Ismo Kauppinen

Frequency warped Burg's method for AR-modeling

Auto-regressive modeling of measured data is commonly used in numerous signal processing applications. When aiming for high accuracy, Burgs method has been found to give a suitable model. It has been shown that when the signal energy is non-uniformly distributed in a frequency range, the use of a modified frequency scale is advantageous. This is often the case with audio signals. We introduce a frequency warped version of Burgs method for calculating the auto-regressive filter parameters. A bilinear frequency mapping can be embedded in Burgs method by replacing the unit-delays of the lattice structure used in Burgs method with first-order allpass filters. The benefits of the frequency-warped Burgs method are demonstrated by comparing its signal modeling performance against those of the conventional Burgs method and the warped Yule-Walker method.

Interferometers Based on the Rotational Motion

Abstract This review‐type article deals with small, stable, compact, inexpensive, and low‐resolution interferometers mainly based on continuous or back and forth rotational motion to create the optical path difference. These interferometers are suitable for low‐resolution Fourier Transform Infrared (FTIR) spectroscopy. The paper introduces the most typical interferometers illustrated with figures. For example, interferometers with retroreflectors such as corner cubes are presented. This work aims at developing a tilt compensated optical design where the optical path difference of the interferometer will be achieved with rotational scanning. The most important property of this optical design is large mechanical tolerances to make manufacturing easy and inexpensive. New interferometers based on rotational motion are described. The mechanical angle tolerances of these interferometers can be up to three orders of magnitude larger than the angle tolerances in commonly used corner cubes.

Improved noise reduction in audio signals using spectral resolution enhancement with time-domain signal extrapolation

In this paper, we present significant improvement to frame-by-frame noise reduction methods which are based on spectral domain processing. This work mainly focuses on the analysis stage of the noise reduction process. It is common knowledge that better performance is obtained by increasing the spectral resolution. However, effective spectral resolution depends directly on the number of samples in the processing frame. This leads to the well-known tradeoff between spectral and temporal resolution. Our approach is to use a novel discrete signal extrapolation method to prolong the given signal frame, resulting in increased spectral resolution without losing any temporal quality. The improvement in the noise reduction is tested with objective and subjective evaluations. The effect of the human auditory system is taken into account in the objective measures. Subjective tests are performed to determine the quality improvement in a perceptual sense.

Extremely sensitive CWA analyzer based on a novel optical pressure sensor in photoacoustic gas analysis

Major improvement into the sensitivity of broadband Fourier transform infrared (FTIR) spectrometers, used in gas analysis, can be achieved by a photoacoustic detection system, which bases on a recently introduced optical pressure sensor. The sensor is a cantilever-type microphone with interferometric measurement of its free end displacement. By using a preliminary prototype of the photoacoustic gas detector, equipped with the proposed sensor and a black body radiation source, a detection limit in the sub-ppb range was obtained for e.g. methane gas. The limit, obtained in non-resonant operation mode, is very close to the best photoacoustic results achieved with powerfull laser sources and by exploiting the cell resonances. It is also orders of magnitude better than any measurement with a black body radiation source. Furthermore, the ultimate sensitivity leads on to very small detection limits also for several chemical warfare agents (CWA) e.g. sarin, tabun and mustard. The small size of the sensor and its great thermal stability enables the construction of an extremely sensitive portable CWA analyzer in the near future.

Sensitive and Fast Gas Sensor for Wide Variety of Applications Based on Novel Differential Infrared Photoacoustic Principle

Abstract A modular sensor concept for various gas measurement applications requiring high sensitivity and fast response time is presented. The proposed differential photoacoustic detection combines the selectivity of the traditional absorption method and the high sensitivity of the novel cantilever enhanced photoacoustic detector. High precision is achieved using short optical path length resulting in fast response time and wide dynamic measurement range. An example realization for greenhouse gas flux measurement is presented. Zusammenfassung Für verschiedenartige Gasmessungen, die hohe Empfindlichkeiten und schnelle Reaktionszeiten erfordern, wird ein modulares Sensorkonzept vorgestellt. Das dabei verwendete differentielle photoakustische Messprinzip kombiniert die Selektivität konventioneller Absorptionsmethoden mit der hohen Empfindlichkeit neuartiger photoakustischer Detektion mittels Schwingbalken. Hohe Messgenauigkeit wird durch eine kurze optische Weglänge erzielt und resultiert in schneller Reaktionszeit und hoher Dynamik. Als Beispielanwendung wir die Flussmessung von Treibhausgasen beschrieben.

2.5.5 Microimmersion lens LEDs for portable photoacoustic methane sensors

Microimmersion lens mid-infrared light emitting diodes (mid-IR LEDs) operating in the 3.4 m wavelength range, which covers the absorption bands of many important hydrocarbons (CH4, C3H8…), have been studied with respect to their application in cantilever enhanced photoacoustic trace gas detection technique

3.4 - Sensitive and Fast Gas Sensor for Wide Variety of Applications Based on Novel Differential Infrared Photoacoustic Principle

With the increase in the emission of gases from the industries and vehicles, there is a strongly growing need of a solution to monitor the emissions in order to have a cleaner and a greener atmosphere. Furthermore, a fast and sensitive gas sensor with compact size is needed in several applications such as greenhouse gas flux measurements, breath measurements in medical diagnostics, tail pipe measurements in engine development, leak detection and homeland security. There exist a lot of gas measurement solutions, but they do not satisfy the demand of growing effect of the global warming. Moreover the existing technologies are not sensitive, fast, compact and cost efficient enough.