SA3229 Datasheet, PDF (8/16 Pages) ON Semiconductor – Preconfigured DSP System

RHYTHM SA3229
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 4. Independent Channel I/O Curve Flexibility
Channel I/O processing is specified by the Squelch
threshold (SQUELCHTH) and any four of the following
five parameters (only four of the five properties are
independent):
• Low level gain (LLGAIN)
• Lower threshold (LTH)
• High level gain (HLGAIN)
• Upper threshold (UTH)
• Compression ratio (CR)
During the Parameter Map creation, constraints are
applied to the compression parameters to ensure that the I/O
characteristics are continuous. Parameter adjustments
support two popular styles of compression ratio adjustment:
• The compression region of the I/O curve pivots about
the upper threshold. As the compression ratio trimmer
is adjusted, high−level gain remains constant while
the low−level gain changes.
• The compression region of the I/O curve pivots about
the lower threshold. Low−level gain remains constant
as the compression ratio trimmer is adjusted.
The squelch region within each channel implements a low
level noise reduction scheme (1:3 expansion) for listener
comfort. This scheme operates in quiet listening
environments (programmable threshold) to reduce the gain
at very low levels.
Automatic Telecoil
The automatic telecoil feature in SA3229 is to be used
with memory D programmed with the telecoil or
MIC + TCOIL front end configuration. The feature enables
the part to transition to memory D upon the closing of
a switch connected to MS2. With the feature enabled and
a reed switch connected to MS2, the static magnetic field of
a telephone handset will close the switch whenever the
handset is brought close to the device, causing the hybrid to
change to memory D. The part will transition back to the
initial memory once the switch is deemed opened after
proper debouncing.
A debounce algorithm with a programmable debounce
period is used to prevent needless switching in and out of
memory D due to physical switch bounces when MS2 is
configured for automatic telecoil. Upon detecting a close to
open switch transition, the debounce algorithm monitors the
switch status. The debounce algorithm switches the device
out of memory D only once the switch signal has been
continuously sampled open over the specified debounce
period.
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. The forward path of the hearing aid is not
affected. Unlike adaptive notch filter approaches, SM3229’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.
1. 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
Feedback path
H
+
Σ
G
−
H’
Estimated feedback
Figure 5. Adaptive Feedback Canceller (AFC)
Block Diagram
Volume Control, Trimmers and Switches
External Volume Control
The volume of the device can either be set statically via
software or controlled externally via a physical interface.
SA3229 supports both analog and digital volume control
functionality, although only one can be enabled at a time.
Digital control is supported with either a momentary switch
or a rocker switch. In the latter case, the rocker switch can
also be used to control memory selects.
Analog Volume Control
Both the external (analog) volume control and trimmers
work with a three−terminal 100 kW – 360 kW variable
resistor. The volume control can have either a log or linear
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