Download A HIGH DEFINITION COMPRESSION SYSTEM FOR OPEN FITTINGS

A HIGH DEFINITION COMPRESSION SYSTEM
FOR OPEN FITTINGS
system's unique choice of filter structures, which has these demands. In addition, the ingenious system
a fixed propagation delay regardless of the input level architecture makes certain that the desired gains for
of incoming sounds.
varying sound levels are actually achieved. In concert
with the ReSoundAIR's inventive mechanical,
Low noise and distortion
acoustic and signal processing design, this allows
The WarpOpen compressor was designed to keep hearing instrument dispensers to confidently fit
distortion as low as possible. Normally a compres- patients in this traditionally hard-to-fit group.
sion system would implement an infinite compression point in each of the bands, but in compressors References
with few and non-overlapping bands, such as the Byrne D, Sinclair S & Noble W. (1998). Open earWarpOpen system, this results in bumps in the fre- mould fittings for improving aided auditory localizaquency response at the cross-over frequencies. This tion for sensorineural hearing losses with good highcan cause distortion at those frequencies. Therefore frequency hearing. Ear & Hearing 19(1): 61-71.
the WarpOpen compressor has a single broadband
output compressor (MPO – Maximum Power Groth J, Olsen SØ & Søndergaard M. (2003).
Output limiter) to make sure that no such bumps Disturbance of DSP group delays for non-occluding
occur.
fittings. Paper presented at annual convention of
American Academy of Audiology, San Antonio.
As a result of all the design advantages described
above, the WarpOpen compressor has very low inter- Noble W, Sinclair S & Byrne D. (1998). Improvenal noise and distortion. As a further precaution ment in aided sound localization with open earagainst circuit and microphone noise a low level moulds: Observations in people with high-frequency
expansion system has been implemented in the hearing loss. Journal of the American Academy of
WarpOpen. This system reduces gain for very low Audiology 9: 25-34.
input levels and ensures that internal noise is inaudible. It also ensures that very low level environmental
noise is not overamplified.
GN ReSound as • Mårkærvej 2A
P.O. Box 224 • DK-2630 Taastrup • Denmark
Tel.: +45 72 11 11 11 • Fax: +45 72 11 11 88
[email protected] • www.gnresound-group.com
2905-GB-03.02
Summary
Successful provision of amplification to individuals
with high frequency hearing losses requires open fittings, which in turn places demands on the amplification system. These include flexible frequency shaping, compression to compensate for reduced dynamic range, extremely low noise and distortion, and a
low propagation delay. The WarpOpen system
employs innovative technology including minimumphase FIR filtering and frequency warping to meet
Providing amplification to individuals with hearing
loss in the high frequencies presents a challenge to
hearing instrument designers. The patient with good
hearing in the low frequencies requires a fitting
which allows the escape of low frequency bone-conducted sounds – like the wearer's own voice – from
the ear canal to avoid the sensation of occlusion. In
addition, the amplification system should not add
noise which the wearer will be able to hear. Finally,
the open nature of such a fitting means that unamplified sound will enter the ear canal directly and
mix with amplified sound from the hearing instrument. Depending on the lag between the direct and
amplified sound, the result can range from a slight
alteration in the timbre of the incoming sound to a
perception of reverberation.
Frequency shaping system
A prerequisite for successful fitting of a wide range of
high frequency hearing losses is flexibility and precision in shaping the frequency response. The
WarpOpen utilizes a high resolution minimumphase Finite Impulse Response (FIR) filter to this
end. Minimum-phase FIR filtering is a digital filtering technique that is particularly suitable for use
where both continuous and transient signals are
processed, and offers the lowest possible processing
delay in a digital application. Speech recognition,
pro-audio, video, and data transmission are some of
the applications for which this type of filters are
advantageous. One particular benefit of the minimum-phase FIR filtering is that the manner in
which transient sounds are processed correspond
better to auditory system processing than other digital filter techniques. The WarpOpen is the first
hearing aid compression system to make use of this
technique. The minimum-phase FIR filter used in
the WarpOpen system corresponds to the resolution
of the Cochlea Dynamics compression system used
in the GN ReSound Canta series of hearing instruments. The settings of the filter determine the gain
for soft and moderate level inputs to the hearing
instrument with great precision.
A unique device intended for fitting mild or high
frequency hearing losses, the ReSoundAIR from
GN ReSound, effectively addresses these challenges.
