SA3291 Datasheet, PDF (13/20 Pages) ON Semiconductor – Preconfigured Wireless DSP System for Hearing Aids

AYRE SA3291
necessary to select four parameters as user adjustable, or
fixed, and to allow one parameter to be calculated.
The squelch region within each channel implements a low
level noise reduction scheme (1:2 or 1:3 expansion ratio) for
listener comfort. This scheme operates in quiet listening
environments (programmable threshold) to reduce the gain at
very low levels. When the Squelch and AFC are both enabled
it is highly recommended that the Squelch be turned on in all
channels and that the Squelch thresholds be set above the
microphone noise floor (see Adaptive Feedback Canceller).
The number of compression channels is programmable in
ARKonline^® and can be 1, 2, 4, 6 or 8.
Telecoil Path
The telecoil input is calibrated during the Cal/Config
process. To compensate for the telecoil/microphone
frequency response mismatch, a first order filter with
500 Hz corner frequency is implemented. Through
ARKonline, it is possible to implement a telecoil
compensation filter with an adjustable corner frequency. To
accommodate for the gain mismatch, the telecoil gain is
adjusted to match the microphone gain at 500 Hz or 1 kHz
(default) and is selectable in ARKonline.
There is also a telecoil gain adjustment parameter that can
be enabled in ARKonline and set in IDS, enabling manual
adjustment of the telecoil gain compensation.
Automatic Telecoil
The Ayre SA3291 is equipped with an automatic telecoil
feature, which causes the hybrid to switch to a specific
memory upon the closing of a switch connected to MS2.
This feature is useful when MS2 is connected to a switch,
such as a reed switch, that is open or closed depending on the
presence of a static magnetic field. Memory D can be
programmed to be the telecoil or mic+telecoil memory so
that, when a telephone handset is brought close to such
a switch, its static magnetic field closes the switch and
causes the hybrid to change to memory D. However, it is
possible that the hearing aid wearer may move his or her
head away from the telephone handset momentarily, in
which case it is undesirable to immediately change out of
telecoil mode and then back in moments later.
The Ayre SA3291 has a debounce circuit that prevents this
needless switching. The debounce circuit delays the device
from switching out of memory D when MS2 is configured
as a static switch in ‘D−only’ mode. The debounce time is
programmable to be 1.5, 3.5 or 5.5 seconds after the switch
opens (i.e., the handset is moved away from the hearing
instrument) or this feature can be disabled.
DAI Path
The DAI input can be adjusted using a first order filter
with a variable corner frequency similar to the telecoil
compensation filter. Through ARKonline, it is possible to
implement this DAI filter to set either a static or adjustable
corner frequency.
The Mic plus DAI mode mixes the Mic1 and DAI signals.
The Mic1 input signal is attenuated by 0, −6 or −12 dB before
being added to the DAI input signal. The DAI input also has
gain adjustment in 1 dB steps to assist in matching it to the
Mic1 input level.
Graphic Equalizer
The Ayre SA3291 has a 16−band graphic equalizer. The
bands are spaced linearly at 500 Hz intervals, except for the
first and the last band, and each one provides up to 24 dB of
gain adjustment in 1 dB increments.
Biquad Filters
Additional frequency shaping can be achieved by
configuring generic biquad filters. The transfer function for
each of the biquad filters is as follows:
H(z)
+
b0 ) b1
1 ) a1
z^−1 ) b2
z^−1 ) a2
z^−2
z^−2
Note that the a0 coefficient is hard−wired to always be ‘1’.
The coefficients are each 16 bits in length and include one
sign bit, one bit to the left of the decimal point, and 14 bits
to the right of the decimal point. Thus, before quantization,
the floating−point coefficients must be in the range −2.0 ≤ x
< 2.0 and quantized with the function:
round ^ǒx 2^14Ǔ
After designing a filter, the quantized coefficients can be
entered into the PreBiquads or PostBiquads tab in the
Interactive Data Sheet. The coefficients b0, b1, b2, a1, and
a2 are as defined in the transfer function above. The
parameters meta0 and meta1 do not have any effect on the
signal processing, but can be used to store additional
information related to the associated biquad.
The underlying code in the product components
automatically checks all of the filters in the system for
stability (i.e., the poles have to be within the unit circle)
before updating the graphs on the screen or programming
the coefficients into the hybrid. If the Interactive Data Sheet
receives an exception from the underlying stability checking
code, it automatically disables the biquad being modified
and display a warning message. When the filter is made
stable again, it can be re−enabled.
Also note that in some configurations, some of these
filters may be used by the product component for
microphone/telecoil compensation, low−frequency EQ, etc.
If this is the case, the coefficients entered by the user into
IDS are ignored and the filter designed by the software is
programmed instead.
Rocker Switch
The Ayre SA3291 is equipped with a rocker switch feature
that can perform both VC adjustments or an audio memory
switch.
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