Jussi Rämö

High-precision parallel graphic equalizer

This paper proposes a high-precision graphic equalizer based on second-order parallel filters. Previous graphic equalizers suffer from interaction between adjacent band filters, especially at high gain values, which can lead to substantial errors in the magnitude response. The fixed-pole design of the proposed parallel graphic equalizer avoids this problem, since the parallel second-order filters are optimized jointly. When the number of pole frequencies is twice the number of command points of the graphic equalizer, the proposed non-iterative design matches the target curve with high precision. In the three example cases presented in this paper, the proposed parallel equalizer clearly outperforms other non-iterative graphic equalizer designs, and its maximum global error is as low as 0.00-0.75 dB when compared to the target curve. While the proposed design has superior accuracy, the number of operations in the filter structure is increased only by 23% when compared to the second-order Regalia-Mitra structure. The parallel structure also enables the utilization of parallel computing hardware, which can nowadays easily outperform the traditional serial processing. The proposed graphic equalizer can be widely used in audio signal processing applications.

Digital Augmented Reality Audio Headset

Augmented reality audio (ARA) combines virtual sound sources with the real sonic environment of the user. An ARA system can be realized with a headset containing binaural microphones. Ideally, the ARA headset should be acoustically transparent, that is, it should not cause audible modification to the surrounding sound. A practical implementation of an ARA mixer requires a low-latency headphone reproduction system with additional equalization to compensate for the attenuation and the modified ear canal resonances caused by the headphones. This paper proposes digital IIR filters to realize the required equalization and evaluates a real-time prototype ARA system. Measurements show that the throughput latency of the digital prototype ARA system can be less than 1.4 ms, which is sufficiently small in practice. When the direct and processed sounds are combined in the ear, a comb filtering effect is brought about and appears as notches in the frequency response. The comb filter effect in speech and music signals was studied in a listening test and it was found to be inaudible when the attenuation is 20 dB. Insert ARA headphones have a sufficient attenuation at frequencies above about 1 kHz. The proposed digital ARA system enables several immersive audio applications, such as a virtual audio tourist guide and audio teleconferencing.

Assisted Listening Using a Headset: Enhancing audio perception in real, augmented, and virtual environments

Historically, headphones have mainly been used for analytic listening in music production and in homes. During the last decade, with the boom of dedicated music players and mobile phones, the everyday use of light headphones has become highly popular. Current headphones are also paving the way for more sophisticated assisted listening devices. Today, active noise control (ANC), equalization techniques, and a hear-through function are already a standard part of many headphones that people commonly use while traveling. It is not difficult to predict that, in the near future, a headset will be a ?hearing aid for those with normal hearing,? which can improve listening conditions for example in a noisy environment.

Perceptual headphone equalization for mitigation of ambient noise

An adaptive perceptual equalizer for headphones is introduced. It estimates the effect of auditory masking while considering the characteristics of the headphones, ambient noise, and music. The system utilizes a psychoacoustic masking model to estimate the level to which the music should be raised to have the same perceived tonal balance in noise as it has in a quiet environment. Prototype testing showed that the most important task is to make the music audible in each Bark band. The compensation of the partial masking further improves the perceived sound quality. The system uses a microphone of a headset to capture the ambient noise. The equalization is implemented using a high-order graphical equalizer that does not require subband decomposition of the music signal. The proposed equalizer also retains reasonable SPL levels: in an example case, the maximum gain in one Bark band was 11 dB while the overall SPL increase was only 2.5 dB.

Optimizing a High-Order Graphic Equalizer for Audio Processing

A high-order graphic equalizer has the advantage that the gain in one band is highly independent of the gains in the adjacent bands. However, all practical filters have transition bands, which interact with the adjacent bands and create errors in the desired magnitude response. This letter proposes a filter optimization algorithm for a high-order graphic equalizer, which minimizes the errors in the transition bands by iteratively optimizing the orders of adjacent band filters. The optimization of the filter order affects the shape of the transition band, thus enabling the search for the optimum shape relative to the adjacent filter. The optimization is done offline, and during filtering only the gains of the band filters are altered. In an example case, the proposed method was able to meet the given peak-error limitations of ±2 dB, when the total order of the graphical equalizer was 328, whereas the non-optimized filter could not meet the requirements even when the total order was raised to 672. Optimized high-order graphical equalizers can be widely used in audio signal processing applications.