Evaluating analog reconstruction performance of transient digital audio workstation signals at high- and standard- resolution sample frequencies

Given that audio signals in many applications are neither predictable nor guaranteed to be repeated (and hence do not deliver an infinite history of data), they do not completely adhere to the ideal sampling theorems presented by Nyquist and Shannon. Digital-to-analogue (DAC) reconstruction theory is hence used to investigate how accurately digital audio workstation signals are actually reconstructed, and explores whether high-resolution sample frequencies (i.e. frequencies above 44.1 kHz) provide a performance advantage. Ideal reconstruction profiles are then evaluated against the actual reconstruction data observed from three pro-audio DACs and at multiple sample frequencies. The test signal is chosen to evaluate the performance of DACs when presented with transient data that approaches the Nyquist sample frequency. This approach is used because it has the potential to yield information on the suggested benefits of higher-than-Nyquist sample and reconstruction approaches in a real-world music production context

Effects of sound-induced hearing loss and hearing AIDS on the perception of music

This is the final version of the article. It first appeared from the Audio Engineering Society via https://doi.org/10.17743/jaes.2015.0081Exposure to high-level music produces several physiological changes in the auditory system that lead to a variety of perceptual effects. Damage to the outer hair cells within the cochlea leads to a loss of sensitivity to weak sounds, loudness recruitment (a more rapid than normal growth of loudness with increasing sound level) and reduced frequency selectivity. Damage to inner hair cells and/or synapses leads to degeneration of neurons in the auditory nerve and to a reduced flow of information to the brain. This leads to poorer auditory discrimination and may contribute to reduced sensitivity to the temporal fine structure of sounds and to poor pitch perception. Hearing aids compensate for the effects of threshold elevation and loudness recruitment via multi-channel amplitude compression, but they do not compensate for reduced frequency selectivity or loss of inner hair cells/synapses/neurons. Multi-channel compression can impair some aspects of the perception of music, such as the ability to hear out one instrument or voice from a mixture. The limited frequency range and irregular frequency response of most hearing aids is associated with poor sound quality for music. Finally, systems for reducing acoustic feedback can have undesirable side effects when listening to music.This work was supported by the Medical Research Council (UK, grant number G0701870), Action on Hearing Loss, Phonak, and Starkey

The Measurement of Audio Transformer Colouration

This thesis presents and discusses research undertaken into the detection and measurement of colouration produced by audio transformers. It is common for transformers to be subjectively described by audio professionals as ‘warm’, ‘fat’, ‘smooth’, etc. however there is little evidence to show if there is an audible difference and if there is a correlation between these levels of perceptual attributes and the performance of the device. Therefore, the research question was defined as: is there an audible difference with a transformer? A review of the objective and subjective elements of the study was conducted. First the history, application and operation of the audio transformer with an aim to understand the objective measures and performance of the device with focus on the nonlinear distortion response. This also includes the design and testing of a suitable test circuit used in the measurement method. The subjective review concerns perception of distortion and testing methodologies for investigating audible differences. The conclusion of the review is that few pieces of research exist showing the relationship between device, distortion and perception. The testing of each transformer involved the use of a specially designed test circuit using a variety of measures including THD+N and frequency domain analysis, to provide the most information about the operation of the device. Using the same setup, the device responses to a variety of samples were recorded and implemented in a double-blind triple-stimulus with hidden reference test using trained listeners in accordance with the ITU-R BS.1116-3 recommendation. The test results were then analysed for a random distribution using a 1-tailed binomial test. The results of the analysis show a high likelihood that the bass samples were audibly different shown by the significant p-values of all 3 samples at less than 0.001. A slight correlation seems to exist with THD+N, 3rd harmonic distortion and level however with no other obvious trends, it was concluded that the distortion and therefore the audibility is programme dependent. It was concluded that transformers are likely to produce a level of distortion deemed audible although the effect is considered to be programme dependent

