SA3291 Datasheet, PDF (12/20 Pages) ON Semiconductor – Preconfigured Wireless DSP System for Hearing Aids

AYRE SA3291
There are 750 log entries plus 6 memory select counters
which are all protected using a checksum verification.
A new log entry is made whenever there is a change in
memory state, VC, or battery level state. A new log entry can
also be optionally made when the environmental sound level
changes more than the programmed threshold, thus it is
possible to log only significantly large changes in the
environmental level, or not log them at all.
The ARK software iLog graph displays the iLog data
graphically in a way that can be interpreted to counsel the
user and fine tune the fitting. This iLog graph can be easily
incorporated into other applications or the underlying data
can be accessed to be used in a custom display of the
information.
Tinnitus Treatment
The Ayre SA3291 has an internal white noise generator
that can be used for Tinnitus Treatment. The noise can be
attenuated to a level that will either mask or draw attenuation
away from the user’s tinnitus. The noise can also be shaped
using low−pass and/or high−pass filters with adjustable
slopes and corner frequencies.
As shown in Figure 1, the Tinnitus Treatment noise can be
injected into the signal path either before or after the VC or
it can be disabled. If the noise is injected before the VC then
the level of the noise will change along with the rest of the
audio through the device when the VC is adjusted. If the
noise is injected after the VC then it is not affected by VC
changes.
The Tinnitus Treatment noise can be used on its own
without the main audio path in a very low power mode by
selecting the Tinnitus Treatment noise only. This is
beneficial either when amplification is not needed at all by
a user or if the user would benefit from having the noise
supplied to them during times when they do not need
acoustic cues but their sub−conscious is still active, such as
when they are asleep.
The ARK software has a Tinnitus Treatment tool that can
be used to explore the noise shaping options of this feature.
This tool can also be easily incorporated into another
software application.
Narrow−band Noise Stimulus
The Ayre SA3291 is capable of producing Narrow−band
Noise Stimuli that can be used for in situ audiometry. Each
narrow−band noise is centred on an audiometric frequency.
The duration of the stimuli is adjustable and the level of the
stimuli are individually adjustable.
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 64−times
or 128−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 Ayre SA3291 has an enhanced Head Room Expander
(HRX) circuit that increases the input dynamic range of the
Ayre SA3291 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.
Channel Processing
Figure 6 represents the I/O characteristic of independent
AGC channel processing. The I/O curve can be divided into
the following main regions:
• Low input level expansion (squelch) region
• Low input level linear region
• Compression region
• High input level linear region (return to linear)
0
−10
−20
−30 _{Low Level}
−40 ^Gain
−50
−60
Compression
Ratio
Lower
Threshold
High Level
Gain
Upper
Threshold
−70
Squelch
Threshold
−80
−90
−100
−120 −110 −100 −90 −80 −70 −60 −50 −40 −30 −20
INPUT LEVEL (dBV)
Figure 6. Independent Channel I/O Curve Flexibility
The I/O characteristic of the channel processing can be
adjusted in the following ways:
• Squelch threshold (SQUELCHTH)
• Low level gain (LLGAIN)
• Lower threshold (LTH)
• High level gain (HLGAIN)
• Upper threshold (UTH)
• Compression ratio (CR)
To ensure that the I/O characteristics are continuous, it is
necessary to limit adjustment to a maximum of four of the
last five parameters. During Parameter Map creation, it is
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