Marine VHF Audio and Signalling Processor
CMX885
Audio Scrambling
The CMX885 incorporates an optional frequency inversion scrambler in transmit mode. This scrambles
audio band signals, to be de-scrambled in the receiver. The inversion frequency defaults to 3300Hz, but
maybe modified by writing to P4.8.
See:
o
Audio Control – $C2 write
7.6
Inband Signalling
The CMX885 supports DCS, NWR (Rx only) and DTMF signalling and a user-programmable audio tone
generator (288Hz to 3000Hz). Note that if tones below 400Hz are used, the 300Hz HPF should be
disabled.
Selection of the Inband signalling mode is performed by bits 11-8 of the Mode register ($C1). Detection of
the selected Inband signalling mode can be performed in parallel with voice or data reception.
See:
o
o
o
o
Mode Control – $C1 write
Tx Inband Tones – $C3 write
Tone Status – $CC read
Audio Tone – $CD write
7.6.1 Receiving DTMF Tones
DTMF Tone detection may be enabled in the Mode register ($C1) in parallel with other inband tone
modes (however, this is not recommended due to the increased likelihood of false detects). When a
DTMF tone has been detected, b10 of the Tone Status register ($CC) and b12 of the IRQ Status register,
$C6, will be set. This value will overwrite any existing inband tone value that may be present. The DTMF
detector returns the values shown below in Table 3.
7.6.2 Transmitting DTMF Tones
The DTMF signals to be generated are defined in the TxTONE register ($C3). Single tones and twist
(lower frequency tone reduced by 2dB) can be enabled by setting the appropriate bit in the $C3 register to
1. The DTMF level is set in Program Block P1.0. The DTMF tones must be transmitted on their own, the
host µC must disable audio band signals prior to initiating transmission of the DTMF tones and (if
required) restore the audio band signals after the DTMF transmission is complete. Table 3 shows the
DTMF tone pairs, together with the values for programming the ‘Tone Pair’ field of the TxTONE register.
© 2010 CML Microsystems Plc
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D/885/3
CMLMICRO [ CML MICROCIRCUITS ]
