generating ultrasonic sound. The film is at a negative DC bias.
7:12 Modular Design: The transducer array is modular, allowing for different sizes of speakers (e.g., 16x16, 8x16 inches).
8:33 Metallized Plastic: The film is made of metallized plastic, not just aluminum foil.
10:23 Internal Circuit: The speaker contains a small circuit board with diodes, resistors, capacitors, and an inductor.
11:29 Circuit Function: This circuit applies a negative DC bias voltage to the output, which is necessary for proper electrostatic speaker operation, preventing frequency doubling. The inductor present is probably for compensating capacitive reactance.
13:50 Circuit Location Puzzle: The presenter expresses confusion as to why this circuitry is located inside the speaker rather than in the amplifier unit.
14:23 Amplifier/Driver Unit: The "sound beam controller" (amplifier/driver) is opened.
15:42 Circuit Board Components: The controller board contains a Xilinx CPLD (XC9572), a Burr-Brown DAC908, an Atmel ATtiny microcontroller, and analog circuitry.
16:41 Power Stage: Two half-bridges are present, likely to generate the high-frequency ultrasonic signal. Transformers or inductors, along with a capacitor, are probably used as a filter.
19:33 Probing the Circuit: The output voltage is measured with an oscilloscope.
20:09 Output Signal: Without audio input, the output is approximately 17V RMS at 62.186 kHz. With sound input, the amplitude of the carrier wave is modulated and also generally increased.
21:35 Disconnecting Speaker: Disconnecting the speaker while running causes the frequency to jump much higher, about 300 kHz.
22:54 Cable Length Experiment: Doubling the coax cable length between the amplifier and speaker does not change the operating frequency.
27:46 Power stage modulation: Two pulses per half cycle on each half-bridge are present.
27:51 Power Stage Analysis: The power stage is a simple full-bridge design. A series resonance circuit, formed by an inductor and the driver capacitance, likely causes the voltage increase.
29:23 The series resonance inductor may also be present to support multiple sizes of speaker.
30:12 Conclusion: The presenter expresses interest in the transducer's electrostatic design, while the driver board is deemed relatively standard.