B. INTEGRAL PARTS AND FUNCTIONALITY
B-5
BeFREE 20 – TECHNICAL OVERVIEW
B.4.1.
Microphone Amplifier
This is a logarithmic audio amplifier with programmable gain, noise gating threshold
and compression ratio. Noise gating feature enables ambient noise to be suppressed. All
sounds below the threshold are removed inside the amplifier, so with higher threshold the
microphone needs to be used closer to the mouth and speech needs to be louder to get
through the amplifier. The compression feature enables compression of the output signal
dynamics in order to minimize the difference between soft/low and loud speech. The
amplifier is also capable of limiting extremely loud speech thus preventing distortion and
popping. The overall gain can be even lower than one (i.e. attenuation) as well as equal to
zero (i.e. totally muted input). Also the range of the goose-neck microphone can be
adjusted.
A linear regime of the amplifier is a special option which might be activated in full-
duplex operation where acoustic echo is to be cancelled using application software (rather
than optionally integrated hardware echo-canceller – see Chapter D.3).
B.4.2.
Left and Right Speaker Amplifiers
These are linear audio power amplifiers with programmable gain, each driving its own
speaker (Left and Right Speaker in Figure B.4). The gain of each amplifier (i.e. volume
level of the respective speaker) can be adjusted by the user in normal operation via the
respective incremental encoder at the respective side of the module (Left and Right
Encoder in Figure B.1). The selected level is immediately displayed at the respective bar-
graph display in green colour (20 volume mute). Each amplifier can deliver up to
2 W (2 x 1 W) of electric power to its speaker.
By default, the left speaker is driven by the sum of left output of USB Codec A and the
left output of USB Codec B, while the right speaker is driven by the sum of right output of
Codec A and right output of Codec B (so-called
Stereo Mode
). Alternatively, each Codec
can drive its own speaker, i.e. Codec A to left speaker, Codec B to right speaker (so-called
Mono Mode
). In either of the modes the analogue Line Out signals can be optionally
added to the summing amplifiers, Line Out Left to left amplifier, Line Our Right to right
amplifier.
B.4.3.
Central Speaker Amplifier
This is a linear audio power amplifier with programmable gain and cut-off frequency,
so it effectively acts as a programmable low-pass filter (which is a kind of an equalizer)
capable to boost lower frequencies (typically below 300 Hz) and improve naturalness of
the reproduced speech. It drives the Central Speaker by the “equalized” sum of signals to
