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Lars Bramsløw

Research Engineer, PhD, Project Leader

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mail@eriksholm.com

This compression system allows for a freely variable overall compression ratio while maintaining near-linear performance for locally stable environments. This is achieved by combining slow and fast compression into a system that reacts rapidly if necessary and slowly - almost linearly - otherwise.

A new compression strategy based on a guided level estimator

Totally linear amplifiers are nowadays typically considered outdated in hearing aids, primarily due to problems with audibility of soft sounds and discomfort of loud sounds. By addressing these two problems, compression provides greater comfort compared to linear amplification. However, compression can reduce speech intelligibility if used inappropriately1,2.

Studies have also shown that hearing-aid users often prefer less compression due to sound quality issues, even though audibility and speech intelligibility might be reduced. In this study we developed and tested a new compression system (Adaptive Guidance Level Estimator, or AGILE), which is intended to combine the positive characteristics of compression without its drawbacks.

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A compressor basically contains three components: 1) a level estimator/detector; 2) a compressor characteristic, and 3) an amplifier (Figure 1). The output signal is obtained by amplifying the input signal with a time varying factor, which depends on both the level of the input signal and the compressor characteristic. The controversial part of the compressor is the level estimator.

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AGILE has the same building blocks as the traditional compressor, but the crucial level estimator is enhanced (Figure 2). The central element is a difference controller that constantly monitors the difference between the guided and fast-level estimates.

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Figure 3 shows how the difference controller selects time constants based on the estimated difference. If differences between guided and fast level estimates are small, then time constants will be set to large values, and AGILE will operate as a slow-acting compression system. If differences are large, it will be fast-acting. Figure 3 also shows that positive and negative differences are not necessarily treated equivalently, meaning that attack and release times may differ.

If the input signal level remains stable, the compressor moves slowly and acts almost linearly. Any abrupt change in input level will guide the level estimator closer to the level estimated by the fast level estimator. Therefore, in situations requiring protection against sudden level changes, AGILE is capable of reacting very quickly. Similarly, if there is a sudden large drop in input level, the system will swiftly increase gain. Thus, both effective attack and release times of the compressor are controlled by the dynamics of the input signal, and can vary almost instantly from a few milliseconds to several seconds.

Evaluations of the AGILE compressor indicate that it does indeed achieve the desired goals. In objective terms, the global and local compression ratios are more decoupled than in a traditional compressor. In subjective terms, the AGILE compressor has been well received by hearing-aid users.


Further reading

1 Boike KT, Souza PE (2000). Effect of compression ratio on speech recognition and speech-quality ratings with wide dynamic range compression amplification. Journal of Speech, Language, and Hearing Research, 43, p.456-468.

2 Stone MA, Moore BCJ (2008). Effects of spectro-temporal modulation changes produced by multi-channel compression on intelligibility in a competing-speech task. Journal of the Acoustical Society of America, 123, p.1063-1076.

3 Souza PE (2002). Effects of compression on speech acoustics, intelligibility, and sound quality. Trends in Amplification, 6, p. 131-165.

Simonsen CS, Behrens T (2009). A New Compression Strategy Based on a Guided Level Estimator. Hearing Review, 16(13), p. 26-31.