Perceptual study of loudspeaker crossover filters

Digitaalinen suodatus tarjoaa kiinnostavia mahdollisuuksia äänentoistossa. Monitiekaiuttimien jakosuotimien digitaalinen toteutus on mahdollista suodinratkaisuilla, jotka eivät analogisissa suotimissa ole mahdollisia. Digitaalijakosuotimista on julkaistu monia artikkeleita, mutta havaintotutkimukset ovat harvassa. Diplomityössä on esitelty analogisen ja digitaalisen suodatuksen teoriaa, käytännön ratkaisuja, sekä pohdittu eroja menetelmien välillä. Työssä on myöhemmin tutkittu monitiekaiuttimien digitaalisten jakosuodinten ominaisuuksia kahdella eri jakosuodintyypillä: digitaalisella lineaarivaiheisella FIR-jakosuotimella sekä niin kutsutun Linkwitz-Riley analogisen jakosuotimen digitaalisella toteutuksella. Tutkimus suoritettiin havaintotutkimuksena kuuntelukokeiden avulla sekä kuulokesimulaationa että oikealla kaiuttimella kuunteluhuoneessa. Tavoitteena oli selvittää jakosuotimien aiheuttamien vaihevirheiden havaittavuutta ja niiden havaintokynnyksiä eri ääninäytteillä. Tuloksia analysoitiin auditoristen korrelaattien, ryhmäviivepoikkeaman (vaihevirhe) ja pehmennetyn terssikaistaspektrin (magnitudivirhe) avulla. Nämä korrelaatit selittävät havaittuja ilmiöitä tiettyyn pisteeseen asti, mutta lineaarivaiheisten FIR-jakosuotimien tapauksessa korrelaatiota ei aina esiintynyt. Tämän vuoksi tutkimuksen loppuvaiheessa tuloksia analysoitiin kuulon toimintaan perustuvan auditorisen analyysin avulla, mikä selittää ilmiöt kvalitatiivisesti. Tutkimuksen perusteella havaittiin, että korkean asteen lineaarivaiheiset FIR-jakosuotimet tarjoavat näennäisesti "ideaaliset" jakosuotimen ominaisuudet sekä vaihe- että magnituditoiston osalta, mutta aiheuttavat selvästi kuultavia häiriötä ("soimista") ääninäytteisiin, kun aikaero matalien ja korkeiden taajuuksien kaiutinelementtien välillä ei ole nolla. Matalien ja korkeiden taajuuksien kaistojen jakotaajuuden ollessa 3 kHz havaittiin, että yli 600 asteen lineaarivaiheiset FIR-jakosuotimet näyttävät aiheuttavan selkeitä häiriötä ääneen sekä kuulokesimulaatioiden että oikean kaiutinkokeen perusteella.Digital signal processing offers interesting possibilities in audio reproduction. Crossover filtering in a multi-way loudspeaker is possible to implement digitally in a way that is not possible with analog filters. In spite of many publications on the topic, there exists few perceptual studies of digital crossover filters. This Master's thesis presents an introduction to the theory of analog and digital filtering, practical solutions of analog and digital crossover filters and discusses the differences among them. Later in the thesis, a perceptual study is conducted with two digital crossover filters: digital linear-phase FIR crossover filter and a digital implementation of the analog, so called Linkwitz-Riley crossover filter. The experiment was carried out as a listening experiment using both headphone simulation and a real loudspeaker in a listening room. The main goal of the study was to find out the Just Noticeable Difference (JND) limits for phase errors caused by the crossover filters with different sound samples. The results of the listening experiment were analysed with auditory correlates of group delay distortion (phase errors) and smoothed third-octave spectrum (magnitude error). These correlates explain the results of the listening tests to some extent, but with high-order linear-phase FIR crossover filters, correlation seemed not to always exist. Thus auditory analysis that was based on the function of hearing was used for analysis. It seemed to show qualitatively the reasons for perceived phase errors. It was discovered that high-order, linear-phase FIR crossover filters offer apparently "ideal" properties in magnitude and phase reproduction for crossover filters, but they cause clearly audible degradations as "ringing" in the audio samples, when the flight-time difference between low- and highpass outputs is not zero. The crossover frequency between low- and highpass bands being 3 kHz, it was noticed on the grounds of the listening experiments that filter orders above 600 produce audible errors with linear-phase FIR crossover filters

