GA3219 Datasheet, PDF (7/14 Pages) Gennum Corporation – Venture-TM Digital DSP System

VENTURE GA3219
After the processing the signal goes through two more
biquad filters, post1 and post2, the Wideband Gain and
Volume Control.
These biquad filters are followed by the AGC−O block,
two more biquad filters, post3 and post4, and the Peak
Clipper. The last stage in the signal path is the D/A H−bridge.
FUNCTIONAL BLOCK DESCRIPTION
Adaptive Feedback Canceller
The Adaptive Feedback Canceller (AFC) reduces
acoustic feedback by forming an estimate of the hearing aid
feedback signal and then subtracting this estimate from the
hearing aid input. Therefore, the forward path of the hearing
aid is not affected. Unlike adaptive notch filter approaches,
the Venture GA3219’s AFC does not reduce the hearing
aid’s gain. The AFC is based on a time−domain model of the
feedback path.
The third-generation AFC (see Figure 5) allows for an
increase in the stable gain1 of the hearing instrument while
minimizing artefacts for music and tonal input signals. As
with previous products, the feedback canceller provides
completely automatic operation.
(Added stable gain will vary based on hearing aid style
and acoustic setup. Please refer to the Adaptive Feedback
Cancellation Information note for more details.)
When the AFC is enabled, it is highly recommended that
you either have all channels with Squelch ON or all channels
with Squelch OFF. If you choose to have all channels with
Squelch ON, then there is an additional requirement to have
all Squelch thresholds above the microphone noise floor. If
you require any assistance in determining what threshold
levels to set, please contact the applications department at
ON Semiconductor. Squelch ON/OFF does not incur any
current penalty. When Squelch and AFC are both ON, the
Squelch is limited to 1:2 expansion.
Feedback path
H
+
Σ
G
−
H’
Estimated feedback
Figure 5. Adaptive Feedback Canceller (AFC)
Block Diagram
Adaptive Noise Reduction
The noise reduction algorithm is built upon a high
resolution 128−band filter bank enabling precise removal of
noise. The algorithm monitors the signal and noise activities
in these bands, and imposes a carefully calculated
attenuation gain independently in each of the 128 bands.
The noise reduction gain applied to a given band is
determined by a combination of three factors:
• Signal−to−Noise Ratio (SNR)
• Masking threshold
• Dynamics of the SNR per band
The SNR in each band determines the maximum amount
of attenuation to be applied to the band − the poorer the SNR,
the greater the amount of attenuation. Simultaneously, in
each band, the masking threshold variations resulting from
the energy in other adjacent bands is taken into account.
Finally, the noise reduction gain is also adjusted to take
advantage of the natural masking of ‘noisy’ bands by speech
bands over time.
Based on this approach, only enough attenuation is
applied to bring the energy in each ‘noisy’ band to just below
the masking threshold. This prevents excessive amounts of
attenuation from being applied and thereby reduces
unwanted artifacts and audio distortion. The Noise
Reduction algorithm efficiently removes a wide variety of
types of noise, while retaining natural speech quality and
level.
A/D and D/A Converters
The system’s two A/D converters are second order
sigma−delta modulators operating at a 2.048 MHz sample
rate. The system’s two audio inputs are pre−conditioned
with antialias filtering and programmable gain
pre−amplifiers. These analog outputs are over−sampled and
modulated to produce two, 1−bit Pulse Density Modulated
(PDM) data streams. The digital PDM data is then
decimated down to Pulse−Code Modulated (PCM) digital
words at the system sampling rate of 32 kHz.
The D/A is comprised of a digital, third order sigma−delta
modulator and an H−bridge. The modulator accepts PCM
audio data from the DSP path and converts it into a 32−times
over−sampled, 1−bit PDM data stream, which is then
supplied to the H−bridge. The H−bridge is a specialized
CMOS output driver used to convert the 1−bit data stream
into a low−impedance, differential output voltage waveform
suitable for driving zero−biased hearing aid receivers.
HRX Head Room Expander
The Venture GA3219 has an enhanced Head Room
Expander (HRX) circuit that increases the input dynamic
range of Venture GA3219 without any audible artifacts. This
is accomplished by dynamically adjusting the
pre−amplifier’s gain and the post−A/D attenuation
depending on the input level.
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