AYRE SA3291
www.onsemi.com
9
SIGNAL PATH
The integrated NFMI transceiver is designed to provide
many highly desirable features in a wireless hearing aid
product. With minimal processing overhead, the transceiver
and its built−in MAC and link control function provides
networked mode support for 3 or more devices,
unidirectional low latency stereo audio streaming and
high−speed data transfer from one transmitter device to two
hearing instruments. The use of NFMI technology avoids all
the propagation problems that exist in an RF−based wireless
system. While having a small wireless range typically
limited to less than 60 cm, depending on the transmitter
device, the NFMI system has good immunity to the human
body blockage effect and it is not sensitive to RF
interference. The very high Q−factor for its coil antenna
ensures a very small operating system bandwidth in the radio
front end.
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 iSceneDetect, FrontWave, ADM or Automatic 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 iSceneDetect, ADM and Automatic
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, 4, 6 or 8 channel WDRC
• 16 frequency shaping bands (spaced linearly at 500 Hz
intervals, except for first and last bands)
• 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 (VC). 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.
White noise can be shaped, attenuated and then added into
the signal path at two possible locations: before the VC
(between the Wideband Gain and the VC) or after the VC
(between post 4 and the Peak Clipper) as shown in Figure 1.
FUNCTIONAL BLOCK DESCRIPTION
Binaural Synchronization
The binaural synchronization feature allows two hearing
aids to wirelessly synchronize adjustments such as program
modes or volume control.
Binaural Telecoil
The binaural telecoil wirelessly streams telecoil audio
signals from one hearing aid to the other, enabling hearing
aid users to hear phone calls in both ears.
In this mode, the user can also adjust the VC on the ear not
using the telephone. If the hearing aids become disconnected
by going out of range, in 94% of cases they will reconnect
in less than a second.
Wireless Streaming
Stereo audio signals can be streamed wirelessly from a
relay device to hearing aids equipped with Ayre SA3291. A
relay device can use Bluetooth or other far−field wireless
technology to wirelessly connect with TVs, music players,
mobile phones or other audio sources.
Streaming is primarily a one way mode, with the
transmitter streaming audio or commands to all devices
within its range. Streaming devices can operate across
a greater distance due to the larger transmit antenna and
greater transmit power.
A receiver must be configured in IDS to allow streaming
audio or remote control commands to be received.
CROS/BiCROS
To compensate for single sided deafness, AYRE SA3291
can be configured for use as a CROS/BiCROS device. It
does this by transmitting audio from a microphone on a
device on the completely deaf ear to the device on the aided
or unaided ear over a wireless link.
iSceneDetect 1.0 Environment Classification
The iSceneDetect feature, when enabled, will sense the
environment and automatically control the enhancement
algorithms without any user involvement. It will detect
speech in quiet, speech in noise, wind, music, quiet and noise
