Anti-spoofing Methods for Automatic SpeakerVerification System

Growing interest in automatic speaker verification (ASV)systems has lead to significant quality improvement of spoofing attackson them. Many research works confirm that despite the low equal er-ror rate (EER) ASV systems are still vulnerable to spoofing attacks. Inthis work we overview different acoustic feature spaces and classifiersto determine reliable and robust countermeasures against spoofing at-tacks. We compared several spoofing detection systems, presented so far,on the development and evaluation datasets of the Automatic SpeakerVerification Spoofing and Countermeasures (ASVspoof) Challenge 2015.Experimental results presented in this paper demonstrate that the useof magnitude and phase information combination provides a substantialinput into the efficiency of the spoofing detection systems. Also wavelet-based features show impressive results in terms of equal error rate. Inour overview we compare spoofing performance for systems based on dif-ferent classifiers. Comparison results demonstrate that the linear SVMclassifier outperforms the conventional GMM approach. However, manyresearchers inspired by the great success of deep neural networks (DNN)approaches in the automatic speech recognition, applied DNN in thespoofing detection task and obtained quite low EER for known and un-known type of spoofing attacks.Comment: 12 pages, 0 figures, published in Springer Communications in Computer and Information Science (CCIS) vol. 66

Low noise terahertz MgB2 hot-electron bolometer mixers with an 11GHz bandwidth

Terahertz (THz) hot-electron bolometer mixers reach a unique combination of low noise, wide noise bandwidth, and high operation temperature when 6nm thick superconducting MgB2 films are used. We obtained a noise bandwidth of 11GHz with a minimum receiver noise temperature of 930K with a 1.63THz Local Oscillator (LO) and a 5K operation temperature. At 15K and 20K, the noise temperature is 1100K and 1600K, respectively. From 0.69THz to 1.63THz the receiver noise increases by only 12%. Device current-voltage characteristics are identical when pumped with LOs from 0.69THz up to 2.56THz, and match well with IVs at elevated temperatures. Therefore, the effect of the THz waves on the mixer is totally thermal, due to absorption in the π conduction band of MgB2

THz Hot-Electron Bolometer Mixers

We overview the technology of superconducting Hot-Electron Bolometer mixers in the field of terahertz heterodyne detection. HEB mixer performance is analyzed vs competing technologies and application fields, with emphasize on radio astronomy. We discuss materials which have been reported in the past to be used for such devices with both phonon-substrate and electron-diffusion cooling mechanisms. It caused a splash of interesting theories with attempts of describing both dc and RF characteristics of HEB mixers. Development of HEB mixers made possible to extend high spectral resolution radio astronomy beyond 1THz

MgB2 hot electron bolometer mixers for THz heterodyne instruments

In this work we present experimental investigation of the MgB2 hot-electron bolometer (HEB) for low noise mixing at terahertz frequencies. A dedicated MgB2 thin film deposition system was designed and constructed based on Hybrid Physical-Chemical Deposition. Films as thin as 15nm have a superconducting transition at 35K, with a critical current density <10^7 A/cm2 (at 4.2K) in bridges as narrow as 500nm, indicating on good connectivity in the film. The gain bandwidth (GBW) was measured by mixing of two THz sources. The GBW is proportional to the film thickness and it is at least 6GHz for 15nm thick devices. Performance of MgB2 HEBs was compared to performance of one of the NbN HEB mixers made for the Herschel Space Observatory (one of the flight units), for which both the GBW and the Noise Bandwidth (NBW) was measured. MgB2 HEB mixers show a GBW at least a factor of three broader compared to the NbN HEB measured in the same set-u

Study of MgB2 ultra-thin films in submicron size bridges

We discuss a custom built hybrid physical chemical vapour deposition (HPCVD) system for MgB2 ultra-thin film deposition: construction, deposition process development, and optimization. Achieved films on SiC substrates have a critical temperature (Tc) ranging from 35K (10nm thick films) to 41K (40nm thick films). The 20nm thick unpatterned film had a room temperature resistivity of 13μΩ·cm, whereas it becomes 50μΩ·cm in sub-micrometer size bridges with a critical current density Jc (4.2K) up to 1.2×10^8A/cm2. The lower value of resistivity corresponds to the higher of both Tc and Jc. The surface roughness, measured with an atomic force microscope (AFM), is approximately 1.5nm

Wideband THz HEB mixers using HPCVD MgB2 thin films

We present results of experimental study of the gain bandwidth (GBW) of MgB2 hot electron-bolometer (HEB) mixers at 0.1THz and 0.4THz. Antenna integrated 0.25-1.5um2 area devices were made from thin MgB2 films deposited with a custom made HPCVD system. Film as thin as 15-45nm had a Tc from 35K to 40K. The GBW was found to be independent on the bias conditions, the bath temperature, and the LO frequency. The maximum GBW of 6GHz was observed for 15nm thick HEBs. At an 0.7THz LO and a 23K bath temperature the receiver noise temperature of this mixer was 3000K (corrected for optical losses)