CMX882 View Datasheet(PDF) - CML Microsystems Plc

Part Name

Description

Manufacturer

CMX882

FRS Signalling Processor

CML Microsystems Plc 

FRS Signalling Processor

CMX882

1.5.4.1 Processing Voice Signals for Transmission over Analogue Channels

The microphone input(s), with programmable gain, can be selected as the voice input source. Pre-

emphasis is selectable with either version of the 2 analogue Tx audio filters (for 12.5kHz and 25kHz

channel spacing). These are designed for use in ETS-300-086 and/or TIA/EIA-603 compliant

applications. Both filters attenuate sub-audio frequencies below 250Hz by more than 33dB wrt the signal

level at 1kHz. These filters together with a built in limiter help ensure compliance with ETS-300-086

(25kHz and 12.5kHz channel spacing) when levels and gain settings are set up correctly in the target

system.

10

reference 0dB at 1kHz

0

+0.5dB/-2dB wrt ref.

3kHz

-10

-20

-30

-33dB

-40

-14dB/octave

-50

-60

10

100

1000

10000

Frequency (Hz)

Figure 8 25kHz Channel Audio Filter Response Template

100000

The filter characteristics of the 12.5kHz channel filter fits the filter template shown in Figure 9 (solid

outline). This filter also facilitates implementation of systems compliant with TIA/EIA-603 ‘A’ and ‘B’

bands. To achieve attenuation above 3kHz of better than −100dB/decade for TIA/EIA-603 ‘C’ bands

(dashed outline), additional external circuitry is required, such as suggested in section 1.4.2.

10

+0.5dB/-2dB wrt ref.

reference 0dB at 1kHz

0

3kHz

-10

-20

-30

-40

-50

-60

-70

-80

-90

10

-33dB

100

1000

Frequency (Hz)

-60dB/decade wrt ref.

(-50dB)

20kHz

-100dB/decade

(-82.5dB)

20kHz

10000

100000

Figure 9 12.5kHz Channel Audio Filter Response Template

 2004 CML Microsystems Plc

26

D/882/7

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