SA3291 datasheet(13/20 Pages) ONSEMI

13 / 20 page

AYRE SA3291

www.onsemi.com

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 Ayre SA3291 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 Ayre SA3291 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

as 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 Ayre SA3291 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.

Biquad Filters

Additional frequency shaping can be achieved by

configuring generic biquad filters. The transfer function for

each of the biquad filters is as follows:

H(z)

+ b0 ) b1

z−1

) b2

z−2

) a1

z−1

) a2

z−2

Note that the a0 coefficient is hard−wired to always be ‘1’.

The coefficients are each 16 bits in length and include one

sign bit, one bit to the left of the decimal point, and 14 bits

to the right of the decimal point. Thus, before quantization,

the floating−point coefficients must be in the range −2.0

≤ x

< 2.0 and quantized with the function:

round x

214

After designing a filter, the quantized coefficients can be

entered into the PreBiquads or PostBiquads tab in the

Interactive Data Sheet. The coefficients b0, b1, b2, a1, and

a2 are as defined in the transfer function above. The

parameters meta0 and meta1 do not have any effect on the

signal processing, but can be used to store additional

information related to the associated biquad.

The underlying code in the product components

automatically checks all of the filters in the system for

stability (i.e., the poles have to be within the unit circle)

before updating the graphs on the screen or programming

the coefficients into the hybrid. If the Interactive Data Sheet

receives an exception from the underlying stability checking

code, it automatically disables the biquad being modified

and display a warning message. When the filter is made

stable again, it can be re−enabled.

Also note that in some configurations, some of these

filters may be used by the product component for

microphone/telecoil compensation, low−frequency EQ, etc.

If this is the case, the coefficients entered by the user into

IDS are ignored and the filter designed by the software is

programmed instead.

Rocker Switch

The Ayre SA3291 is equipped with a rocker switch feature

that can perform both VC adjustments or an audio memory

switch.

SA3291 Datasheet(PDF) 13 Page - ON Semiconductor

SA3291 Datasheet(HTML) 13 Page - ON Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Datasheet Download

Link URL