CMX649 CML Microcircuits, CMX649 Datasheet - Page 9

• Manufacturer Part Number: CMX649
• Description: ADM Codec
• Manufacturer: CML Microcircuits

ADM Codec
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 type companding algorithms are provided
and use 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 at reduced data rates.
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.
5.1.7 Programmable Anti-alias/image SC Filters
The anti-aliasing (AAF) and anti-imaging (AIF) switched capacitor (SC) filters have a programmable cut-
off frequency to accommodate different input signal bandwidths. Typically, the audio filter bandwidth
2003 CML Microsystems Plc
C6
Figure 9 Electret Microphone - Input Amplifier Schematic
R1, R3
R2, R4
C1, C3
C2, C4
R5
R6
100 pF
0.01 F
100k
100k
C2
C4
Side-tone audio is added to the audio output signal via an
R2
R4
10%
10%
20%
20%
C1
C3
C7
9
R5
R6
C6
C7
R1
R3
MIC -
BIAS
VSS
MIC OUT
MIC +
VDD
1.0 F
1.0 F
100k
100k
+
-
20k
20k
10%
10%
20%
20%
CMX649
D/649/2