CMX148 datasheet(17/75 Pages) CMLMICRO

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PMR Audio and Data Processor

CMX148

D/148/6

7.5.

Audio Functions

The audio signal can be processed in several ways, depending on the implementation required, by

selecting the relevant bits in the Audio Control – $C2 write register. In both Rx and Tx modes, a

selectable channel filter to suit either the 12.5kHz or 25kHz TIA/ETSI channel mask can be selected. This

filter also incorporates a soft limiter to reduce the effects of over-modulation. Other features include

300Hz HPF, pre- and de-emphasis, companding and frequency inversion scrambling, all of which may be

individually enabled1. The order in which these features are executed is selectable to ensure compatibility

with existing implementations and provide optimal performance (see section 8.2.5). The alternate settings

shown in section 8.2.5 may provide better performance than the default configuration

7.5.1.

Audio Receive Mode

The CMX148 operates in half-duplex, so whilst in receive mode the transmit path (MIC input and MOD1/2

amplifiers) can be disabled and powered down if required. The AUDIO output signal level is equalised (to

VBIAS) before switching between the audio port and the modulator ports, to minimise unwanted audible

transients. The Off/Powersave level at MOD1/2 outputs is the same as the VBIAS pin, so the audio output

level must also be at this level before switching.

See:

Audio Control – $C2 write

Receiving Audio Band Signals

When a voice-based signal is being received, it is up to the host µC, in response to signal status

information provided by the CMX148, to control muting/enabling of the audio signal to the AUDIO output.

The discriminator path through the device has a programmable gain stage. Whilst in receive mode this

should normally be set to 0dB (the default) gain.

Receive Filtering

The incoming signal is filtered, as shown in Figure 7 (with the 300Hz HPF also active), to remove sub-

audio components and to minimise high frequency noise. When appropriate, the audio signal can then be

routed to the AUDIO output. Separate selectable filters are available for:

•

300Hz High Pass (to reject sub-audible signalling)

•

2.55kHz Low Pass (for 12.5kHz channel operation)

•

3.0kHz Low Pass (for 25kHz channel operation)

Note that with no filters selected, the low frequency response extends to below 5Hz at the low end but still

rolls off above 3.3kHz at the top end.

The cut-off point of the 300Hz HPF can be set to either 280Hz, 300Hz or 320Hz by setting the relevant

bits of Program Block P2.5, as shown in Figure 6.

1 The typical responses shown in Figure 7, Figure 10, Figure 11 were recorded using the EV1480 Evkit,

DISC to AUDIO and MOD1.

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