HIS 140 - HI Compression Strategies Overview


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HIS 140 - HI Compression Strategies Overview

  1. 1. HI Compression Strategies Compression’s major role is to decrease sound levels to fit the restricted dynamic range of the patient/client.
  2. 2. HI Compression Strategies Due to hearing loss, the patient/client’s dynamic range (the level difference between discomfort and the threshold of audibility) is reduced compared to a “normal hearing” dynamic range.
  3. 3. HI Compression Strategies The patient/client with sensorineural hearing loss will have difficulty with reduced dynamic range and also, reduced frequency resolution.
  4. 4. HI Compression Strategies Reduced dynamic range and frequency resolution are the primary considerations in the development of various compression strategies and digital algorithms.
  5. 5. HI Compression Strategies  Multi-channel Compression With today’s digital hearing instruments, there are often more than one type of compressor written into various frequency channels of a digital algorithm.
  6. 6. HI Compression Strategies To meet these frequency by channel signal challenges, three fundamental compression strategies are in use. They are: 1. Low level compression (input compression) 2. Compression limiting (output compression) 3. Wide dynamic range compression (combination)
  7. 7. HI Compression Strategies  Compression Characteristics These three compressor strategies are basically controlled by either the input signal (input compression) or the output signal (output compression) or a combination of the two.
  8. 8. HI Compression Strategies  Compressor Characteristics 1. Attack and release times 2. Spectral envelope 3. Compression threshold 4. Compression ratio
  9. 9. HI Compression Strategies  Attack and Release Time Attack time is the time it takes for a compressor to react to an increased input level. Release time it the time it takes for a compressor to react to a decreased input level.
  10. 10. HI Compression Strategies  Spectral Envelope Different syllables of speech have characteristic spectral information (peaks and valleys over a time period). The components of this spectral information (peaks and valleys) are placed inside an envelope (time period).
  11. 11. HI Compression Strategies  Spectral Envelope Envelopes are used to define the effects of various attack and release times upon spectral information.
  12. 12. HI Compression Strategies  Compression Threshold This is defined as the point when compression commences. It is also the threshold where definition begins for static compression characteristics
  13. 13. HI Compression Strategies  Static Compression Characteristics Attack and release times provide information regarding how quickly a compressor operates. Input-output (static characteristics) tell us how much a compressor decreases gain as the input level rises.
  14. 14. HI Compression Strategies  Compression Ratio The static characteristics of input vs. output are described with compression ratios. For example, the compression ratio for a linear amplifier would be 1:1.
  15. 15. HI Compression Strategies  Compression Ratios There are compression ratios which are less than 1:1. This is then termed expansion instead of compression.
  16. 16. HI Compression Strategies  Compression Use Rationale There are primarily six patient/client performance strategies addressed by compressors. Any or all, may be written into various digital algorithms.
  17. 17. HI Compression Strategies  Compression Use Rationale The six strategies are: 1. Avoiding distortion, discomfort, and damage 2. Reduce inter-syllabic & inter-phonemic intensity differences 3. “Normalize” loudness 4. Increase sound comfort 5. Maximize intelligibility 6. Reduce background noise
  18. 18. HI Compression Strategies  Compression Use Rationale These six rationale use various configurations of attack and release times, spectral envelope, compression ratios, and compression thresholds to modify the gain and output of the frequency response in each channel.
  19. 19. HI Compression Strategies  Compressor Use Rationale Various combinations of these compressors may be written into digital hearing instrument algorithms and appropriately applied for best patient/client hearing outcomes.
  20. 20. HI Compression Strategies  Compression Use Rationale A good reference summary may be found in Dillon page #183.