CMX649 查看數據表（PDF） - CML Microsystems Plc

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CMX649

ADM Codec

CML Microsystems Plc 

ADM Codec

CMX649

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. Side-tone audio is added to the audio output signal via an

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.

VDD

R5 C2

C1

R2

R4

R6 C4

C3

MIC OUT

R1

MIC -

-

+

R3 MIC +

C6

20k

BIAS

C7

20k

VSS

Figure 9 Electret Microphone - Input Amplifier Schematic

R1, R3

R2, R4

C1, C3

C2, C4

100kΩ

100kΩ

100 pF

0.01µF

±10%

±10%

±20%

±20%

R5 100kΩ

R6 100kΩ

C6 1.0µF

C7 1.0µF

±10%

±10%

±20%

±20%

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

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