MT9092
voice side-tone path is also included to provide
proportional transmit speech feedback to the
handset receiver so that a dead sounding handset is
not encountered. Figure 3 depicts the nominal half-
channel and side-tone gains for the HPhone-II.
On PWRST (pin 6) the Filter/CODEC defaults such
that the side-tone path, dial tone filter and 400Hz
transmit filter are off, all programmable gains are set
to 0dB and µ-Law companding is selected. Further,
the Filter/CODEC is powered down due to the PuFC
bit (Transducer Control Register, address 0Eh) being
reset. This bit must be set high to enable the Filter/
CODEC.
The internal architecture is fully differential to provide
the best possible noise rejection as well as to allow a
wide dynamic range from a single 5 volt supply
design. This fully differential architecture is
continued into the Transducer Interface section to
provide full chip realization of these capabilites.
A reference voltage (V
Ref
), for the conversion
requirements of the CODER section, and a bias
voltage (V
Bias
), for biasing the internal analog
sections, are both generated on-chip. V
Bias
is also
brought to an external pin so that it may be used for
biasing any external gain plan setting amplifiers. A
0.1µF capacitor must be connected from V
Bias
to
analog ground at all times. Likewise, although V
Ref
may only be used internally, a 0.1µF capacitor from
the V
Ref
pin to ground is required at all times. It is
suggested that the analog ground reference point for
these two capacitors be physically the same point.
SERIAL
PORT
DSP GAIN*
FILTER/CODEC
TRANSDUCER INTERFACE
µ-Law
–6.3 dB
Α-Law
–3.7 dB
-6 dB
HSPKR+
75
Handset
Receiver
(150Ω)
Receive
PCM
–72 to
+22.5 dB
(1.5dB
steps)
Receive
Filter Gain
0 to –7 dB
(1 dB steps)
-6 dB
Receiver
Driver
HSPKR–
75
Side-tone
–9.96 to
+9.96dB
(3.32 dB steps)
DTMF,
Tone
Ringer &
Handsfree
Speaker
Phone
Driver
0.2dB*
SPKR+
SPKR–
Speaker Gain
0 to –24 dB
(8 dB steps)
Tone
Ringer
(input
from DSP)
Speakerphone
Speaker
(40Ω nominal)
(32Ω min)
Side-tone
Nominal
Gain
µ-Law
–11 dB
Α-Law
–18.8 dB
PCM
–72 to
+22.5 dB
(1.5dB
steps)
Transmit
DIGITAL DOMAIN
Transmit
Filter Gain
0 to +7dB
(1 dB steps)
µ-Law
6.1dB
Α-Law
15.4dB
Transmit
Gain
M
U
X
MIC+ Handsfree
MIC– mic
M+
M–
Transmitter
microphone
ANALOG DOMAIN
Internal to Device
Note: *gain the same for A-Law and
µ−
Law
External to Device
Figure 3 - Audio Gain Partitioning
7-7
MITEL [ MITEL NETWORKS CORPORATION ]