This hearing instrument is fit without occluding the
ear canal, and features Stabilizer digital feedback
suppression, which extends the amount of usable
gain to allow fitting a large range of high frequency
hearing losses. This device also utilizes an innovative
compression system design that provides excellent
frequency shaping flexibility with minimal internal
noise and distortion. Further, the sound processing Compression technique
time is exceptionally low.
Sensorineural hearing losses are characterized by
reduced sensitivity for low sound levels, while loudThis article describes the GN ReSound WarpOpen ness perception at high levels is similar to that of norcompression system for open hearing instrument fitt- mal hearing individuals. The capability to provide
ings. The WarpOpen consists of a unique high-resol- input level dependent gain is the main rationale for
ution frequency response shaping system for soft use of wide dynamic range compression in hearing
sounds combined with a 3-band low distortion com- instruments. The WarpOpen employs a 3-band filter
pressor, and low level expansion to ensure that no bank system to achieve fast-acting wide dynamic
internal noise is audible.
range compression separately from the frequency
Efficient processing
The WarpOpen compressor was designed especially
for open fittings. As mentioned, in the case of an
open fitting, the combination of direct and amplified sound in the ear canal may negatively affect
sound quality if the time taken by the system to
process the sound is too long. This means that a low
propagation delay is mandatory. By using the warped
filter bank and the minimum-phase FIR filter, the
propagation delay is kept at the theoretical minimum. The WarpOpen compressor has a propagation
delay of less than 3.5 ms at 1600 Hz, which has been
shown to be imperceptible to listeners with high frequency hearing losses (Groth et al, 2003).
shaping system described above. A signal processing
technique called frequency warping enables variable
cross-over frequencies in the compressor filter bank.
Depending on the desired gain settings, the crossover frequencies are automatically adjusted to best
approximate the response. Examples of how the
WarpOpen compressor's band structure changes
based on the gain prescription are shown in Figure 1.
The frequency warping technique offers better processing efficiency and virtually no cross-over region
distortion. This means that there are no dips or
bumps in the frequency response of the compression
part of the WarpOpen system.
Like the compressor used in the existing Canta
series, the WarpOpen compressor is controlled by a
number of gain handles for soft and loud gain. The
fitter sets the gain for these inputs using a number of
frequency handles as shown in Figure 2, and these
settings are transformed to control the system. The
cross-over frequencies in the filter bank are automatically optimized so it is not necessary – or possible –
for the fitter to adjust them manually.
WarpOpen architecture
Figure 3 schematizes the structure of the WarpOpen
system. A unique characteristic of this structure is
that the part of the system which determines the
compression is placed before the frequency shaping
part of the system. The 3-band filter bank is utilized
to provide power estimates of the incoming sound.
The output of the filter bank is used to compute
how much gain should be reduced compared to the
gain for low sound levels. That is, negative gain values are derived, and these are added to the gain
determined by the frequency shaping part of the
system.
The great advantage of placing the compression part
of the system prior to the frequency shaping part of
Figure 2.
Control of the WarpOpen in the fitting software. The fitter specifies the gain for soft and loud input levels at different frequencies.
System parameters are automatically adjusted to achieve the
desired response.
Figure 1.
An example of the way in which the WarpOpen compressor's crossover bands change based on three different gain prescriptions.
A HIGH DEFINITION COMPRESSION SYSTEM FOR OPEN FITTINGS
the system is that the estimates of the input power
are thereby accurate. The danger in performing the
frequency shaping first, as done in other commercially available systems where frequency shaping and
compression are implemented separately, is contamination of the input
power estimation. If the
input power estimation
is not accurate, the compression applied will not
be based solely on the
input signal level, but
will be heavily influenced by how the system is set to amplify low level
sounds. This has the important implication that
gains for all sound levels can be lower or higher than
desired from an audiological standpoint.
However, a low propagation delay does not guarantee optimum conditions for an open fitting. As one
of the potential advantages of an open fitting is preserved or better localization ability (Noble et al,
1998; Byrne et al, 1998), it is important to ensure
that interaural directional cues are not lost in the signal processing. To accomplish this it is essential that
the delay does not change over time. Traditional lowdelay filter bank structures change their delay quite
drastically when the compressor alters gains due to
changes in input power. This phenomenon is completely avoided with the WarpOpen because of the
Figure 3.
A unique design. Unlike other compressors, the WarpOpen design
places frequency shaping after determination of compression, thereby eliminating contamination of input power estimates.
