An automatic design procedure for low-order IIR parametric equalizers

Parametric equalization of an acoustic system aims to compensate for the deviations of its response from a desired target response using parametric digital filters. An optimization procedure is presented for the automatic design of a low-order equalizer using parametric infinite impulse response (IIR) filters, specifically second-order peaking filters and first-order shelving filters. The proposed procedure minimizes the sum of square errors (SSE) between the system and the target complex frequency responses, instead of the commonly used difference in magnitudes, and exploits a previously unexplored orthogonality property of one particular type of parametric filter. This brings a series of advantages over the state-of-the-art procedures, such as an improved mathematical tractability of the equalization problem, with the possibility of computing analytical expressions for the gradients, an improved initialization of the parameters, including the global gain of the equalizer, the incorporation of shelving filters in the optimization procedure, and a more accentuated focus on the equalization of the more perceptually relevant frequency peaks. Examples of loudspeaker and room equalization are provided, as well as a note about extending the procedure to multi-point equalization and transfer function modeling

Directional perfect absorption using deep subwavelength low-permittivity films

We experimentally demonstrate single beam directional perfect absorption (to within experimental accuracy) of p-polarized light in the near-infrared using unpatterned, deep subwavelength films of indium tin oxide (ITO) on Ag. The experimental perfect absorption occurs slightly above the epsilon-near-zero (ENZ) frequency of ITO, where the permittivity is less than 1 in magnitude. Remarkably, we obtain perfect absorption for films whose thickness is as low as similar to 1/50th of the operating free-space wavelength and whose single pass attenuation is only similar to 5%. We further derive simple analytical conditions for perfect absorption in the subwavelength-film regime that reveal the constraints that the thin layer permittivity must satisfy if perfect absorption is to be achieved. Then, to get a physical insight on the perfect absorption properties, we analyze the eigenmodes of the layered structure by computing both the real-frequency/complex-wavenumber and the complex-frequency/real-wavenumber modal dispersion diagrams. These analyses allow us to attribute the experimental perfect absorption condition to the crossover between bound and leaky behavior of one eigenmode of the layered structure. Both modal methods show that perfect absorption occurs at a frequency slightly larger than the ENZ frequency, in agreement with experimental results, and both methods predict a second perfect absorption condition at higher frequencies, attributed to another crossover between bound and leaky behavior of the same eigenmode. Our results greatly expand the list of materials that can be considered for use as ultrathin perfect absorbers and provide a methodology for the design of absorbing systems at any desired frequencyopen9

Enhancement of terahertz photoconductive antenna operation by optical nanoantennas

Photoconductive antennas are promising sources of terahertz radiation that is widely used for spectroscopy, characterization, and imaging of biological objects, deep space studies, scanning of surfaces, and detection of potentially hazardous substances. These antennas are compact and allow for generation of both ultrabroadband pulses and tunable continuous wave terahertz signals at room temperatures, with no need for high-power optical sources. However, such antennas have relatively low energy conversion efficiency of femtosecond laser pulses or two close pump wavelengths (photomixers) into the pulsed and continuous terahertz radiation, correspondingly. Recently, an approach to solving this problem that involves known methods of nanophotonics applied to terahertz photoconductive antennas and photomixers has been proposed. This approach comprises the use of optical nanoantennas for enhancing the absorption of pump laser radiation in the antenna gap, reducing the lifetime of photoexcited carriers, and improving the antenna thermal efficiency. This Review is intended to systematize the main results obtained by researchers in this promising field of hybrid optical-to-terahertz photoconductive antennas and photomixers. We summarize the main results on hybrid THz antennas, compare the approaches to their implementation, and offer further perspectives of their development including an application of all-dielectric nanoantennas instead of plasmonic ones

P2SF: Physically-based Point Spread Function for digital image processing

Abstract: P2SF is a tool aimed at estimating the spatially variant point spread function based on the ray-tracing computation of the pupil function. Only 6 parameters are required to describe a monochromatic PSF

P2SF: Physically-based Point Spread Function for digital image processing

P2SF is a tool aimed at estimating the spatially variant point spread function based on the ray-tracing computation of the pupil function. Only 6 parameters are required to describe a monochromatic PSF

The impact of influent total ammonium nitrogen concentration on nitrite-oxidizing bacteria inhibition in moving bed biofilm reactor

The application of nitrification–denitrification over nitrite (nitritation–denitritation) with municipal (i.e. diluted and cold (or low-temperature)) wastewater can substantially improve the energy balance of municipal wastewater treatment plants. For the accumulation of nitrite, it is crucial to inhibit nitriteoxidizing bacteria (NOB) with simultaneous proliferation of ammonium-oxidizing bacteria (AOB). The present study describes the effect of the influent total ammonium nitrogen (TAN) concentration on AOB and NOB activity in two moving bed biofilm reactors operated as sequencing batch reactors (SBR) at 15°C (SBR I) and 21°C (SBR II). The reactors were fed with diluted reject water containing 600, 300, 150 and 75 mg TAN L-1. The only factor limiting NOB activity in these reactors was the high concentrations of free ammonia and/or free nitrous acid (FNA) during the SBR cycles. Nitrite accumulation was observed with influents containing 600, 300 and 150 mg TAN L-1 in SBR I and 600 and 300 in SBR II. Once nitrate production established in the reactors, the increase of influent TAN concentration up to the original 600 mg TAN L-1 did not limit NOB activity. This was due to the massive development of NOB clusters throughout the biofilm that were able to cope with faster formation of FNA. The results of thefluorescencein situ hybridization analysis preliminarily showed the stratification of bacteria in the biofilm