GA3284 Datasheet, PDF (7/15 Pages) ON Semiconductor – Pre-configured DSP System

INSPIRIA GA3284
SIGNAL PATH
There are two main audio input signal paths. The first path
contains the front microphone and the second path contains
the rear microphone, telecoil or direct audio input as selected
by a programmable MUX. The front microphone input is
intended as the main microphone audio input for single
microphone applications.
In FrontWave operation or ADM operation, a
multi−microphone signal is used to produce a directional
hearing instrument response. The two audio inputs are
buffered, sampled and converted into digital form using dual
A/D converters. The digital outputs are converted into a
32 kHz or 16 kHz, 20−bit digital audio signal. Further IIR
filter blocks process the front microphone and rear
microphone signals. One biquad filter is used to match the
rear microphone’s gain to that of the front microphone. After
that, other filtering is used to provide an adjustable group
delay to create the desired polar response pattern during the
calibration process. In ADM the two microphone inputs are
combined in an adaptive way while in FrontWave operation
the combination is static.
In the Telecoil mode gains are trimmed during Cal/Config
process to compensate for microphone/telecoil mismatches.
The FrontWave block is followed by four cascaded biquad
filters: pre1, pre2, pre3 and pre4. These filters can be used
for frequency response shaping before the signal goes
through channel and adaptive processing.
The channel and adaptive processing consists of the
following:
• Frequency band analysis
• 1, 2 or 4 channel WDRC
• 12 logarithmically spaced band frequency shaping
(graphic EQ)
• 128 frequency band adaptive noise reduction
• Frequency band synthesis
After the processing the signal goes through two more
biquad filters, post1 and post2, which are followed by the
AGC−O block. The AGC−O block incorporates the
Wideband Gain and the Volume Control. There are also 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 Inspiria GA3284’s AFC does not reduce the hearing aid’s
gain. The AFC is based on a time−domain model of the
feedback path.
The Inspiria GA3284 third−generation AFC provides
added stable gain similar to previous VENTURE products
but offers significantly reduced artefacts for music and tonal
input signals. As with previous VENTURE products, the
feedback canceller in the Inspiria GA3284 provides
completely automatic operation. The feedback canceller can
be activated in Mic1, Mic2, Mic+Telecoil or Mic+DAI
mode and cannot be activated in Telecoil−only or DAI−only
mode.
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,
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