SA3286 Datasheet, PDF (10/16 Pages) ON Semiconductor – Pre-configured DSP System

INSPIRIA SA3286
HRX Head Room Expander
The Inspiria SA3286 has an enhanced Head Room
Expander (HRX) circuit that increases the input dynamic
range of the Inspiria SA3286 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
−80
Threshold
−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
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 Inspiria SA3286 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 Inspiria SA3286 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 at 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 Inspiria SA3286 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.
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