BelaSigna R262
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13
Microphone Placement & Selection
The flexibility of the BelaSigna R262 noise reduction
algorithm doesn’t restrict microphone placements, but the
default algorithm will operate optimally with
omnidirectional microphones placed in the following
configuration:
• The two microphones are facing the user’s mouth
• The microphone centers are located within 10 to 25 mm
from each other
As mentioned, other configurations that differ from the
above guidelines are supported. For example, a 15 cm
distance between the two microphones will not degrade the
performance as long as the microphones are both facing the
user’s mouth. Alternatively, a configuration with one
microphone at the front and one microphone at the back will
not degrade the performance either, as long as the distance
between the microphones is no more than 2 cm.
BelaSigna R262 does not require any acoustic
microphone calibration procedure.
When selecting microphones to be used with
BelaSigna R262, the following guidelines should be used:
• Two omni−directional microphones with similar
characteristics should be used
• The microphone sensitivity should be approximately
−42 dB (where 0 dB = 1 V/Pa, at 1 kHz)
• The microphones are two−terminal microphones
• The microphone power supply is either 1 V
(recommended), or 2 V if it is to be provided by
BelaSigna R262
• The dynamic range of BelaSigna R262 on its analog
input channels is 2.0 V peak−to−peak, after
amplification by the default gain value of 24 dB using
BelaSigna R262’s input preamplifiers
• When higher sensitivity microphones are used, the
preamp gain should be adjusted to match the 2.0 Vpp
input voltage swing on BelaSigna R262, but this will
require special configuration of the ROM application,
as described later. As an example, using microphones
with a −22 dB sensitivity typically requires that the
preamplifier gains be changed down to 12 dB.
• When MEMS microphone are to be used, a general
increase of the algorithm performance can be expected
due to the improved self noise of these microphones,
compared to conventional electret microphones. For
applications requiring microphone configurations
differing significantly from the above recommendations,
contact your local ON Semiconductor support
representative.
Operating Modes
The default application stored in the ROM of
BelaSigna R262 has four Operating Modes. The Operating
Modes are summarized in Table 7.
Table 7. OPERATING MODES SUMMARY
Operating
Mode
Switching Description
Active Active mode can be entered at boot time,
depending on the BOOT_SEL configuration and
when exiting Sleep mode. Active mode can also
be entered via an I
2
C command from another
mode.
In Active mode, the noise reduction algorithm is executed. While
in Active mode, BelaSigna R262 collects statistics on the input
signals that can be retrieved via I
2
C. These signal statistics can
be used for level calibration and other debugging. For more
information using Active mode for calibration and debugging see
the BelaSigna R262 Communications and Configuration Guide.
Bypass Bypass mode can be entered at boot time,
depending on the BOOT_SEL configuration. It
can also be entered via an I
2
C command from
another mode.
In Bypass mode, no signal processing is done on the audio
inputs. The inputs are passed directly to the audio outputs. While
in Bypass mode, BelaSigna R262 collects statistics on the input
signals that can be retrieved via I
2
C. These signal statistics can
be used for level calibration and other debugging. For more
information using Bypass mode for calibration and debugging see
the BelaSigna R262 Communications and Configuration Guide.
Sleep Sleep mode can be entered via I
2
C commands.
When Sleep mode is entered via I
2
C, the chip will
exit Sleep mode only based on activity on the
I
2
C_SCL pin. Sleep mode will be automatically
entered if BelaSigna R262 detects that a required
external clock is no longer present. For more
information, see the Sleep Control section below.
In Sleep mode no signal processing is done. All analog blocks of
the chip are disabled and the digital core continues to run off an
internal low−speed oscillator, thereby allowing the external clock
to be disabled when the chip is asleep. This is BelaSigna R262’s
lowest power operating mode.
Stand−By Stand−By mode is an intermediate mode that is
only used when exiting sleep mode by an I
2
C
command.
When I
2
C is used to exit Sleep mode, the application will
transition to Stand−By mode, and will wait until the master I
2
C
device issues a Switch_Mode command to enter another
processing mode like Active or Bypass. If no such command is
issued, BelaSigna R262 will return to Sleep mode and wait for a
valid wake−up sequence.
