CMX649E3 CML Microcircuits, CMX649E3 Datasheet - Page 9

• Manufacturer Part Number: CMX649E3
• Description: ADM Codec
• Manufacturer: CML Microcircuits

ADM Codec
(note that the encoder can also be configured to do this function and in this example
5.1.4 Non-Linear Instantaneous Companding
When using the device over its standard PCM codec style interface, instantaneous companding can be
enabled to cut in half the PCM word size. Either µ-law or A-law companding are available using their
standard 16-chord piecewise linear approximations. Essentially the companded 8-bit PCM word is a
simple floating-point representation with a sign bit, a 3-bit exponent and a 4-bit mantissa. This approach
yields toll quality speech.
5.1.5 Digitally Controlled Amplifiers
There are three Digitally Controlled Amplifiers (DCA) on-chip, which are used to set the signal levels for
transmit-audio-in, side-tone-audio, and receive-audio-out (volume control). The transmit-audio DCA is
adjustable in 0.5dB steps over a +7.5dB to –7.5dB range. The side-tone DCA is adjustable in 6.0dB
steps over a 0dB to –21.0dB range.
operational amplifier configured as a summing amplifier. This feeds the receive-audio DCA, which is
adjustable in 1.5dB steps over a +12.0dB to –33.0dB range.
5.1.6 Microphone Amplifier
The input amplifier is a high gain low-noise operational amplifier capable of interfacing with a variety of
different microphones. Figure 9 is a simplified schematic showing the external components required for
typical application with an electret condenser microphone. Typical values for R1, R3, C1 and C3 should
be set according to microphone sensitivity requirements, those shown are for unity gain. Note also that
the microphone biasing resistors (R5 and R6) are microphone specification dependent.
all data is read and written via C-BUS registers $D6 ($E6) and $D8 ($E8) respectively (for encoder))
2002 CML Microsystems Plc
C6
Figure 9 Electret Microphone - Input Amplifier Schematic
R1, R3
R2, R4
R5
R6
100k
100k
C2
C4
Side-tone audio is added to the audio output signal via an
R2
R4
10%
10%
C1
C3
C7
9
R5
R6
R1
R3
MIC -
BIAS
VSS
MIC OUT
MIC +
VDD
100k
100k
+
-
20k
20k
10%
10%
CMX649
D/649/1