SA3286 Datasheet, PDF (11/16 Pages) ON Semiconductor – Pre-configured DSP System

INSPIRIA SA3286
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
1 ) a1
z^−1 ) b2
z^−1 ) a2
z^−2
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 ^ǒx 2^14Ǔ
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. For more information on filter design
refer to the Biquad Filters In PARAGON Digital Hybrid
information note.
Rocker Switch
The Inspiria SA3286 is equipped with a rocker switch
feature that can perform both volume control (VC)
adjustments or an audio memory switch.
There are 3 modes of operation:
• Digital VC
• Momentary Memory Select
• Mixed Mode
In Mixed Mode, the switches behaviour is configurable to
be set to that a short or long press of the switch will invoke
either a memory or VC change (i.e., a short press is a
memory select, a long press is a VC change).
There is a programmable threshold that can be used to set
the timing behaviour.
Volume Control
The Volume Control (VC) can be either external or
programmable. If external VC operation is selected, a
further decision is required as to whether a variable resistor
(analog VC) or a Digital Volume Control (DVC) will be
connected to the 9−bit A/D converter.
Analog Volume Control
The external VC can be configured to work with either a
two−terminal 200 kW variable resistor or a three−terminal
0.1 MW − 1 MW variable resistor. In two−terminal
configuration, the VC is connected between GND and the
VC input. In three−terminal configuration, it is connected
between GND, Vreg and the VC input.
If using a two−terminal VC, it must be calibrated before
use. Calibration is not necessary with a three−terminal
connection. Hysteresis is built into the VC circuitry to
prevent unintentional volume level toggling. A log taper
potentiometer is recommended so that gain in dB would be
linear with potentiometer rotation. The range of VC is
adjustable and can be set between 1 dB (min) and 42 dB
(max).
Digital Volume Control
If using a Digital Volume Control with the Inspiria
SA3286, a resistor must be connected between the VC input
and Vreg, and another resistor of the same value must be
connected between the VC input and GND. The values of
both resistors can be between 50 kW and 0.5 MW.
A toggle switch can be used as a DVC, momentarily
connecting the VC to either Vreg or GND. By connecting the
VC to Vreg, the volume will be increased one step, and by
connecting the VC to GND, the volume will be decreased
one step.
The following parameters can be programmed into the
hybrid to specify the DVC functionality:
• DVC enable or disable
• Volume up/down step size of 1 dB, 2 dB, 3 dB or 4 dB
• Volume up/down beep frequency and volume
• DVC range between 6 dB and 42 dB in 6 dB steps
• Default DVC value when the hybrid is powered up
• Volume up/down beep enable
• Max/Min beep enable
• Max/Min beep frequency & volume
If the Max/Min beep is enabled then when the volume has
been incremented to the maximum value of the specified
DVC range the device will play two beeps to indicate that it
cannot increase the volume any more. The same is true for
decrementing the volume and reaching the minimum value
of the DVC range.
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