INSPIRIA SA3286

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10

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:

Figure 6. Independent Channel I/O Curve Flexibility

−100

−90

−80

−70

−60

−50

−40

−30

−20

−10

0

−120 −110 −100 −90 −80 −70 −60 −50 −40 −30 −20

INPUT LEVEL (dBV)

Low Level

Gain

Compression

Ratio

High Level

Gain

Squelch

Threshold

Lower

Threshold

Upper

Threshold

The I/O characteristic of the channel processing can be

adjusted in the following ways:

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

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.