Virtualized loudspeaker testing

Kaiuttimien vertaileminen huoneolosuhteissa on vaikeaa, koska pienetkin erot kaiuttimien sijoittelussa vaikuttavat lopputulokseen. Ihmisen kuulomuistin lyhyys rajoittaa vertailtavien kaiuttimien määrää, jos kaiuttimet sijoitetaan vuorotellen samaan pisteeseen. Työn kirjallisuusosiossa esitellään tilakuulon ja äänentoiston perusteita. Myöhemmässä osassa esitetään binauraalitekniikkaan perustuva menetelmä kaiutinten vertailuun. Menetelmä perustuu mitattujen tosi- ja keinopäävasteiden käyttöön yhdessä kuulokkeiden kanssa, ja sen avulla voidaan verrata useita kaiutinpareja viiveettömästi. Vaikka työssä esitetty menetelmä ohittaa perinteisiin kuuntelukokeisiin liittyvät ongelmat, sen käyttökelpoisuutta rajaavat mittausepävarmuus ja toiston tarkkuus. Menetelmän toimivuutta tutkittiin kuuntelukokeessa. Kokeella selvitettiin, kuinka paljon binauraalinen toisto eroaa todellisesta kuuntelutilanteesta. Puhesignaalilla menetelmä oli lähes erottamaton todellisuudesta. Musiikki- ja kohinasignaaleilla koehenkilöt arvioivat eron olevan havaittava tai hieman häiritsevä. Koe osoitti, että yksilöllisesti korjatut keinopäävasteet soveltuvat binauraalisynteesiin lähes yhtä hyvin kuin tosipäävasteet.Loudspeakers cannot be accurately compared to each other in room conditions since the placement of loudspeakers has a significant effect on the results. Also, the comparison of multiple loudspeakers is limited by the poor auditory memory of humans since the repositioning of the loudspeakers cannot be done instantly. In this thesis, the fundamentals of spatial hearing and sound reproduction are reviewed and a binaural method for loudspeaker comparisons is developed. The proposed method utilizes individual binaural responses together with an artificial head and headphones to enable fast, seamless and place-independent switching between different loudspeakers. The method overcomes the worst drawbacks of traditional loudspeaker listening tests but new problems related to the measurement accuracy and reproduction arise. Formal listening tests were conducted to examine the differences between the binaural method and the real loudspeakers in a standard listening room. It was found out that the binaural method is close to imperceptible compared to reality when a speech signal is listened to. However, the difference to the real loudspeakers increases when audio material with more energy at high frequencies is used. The listening tests revealed that individually equalized responses of an artificial head are applicable to binaural synthesis almost as well as individual true-head responses

Distortion Product Otoacoustic Emissions Evoked by Tone Complexes

Distortion product otoacoustic emissions (DPOAEs) are traditionally evoked by two-tone stimuli. In this study, emission data from Mongolian gerbils are reported that were obtained with stimuli consisting of six to 10 tones. The stimuli were constructed by replacing one of the tones of a tone pair by a narrowband multitone complex. This produced rich spectra of the ear canal sound pressure in which many of the third-order DPOAEs originated from the interaction of triplets of stimulus components. A careful choice of the stimulus frequencies ensured that none of these DPOAE components coincided. Three groups of DPOAEs are reported, two of which are closely related to DPOAEs evoked by tone pairs. The third group has no two-tone equivalent and only arises when using a multitone stimulus. We analyzed the relation between multitone-evoked DPOAEs and DPOAEs evoked by tone pairs, and explored the new degrees of freedom offered by the multitone paradigm

Distortion product otoacoustic emissions evoked by tone complexes

Distortion product otoacoustic emissions are a manifestation of nonlinear interaction between two or more stimulus components within the cochlea. Most studies employ a two-tone stimulus to evoke them. In this study we used a more complex stimulus, in which one of the primaries of the customary two-tone stimulus was replaced by an irregularly spaced tone complex. We obtained data from Mongolian gerbils, and investigated whether the novel stimulus has added value in terms of measurement efficiency, and whether it allows the derivation of information on cochlear mechanics that cannot be derived from two-tone stimuli