Yasuhiro Riko

A basic concept of direct converting digital microphone

An electroacoustic system which directly converts analog acoustic signals to digital electric signals is described. The system consists of a subtractor, a sampling and holding circuit, a sigma–delta modulator as a comparator, an accumulator, and a local direct digital-to-analog converting transducer similar to a typical electronic analog-to-digital converter. The subtractor is an electrostatic device which has a diaphragm, driving electrodes, and a detecting electrode. The surface area of the driving electrodes corresponds to the significant bits in the digital signal, as an electroacoustic digital-to-analog converter. The detecting electrode produces an electrical signal proportional to the displacement of the diaphragm driven by subtracting the received acoustic signal from the electrostatically driven force. This is regarded as a subtractor. The detected signal is amplified and sampled-held and modulated by the sigma–delta procedure and generates a signal of ±1 bit, which is added to the accumulator me...

Environmentally Robust Electret Condenser Microphone

A new structure of front filter of ECMs has been introduced to protect in practical use from such common hazards as water or alcohol. This protection structure is composed of acoustically transparent mesh with a water-repellent coating on an electrically-conductive layer. This water-repellent coating has a contact angle of more than 150 degree for water on the filter having less than 50 /spl mu/m aperture which chokes water invasion but is enough for acoustical transmission. The new protection structure also has an electrically conductive layer inside which copes with the induction noise problem by the effect of electrostatic shielding. The reliability of the coating was confirmed by several accelerated tests such as long term immersion in water, heat cycling in the presence of both high and low moisture, heat shocks and surface scratch. These tests showed no damage to the coating. The fabricated electret condenser microphone has met the requirements for the IEC 60526 class 7 which is 30 minutes under water at a depth of 1 meter. The induction noise was reduced by approximately 10 dB at 2 GHz.

A conceptual experiment of direct converting digital microphone

A transducer which directly converts signals from analog acoustic to digital electric is described. This consists of a subtracter, a sampling and holding circuit, a sigma‐delta modulation circuit as a comparator, an accumulator, and a local digital‐to‐analog converter similar to a typical electric analog‐to‐digital converter. The subtracter is an electrostatic device which has a diaphragm, driving electrode, and detecting electrode. The driving electrode is divided into corresponding digital signal bits by the ratio of each area which enables one to drive the diaphragm digitally and is regarded as an electroacoustic digital‐to‐analog converter. The detecting electrode makes electric signals proportional to the displacement of the diaphragm driven by subtracting the incoming acoustic signal and electrically driven force. This is regarded as the subtracter. This detected signal is amplified and sampled‐held and modulated by the sigma‐delta procedure and generates subtracted signal to + or −1 bit which is ad...

Localization of moving reinforced sound source by superposed spread spectrum reference signal

A localization of moving reinforced sound source is described. A sound reinforcement system, in which microphones and loudspeakers are installed in the same sound field such as a conference room, has a drawback of insufficient reproduced sound pressure due to an adjustment to prevent howling. The fundamental technique of howling suppression that decreases coupling between microphones and loudspeakers is well known. The algorithm for decreasing coupling proposed here is based on a synthesis of directivity of microphone arrays to meet its dips in the directions of loudspeakers automatically. The direction of loudspeaker includes not only direct propagation but propagation through reflection and diffraction. In order to localize the direction in real time, a reference signal that is superposed on the speech signal is continuously detected by microphone. As a result, the reference signal whose sound pressure is low enough to be the same as the room noise of a typical broadcasting studio, will be detected without interference for speech communication by using spread spectrum technology.

A study on designing the amplitude gain of the feedback loop of an electroacoustic system.

The acoustic feedback from a loudspeaker to a microphone forms a closed loop and is the cause of instability (howling). When the electrical amplifier gain is fixed on maximum within a designed range, it is the excess amplitude of the open‐loop gain over the unity (0 dB) that should be minimized by a howling suppression technique. It can be designed using a diffuse sound field model [Diestel, J. Acoust. Soc. Am. 35, 2019–2022 (1963)]. According to this method, the designed value was 18–24 dB in the case of a single loudspeaker and single microphone system located in a room with dimensions of 10×20×6 m and sound absorption coefficient of 0.05–0.5 and the gain of the electrical amplifier being fixed so that the farthest located attendant from the loudspeaker could perceive 62.9 dB (SPL). In order to evaluate the result, measurements were carried out in a conference room.