A HIGH DEFINITION COMPRESSION SYSTEM FOR OPEN FITTINGS
Efficient processing
The WarpOpen compressor was designed especially
for open fittings. As mentioned, in the case of an
open fitting, the combination of direct and amplified sound in the ear canal may negatively affect
sound quality if the time taken by the system to
process the sound is too long. This means that a low
propagation delay is mandatory. By using the warped
filter bank and the minimum-phase FIR filter, the
propagation delay is kept at the theoretical minimum. The WarpOpen compressor has a propagation
delay of less than 3.5 ms at 1600 Hz, which has been
shown to be imperceptible to listeners with high frequency hearing losses (Groth et al, 2003).
shaping system described above. A signal processing
technique called frequency warping enables variable
cross-over frequencies in the compressor filter bank.
Depending on the desired gain settings, the crossover frequencies are automatically adjusted to best
approximate the response. Examples of how the
WarpOpen compressor's band structure changes
based on the gain prescription are shown in Figure 1.
The frequency warping technique offers better processing efficiency and virtually no cross-over region
distortion. This means that there are no dips or
bumps in the frequency response of the compression
part of the WarpOpen system.
Like the compressor used in the existing Canta
series, the WarpOpen compressor is controlled by a
number of gain handles for soft and loud gain. The
fitter sets the gain for these inputs using a number of
frequency handles as shown in Figure 2, and these
settings are transformed to control the system. The
cross-over frequencies in the filter bank are automatically optimized so it is not necessary – or possible –
for the fitter to adjust them manually.
WarpOpen architecture
Figure 3 schematizes the structure of the WarpOpen
system. A unique characteristic of this structure is
that the part of the system which determines the
compression is placed before the frequency shaping
part of the system. The 3-band filter bank is utilized
to provide power estimates of the incoming sound.
The output of the filter bank is used to compute
how much gain should be reduced compared to the
gain for low sound levels. That is, negative gain values are derived, and these are added to the gain
determined by the frequency shaping part of the
system.
The great advantage of placing the compression part
of the system prior to the frequency shaping part of
Figure 2.
Control of the WarpOpen in the fitting software. The fitter specifies the gain for soft and loud input levels at different frequencies.
System parameters are automatically adjusted to achieve the
desired response.
Figure 1.
An example of the way in which the WarpOpen compressor's crossover bands change based on three different gain prescriptions.
A HIGH DEFINITION COMPRESSION SYSTEM FOR OPEN FITTINGS
the system is that the estimates of the input power
are thereby accurate. The danger in performing the
frequency shaping first, as done in other commercially available systems where frequency shaping and
compression are implemented separately, is contamination of the input
power estimation. If the
input power estimation
is not accurate, the compression applied will not
be based solely on the
input signal level, but
will be heavily influenced by how the system is set to amplify low level
sounds. This has the important implication that
gains for all sound levels can be lower or higher than
desired from an audiological standpoint.
However, a low propagation delay does not guarantee optimum conditions for an open fitting. As one
of the potential advantages of an open fitting is preserved or better localization ability (Noble et al,
1998; Byrne et al, 1998), it is important to ensure
that interaural directional cues are not lost in the signal processing. To accomplish this it is essential that
the delay does not change over time. Traditional lowdelay filter bank structures change their delay quite
drastically when the compressor alters gains due to
changes in input power. This phenomenon is completely avoided with the WarpOpen because of the
Figure 3.
A unique design. Unlike other compressors, the WarpOpen design
places frequency shaping after determination of compression, thereby eliminating contamination of input power estimates.
A HIGH DEFINITION COMPRESSION SYSTEM FOR OPEN FITTINGS
A HIGH DEFINITION COMPRESSION SYSTEM
FOR OPEN FITTINGS
system's unique choice of filter structures, which has these demands. In addition, the ingenious system
a fixed propagation delay regardless of the input level architecture makes certain that the desired gains for
of incoming sounds.
varying sound levels are actually achieved. In concert
with the ReSoundAIR's inventive mechanical,
Low noise and distortion
acoustic and signal processing design, this allows
The WarpOpen compressor was designed to keep hearing instrument dispensers to confidently fit
distortion as low as possible. Normally a compres- patients in this traditionally hard-to-fit group.
sion system would implement an infinite compression point in each of the bands, but in compressors References
with few and non-overlapping bands, such as the Byrne D, Sinclair S & Noble W. (1998). Open earWarpOpen system, this results in bumps in the fre- mould fittings for improving aided auditory localizaquency response at the cross-over frequencies. This tion for sensorineural hearing losses with good highcan cause distortion at those frequencies. Therefore frequency hearing. Ear & Hearing 19(1): 61-71.
the WarpOpen compressor has a single broadband
output compressor (MPO – Maximum Power Groth J, Olsen SØ & Søndergaard M. (2003).
