5[1][1].compression of dilon

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Concepts of Compression

Harvey Dillon

National Acoustic Laboratories.

CRC for Cochlear Implant and Hearing Aid Innovations

ASA

Brisbane, 2001



Audiology Research Team Pierce Lee/이 원

육프로그램 Outline: Concepts of Compression

Basics

Rationales for using compression

Evidence for the benefit of compression



Why do we need compression, andwhat is it ?



Whole Range

0

1

2

3

4

5

6

7

O u t p u t L e v e l

WeakModerate

Intense

0

1

2

3

4

5

6

7

O u t p u t L e v e

l

WeakModerate

Intense

High Level

0

1

2

3

4

5

6

7

O u t p u t L e v e l

WeakModerate

Intense

Low Level



Feedback point

OutputInput

Level detecting

device

Variable gain

amplifier



Feedback point

OutputInput

Rules

Variable gain

amplifier


http://slidepdf.com/reader/full/511compression-of-dilon 8/63 Audiology Research Team Pierce Lee/이 원

육프로그램

50

60

70

80

90

100

110

30 40 50 60 70 80 90 100 110

Input (dB SPL)

O u t p u t ( d B

S P L )

Compression, Expansion, Gain and Attenuation

0102030



육프로그램

50

60

70

80

90

100

110

30 40 50 60 70 80 90 100 110

Input (dB SPL)

O u t p u t ( d B

S P L )

0102030

Compression, Expansion, Gain and Attenuation



육프로그램 Seven Rationales for Using Compression

1. Avoiding discomfort and distortion

2. Increasing comfort

3. Reducing inter-syllabic level differences

4. Reducing long term level differences

5. Normalising loudness

6. Empirical evidence

7. Maximising speech intelligibility



육프로그램

Avoid Discomfort, Damage, and

Distortion

byControlling maximum output (SSPL)

using compression limiting

Rationale 1:



육프로그램 Avoid Discomfort, Damage and Distortion

AGCo, not AGCi

High compression ratio (>10 ?)

Fast attack time (<5 ms)

Fast, or adaptive , release time (<100 ms)

Peak clipping for very profound losses

Compression parameters:



육프로그램

Increase comfort

byhigh level gradual comression

Rationale 2:



육프로그램 Increase comfort

Increase comfort

Decrease need for volume control

Aims:



육프로그램

O u t p u t d B

S P L

Input dB SPL65

linear

Compression

Increasing comfort



육프로그램 Increased comfort

Decreased signal to noise ratio for

sequential signal and noise

(Distorted envelope cues)

Disadvantages:



육프로그램

Increase Audibility of Soft Phonemes

by

fast acting wide dynamic range

compression

Rationale 3:



육프로그램 Increase audibility of soft phonemes

Increase intelligibility

Decrease need for volume control

Aims:



육프로그램 Effect of compression on waveform

Linear

time

Compressed

time

CVR increased




육프로그램

O u t p u t d B

S P L

Input dB SPL65

Increasing audibility




육프로그램 Increase audibility of soft phonemes

AGCi (not AGCo)

Release time < 50 ms

Low (enough) compression threshold

Single channel or multi-channel

compression

Compression parameters:




육프로그램 Increased audibility of soft phonemes

Increased risk of feedback

Decreased signal to noise ratio for

sequential signal and noise

(Distorted envelope cues)

Disadvantages:




육프로그램 Preferred Threshold

40 dB

SPL

66 dB

SPL

don't

know

0

2

4

6

8

10

12

14

40 dB

SPL

66 dB

SPL

don't

know

N o o f p e o p l e

statistically

significant




육프로그램 Preferred Threshold

46 dB

SPL

62 dB

SPL

don't

know

0

10

20

30

40

5060

70

80

90

46 dB

SPL

62 dB

SPL

don't

know

60%

31%

9%

N o o f p e o p l e

statistically

significant



CT6

2

65

Summary: High CT preferred to a low CT

because its “clearer”

O u t p u t d B

S P L

Input dB SPL

CT46



Summary: High CT preferred to a low CT

because its “clearer”

2:1 compression preferred to linear because it’s more “comfortable”

O u t p u t d B

S P L

Input dB SPL

CT46

CT62

linear

65

R ti l 4




육프로그램

Reduce Long Term Level

Differences

by

slow acting automatic volume control

Rationale 4:




육프로그램 Reduce Long Term Level Differences

To avoid (or reduce) the need for volume

control adjustments

To improve intelligibility in quiet

environments(compared to linear amplifier with no volume

control)

Aims:





Slow acting compression

Release time > 1 sec ?

Low compression threshold

< 55 dB SPL ?

