All-pass Filter Sections Research Articles

Design and realisation of a fractional‐order sinusoidal oscillator

In this work, a new fractional-order sinusoidal oscillator is proposed. The proposed oscillator consists of one fractional-order all-pass filter and one fractional-order lossless integrator blocks. In order to emulate fractional-order capacitors, three different approximation methods the R–C pair, Matsuda and Oustaloup are employed and the results are compared. Three sinusoidal voltage signals with different phases that are controlled by fractional orders are provided by the proposed oscillator topology that is not possible for the classical integer-order case. Grounded passive components are used in the introduced oscillator circuit. Additionally, the output voltage signals are available at the low impedance terminal W of AD844ANs. Moreover, the time constant of the fractional-order all-pass filter can be adjusted accurately by means of second resistor Rx in the all-pass filter section. In addition to the simulation results, the proposed fractional oscillator is also implemented to verify the oscillator circuit experimentally. For the case of fractional orders α = 1 and β = 0.5, the measurement results are 10.3 kHz, 43.22° and 67.99°, respectively, while the desired values are 10 kHz, 45° and 67.36°, respectively.

Efficient digital filters with variable cut-off based on all-pass and sharpening digital filters

All-pass digital filters are known as excellent prototype filters in many applications, such as control systems, signal processing and communications. Efficient filters can be designed because transfer function is very robust to small element changes. The drawback of digital filters based on all-pass filter sections is that they are not suitable for tuning and the design of filters with variable cut-off frequency. In this paper there is proposed a new design method suitable for the design of filters that may satisfy different attenuation specifications and variable cut-off based on low-order wave digital filters. The method is derived using computer algebra system.

Optimum design of group delay equalizers

This paper introduces a novel optimization procedure for the design of group delay equalizers, which is simple to implement, robust and of fast convergence. A pole-zero placement technique is applied to ensure filter stability and direct updating equations. By cascading second-order allpass filter sections to the IIR filter to be equalized, the technique is able to produce group delay responses that are practically as close to a flat response as desired. The new optimization scheme is shown to avoid local minima often encountered by other algorithms used in the design of delay equalizers. Simulation results are presented to verify the effectiveness of the proposed approach.

Multichannel Dynamic-Range Compression Using Digital Frequency Warping

A multichannel dynamic-range compressor system using digital frequency warping is described. A frequency-warped filter is realized by replacing the filter unit delays with all-pass filters. The appropriate design of the frequency warping gives a nonuniform frequency representation very close to the auditory Bark scale. The warped compressor is shown to have substantially reduced group delay in comparison with a conventional design having comparable frequency resolution. The warped compressor, however, has more delay at low than at high frequencies, which can lead to perceptible changes in the signal. The detection threshold for the compressor group delay was determined as a function of the number of all-pass filter sections in cascade needed for a detectible change in signal quality. The test signals included clicks, vowels, and speech, and results are presented for both normal-hearing and hearing-impaired subjects. Thresholds for clicks are lower than thresholds for vowels, and hearing-impaired subjects have higher thresholds than normal-hearing listeners. A frequency-warped compressor using a cascade of 31 all-pass filter sections offers a combination of low overall delay, good frequency resolution, and imperceptible frequency-dependent delay effects for most listening conditions.

Novel All-Pass Filter Section Using Differential Difference Amplifier

A novel high-performance first-order all-pass filter employing a single active element and a minimum number of passive components is presented. The proposed circuit is based on differential difference amplifier and is very suitable for low voltage operation. Also, the use of grounded capacitor enables its implementation with standard CMOS technologies. SPICE simulation and experimental results verifying theoretical analyses are also provided.

Frequency tracking using constrained adaptive notch filters synthesised from allpass sections

New constrained adaptive notch filter structures are proposed. These structures are synthesised from allpass filter sections, which are realised as structurally lossless bounded real functions with a minimum number of delay elements and multipliers. Both structures admit orthogonal tuning of their notch frequency and bandwidth. Frequency tracking is achieved simply by the evaluation of a function of a filter parameter. Connections are shown with the structures used by other workers. Signal enhancement outputs are obtained from both structures and their signal-to-noise improvement ratios are given. The mirror-image pair of polynomials present in a real allpass transfer function is shown to provide a significant simplification in the generation of the necessary gradient terms used in parameter adaptation. Simulations are included to verify the performance of these structures when they are used to track both single and multiple sinusoids in additive broadband noise.

Signal processor implementation of digital all-pass filters

The implementation of digital filter structures of all-pass subfilters using modern high-speed signal processors is investigated. The focus is on digital all-pass filters realized as a cascade of first- and second-order all-pass sections. Wave digital all-pass sections are considered, with special emphasis on developing two-port adaptor structures that are suitable for signal processor implementation. Explicit formulas are developed for scaling and round-off noise analysis of these all-pass sections. Other well-known first- and second-order all-pass filter sections are considered. The performance of the various all-pass filter structures are compared with respect to roundoff noise, parasitic oscillations, and speed of signal processor implementation. >

Realization of all-pass filter sections using current-differencing amplifiers

Improvements in the performance of the basic current-differencing operational amplifier (CDOA) enable a basic all-pass filter section, having high performance in the 300–3400 Hz range, to be simply designed. Furthermore, one amplifier may be eliminated from the basic design as a method of reducing undesirable effects of active device parameters, but with some added design problems. Performance details of the circuits are given, together with a theoretical assessment of the effects of parasities performance.