Output limiter) to make sure that no such bumps Disturbance of DSP group delays for non-occluding
occur.
fittings. Paper presented at annual convention of
American Academy of Audiology, San Antonio.
As a result of all the design advantages described
above, the WarpOpen compressor has very low inter- Noble W, Sinclair S & Byrne D. (1998). Improvenal noise and distortion. As a further precaution ment in aided sound localization with open earagainst circuit and microphone noise a low level moulds: Observations in people with high-frequency
expansion system has been implemented in the hearing loss. Journal of the American Academy of
WarpOpen. This system reduces gain for very low Audiology 9: 25-34.
input levels and ensures that internal noise is inaudible. It also ensures that very low level environmental
noise is not overamplified.
GN ReSound as • Mårkærvej 2A
P.O. Box 224 • DK-2630 Taastrup • Denmark
Tel.: +45 72 11 11 11 • Fax: +45 72 11 11 88
[email protected] • www.gnresound-group.com
2905-GB-03.02
Summary
Successful provision of amplification to individuals
with high frequency hearing losses requires open fittings, which in turn places demands on the amplification system. These include flexible frequency shaping, compression to compensate for reduced dynamic range, extremely low noise and distortion, and a
low propagation delay. The WarpOpen system
employs innovative technology including minimumphase FIR filtering and frequency warping to meet
Providing amplification to individuals with hearing
loss in the high frequencies presents a challenge to
hearing instrument designers. The patient with good
hearing in the low frequencies requires a fitting
which allows the escape of low frequency bone-conducted sounds – like the wearer's own voice – from
the ear canal to avoid the sensation of occlusion. In
addition, the amplification system should not add
noise which the wearer will be able to hear. Finally,
the open nature of such a fitting means that unamplified sound will enter the ear canal directly and
mix with amplified sound from the hearing instrument. Depending on the lag between the direct and
amplified sound, the result can range from a slight
alteration in the timbre of the incoming sound to a
perception of reverberation.
Frequency shaping system
A prerequisite for successful fitting of a wide range of
high frequency hearing losses is flexibility and precision in shaping the frequency response. The
WarpOpen utilizes a high resolution minimumphase Finite Impulse Response (FIR) filter to this
end. Minimum-phase FIR filtering is a digital filtering technique that is particularly suitable for use
where both continuous and transient signals are
processed, and offers the lowest possible processing
delay in a digital application. Speech recognition,
pro-audio, video, and data transmission are some of
the applications for which this type of filters are
advantageous. One particular benefit of the minimum-phase FIR filtering is that the manner in
which transient sounds are processed correspond
better to auditory system processing than other digital filter techniques. The WarpOpen is the first
hearing aid compression system to make use of this
technique. The minimum-phase FIR filter used in
the WarpOpen system corresponds to the resolution
of the Cochlea Dynamics compression system used
in the GN ReSound Canta series of hearing instruments. The settings of the filter determine the gain
for soft and moderate level inputs to the hearing
instrument with great precision.
A unique device intended for fitting mild or high
frequency hearing losses, the ReSoundAIR from
GN ReSound, effectively addresses these challenges.
This hearing instrument is fit without occluding the
ear canal, and features Stabilizer digital feedback
suppression, which extends the amount of usable
gain to allow fitting a large range of high frequency
hearing losses. This device also utilizes an innovative
compression system design that provides excellent
frequency shaping flexibility with minimal internal
noise and distortion. Further, the sound processing Compression technique
time is exceptionally low.
Sensorineural hearing losses are characterized by
reduced sensitivity for low sound levels, while loudThis article describes the GN ReSound WarpOpen ness perception at high levels is similar to that of norcompression system for open hearing instrument fitt- mal hearing individuals. The capability to provide
ings. The WarpOpen consists of a unique high-resol- input level dependent gain is the main rationale for
ution frequency response shaping system for soft use of wide dynamic range compression in hearing
sounds combined with a 3-band low distortion com- instruments. The WarpOpen employs a 3-band filter
pressor, and low level expansion to ensure that no bank system to achieve fast-acting wide dynamic
internal noise is audible.
range compression separately from the frequency