Single channel or multi-channelcompression

Method:





Noise increases during long gaps

May not react sufficiently quickly to sudden

changes in overall level

Disadvantages:




육프로그램

Normalise Loudness

by

compressing most at low levels

and high frequencies

Rationale 5:

N li l d




육프로그램 Normalise loudness

Achieve normal overall loudnessperception

Achieve normal balance of loudnessacross frequencies

Hence to improve intelligibility andnaturalness of sound

Aims:

L d S l




육프로그램 Loudness Scale

Uncomfortable

Very Loud

Loud

Loud but ComfortableComfortable

Soft but Comfortable

SoftVery soft

Inaudible



20 30 40 50 60 70 80 90 100

Hearing

Impaired

(individual)

Normal

Hearing

(average)

Inaudible

Very Soft

Soft

Comfortable

Loud

Very Loud

Uncomfortable




육프로그램 Input-output curve to normalise loudness

40

60

80

100

20 40 60 80 100

Input (dB SPL)

O u t p u t ( d B

S P L )




육프로그램 Loudness normalisation - procedures

FIG6

DSL[i/o]

IHAFF (Contour test and VIOLA)

Scaladapt

LGOB

N li l d





+

-10

0

10

20

30

40

50

60

0.1 0.2 0.5 1.0 2.0 5.0

Frequency (kHz)

G a i n ( d B )

50

90

TILL

Normalise loudness




육프로그램

+

1

2

3

4

Normalise loudness

Normalise loudness





Greatest compression ratio at low levels

Greatest compression ratio at high

frequencies (for most losses)

Release times fast or slow

Two or more channels (or even one)

Compression parameters

Normalise loudness





Increased risk of feedback

Increased noise in gaps (if fast

compression)

Sub-optimal intelligibility

Disadvantages:




육프로그램

Empirically Determined Compression

byusing whichever compression

works best in experiments

Rationale 6:




육프로그램 Empirically determined compression

To use the form of compression that givesthe best intelligibility or quality, as indicated

by empirical experiments.

Conclusion: WDRC for the LFs, especially forpeople with sloping losses and big dynamic

ranges

Aim:




육프로그램

Speech intelligibility maximization

byNAL-NL1

Rationale 7:



Experimental evidence: The effect

of compression on intelligibility

Conclusions




육프로그램 Conclusions

1. Fast acting compression probably does

not significantly increase intelligibility

compared to linear at optimal level.

2. Fast acting compression probably does

not significantly decrease intelligibility

compared to linear at optimal level.




육프로그램 Comparison of linear and non-linear

Input level

O u t p u t l e v e l





Input level






Input level


3. Compression (many sorts) maintains intelligibility

as input level is decreased below typical levels.





Input level


4 Compression (many sorts) maintains loudness comfort

as input level is increased above typical levels.




육프로그램

Questions

References

http://localhost/2002/University/Hallym-2002-1/Lecture/0315/1.Hearing%20aid%20concept-02.ppt




육프로그램 References

Dillon, H. (1996). Compression? Yes, but for low or

high frequencies, for low or high intensities, and with

what response times? Ear & Hearing, 17, 287-307.

Byrne, D. (1997). Hearing aid selection for the 1990s:

Where to? J. Amer Acad Audiol, 7:377-395.

[email protected]



Dillon, H, Storey, L. The National Acoustic

Laboratories procedure for selecting the Saturated

Sound Pressure Level of Hearing Aids: Theoretical

derivation. Ear & Hearing , August, 1998.

Storey, L, Dillon, H, Yeend, I, Wigney, D. The National

Acoustic Laboratories procedure for selecting the

Saturated Sound Pressure Level of Hearing Aids:Experimental validation. Ear & Hearing , August,

1998.

Dillon, H, Storey, L, Grant, F, Phillips, A, Skelt, L,

Mavrias, G, Woytowych, W, and Walsh, M. Preferredcompression threshold with 2:1 wide dynamic

range compression in everyday environments.

Aust. J. of Audiol ., May, 1998.

Question and answer




육프로그램 Question and answer

I am really looking forward to this stupid quiz: True/false

False

I will write my answers down anyway: True/False

True

Question and Answer




육프로그램 Question and Answer

1. The major purpose of compression is to:

Fit a big dynamic range of sound into a small dynamic rangeof hearing

2. Compression is the opposite of gain: True or False?

False

3. Compression makes speech more intelligible than linearamplification when speech .....

is soft

4. Compression makes speech more comfortable than linearamplification when speech .....

is loud

5. TILL amplification makes feedback more likely: True or False?

True

Question and Answer (cont)




육프로그램 Question and Answer (cont)

6. A gradual form of compression is significantly better than

linear amplification + compression limiting: True or False?True

7. Manual volume controls are not needed on automatic hearingaids: True or False?

False 8. Compression threshold should be as low as possible without

feedback occurring: True or False?

False

9. SSPL selection is more critical for people with a mild loss thanfor people with a profound loss: True or False?

False

10. Compression limiting is always better than peak clipping:T/F?






6. A gradual form of compression is significantly better than

linear amplification + compression limiting: True or False?True

7. Manual volume controls are not needed on automatic hearingaids: True or False?

False 8. Compression threshold should be as low as possible without

feedback occurring: True or False?

False

9. SSPL selection is more critical for people with a mild loss thanfor people with a profound loss: True or False?

False

10. Compression limiting is always better than peak clipping:T/F? False






11. Measuring UCL or LDL is critical to accurate prescription of

SSPL: True or False?False

12. NAL-NL1 restores the normal loudness balance across

frequency: True or False?

False

13. Increasing audibility increases intelligibility except when:

.................

SL or SPL is too large

14. NAL-NL1 is the only validated non-linear selection

procedure:

True or False?

False


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