TYING IT ALL TOGETHER

All of the high performance features of the ReSound Azure hearing instrument are unified through the Human Resolution Warp-based processor, the principal component of the hearing instrument system. The purpose of hearing instruments is to amplify sound to compensate for peripheral auditory system dysfunction. Thus the most important element of any hearing instrument is the manner in which it accomplishes this task. The amplification – or compression – system in any hearing instrument can be described in both audiological and engineering terms. The audiological characterization of the system tells what it does to incoming sounds and encompasses compression parameters such as gain and output, compression ratio, compression threshold, and attack and release times.

Combinations of compression parameters form compression schemes intended to achieve a particular goal such as Automatic Volume Control (AVC) or Wide Dynamic Range Compression (WDRC). The engineering characterization of the system tells how the amplification is carried out including system architecture, filtering techniques, and band structure, just to name a few. Although many hearing instruments utilize similar compression schemes, there are radical differences in the digital signal processing techniques they employ. These techniques are increasingly becoming important determinants of hearing instrument sound quality and performance.

One innovative technique introduced to hearing instrument technology by ReSound is frequency warping. This design technique provides logarithmic frequency resolution with high efficiency. A mathematical warping function defines how frequencies are mapped on a logarithmic scale corresponding to the auditory Bark scale (Smith & Abel, 1999). The Bark scale incorporates the human auditory system critical bandwidth as the scale unit (Zwicker et al, 1957).

Most digital signal processing techniques for frequency analysis yield constant bandwidth with uniform spacing of the bands, which is unlike the non-uniform spacing of the auditory filters

Measurable advantages of employing frequency warping include low processing delay and very low distortion. But do these advantages translate to better sound quality for hearing instrument users? Recently, evidence has emerged which supports the idea that perceived audio quality is improved with the frequency warping compressor compared to other techniques.

Dittberner et al (2006) examined the relative impact of frequency warping-based versus Fast Fourier Transform- based compression systems on perceived sound quality of music and speech as a function of degree of hearing loss. They demonstrated a clear preference for the frequency warping-based processing among listeners with moderate sensorineural hearing loss for all types of sounds tested.

The Human Resolution Warp compression system, described more fully in Groth & Nelson (2004), results in 17 smoothly overlapping frequency bands corresponding to human auditory resolution. Not only does this system allow for WDRC with superior sound quality for both conventional and open fittings, it provides the foundation for all other sound processing in the hearing instrument.

Other technologies offered by the ReSound Azure hearing instruments which build on the Warp platform include enhanced Dual Stabilizer II DFS (Digital Feedback Suppression) and NoiseTracker II noise reduction.

Like the preceding generations of ReSound feedback cancellation systems, the Dual Stabilizer II DFS in the ReSound Azure devices identifies acoustic feedback and utilizes two cancellation filters to produce a signal equal in amplitude and opposite in phase, thereby canceling out the feedback. This processing has shown to increase the gain available for any particular fitting by approximately 10 dB.

For ReSound Azure hearing instrument fittings, an even greater degree of extra headroom is afforded by the Dual Stabilizer II DFS. For two-microphone devices, separate DFS systems operate for each microphone to maintain performance even in combination with sophisticated multiband directional processing such as MultiScope.

In the ReSound Azure hearing instruments, the DFS technology has been further tuned to increase its immunity to external tonal sounds, such as music, phones ringing or beeping sounds. Such sounds are known to cause ringing, buzzing or chiming artifacts in less advanced systems. The improvements to the DFS processing are twofold: first and most importantly, the accuracy of the feedback path model which is calculated during the fitting session calibration has been vastly improved through more sophisticated calculations.

Since this model provides the starting point for the adaptive feedback cancellation when the hearing instrument is worn, increasing its accuracy leads to unprecedented performance in terms of extra headroom; secondly, the time constants associated with on-line analysis of feedback occurrence have been tweaked to ensure that the system attacks only true feedback, never environmental sounds. As a result, wearers of ReSound Azure hearing instruments can be provided with the gain they need for audibility and clarity, and still enjoy suberb sound quality.

NoiseTracker II also extends on the spectral subtraction approach utilized by NoiseTracker. The basic goal of NoiseTracker II is still to reduce gain in frequency areas where the signal-to-noise ratio is low, but enhancements have been implemented which serve to add to sound quality.

NoiseTracker technology accurately identifies and characterizes speech and noise, and employs adaptive time constants to maintain noise reduction without affecting speech. As an additional supplement to sound quality in the presence of speech, NoiseTracker II applies a weighted gain reduction function when speech is present, such that the spectral content of noise can effectively be removed and the envelope of the speech signal left virtually intact. The percentage of time that this function has been applied is tracked by the hearing instrument’s datalogging and displayed in the Onboard Analyzer.