CMX138 CML Microcircuits, CMX138 Datasheet - Page 27

• Manufacturer Part Number: CMX138
• Description: Audio Scrambler and Sub-Audio Signalling Processor
• Manufacturer: CML Microcircuits

Audio Scrambler and Sub-Audio Signalling Processor
Tone Cloning™ should not be used in systems where tones 41 to 51 or other “split” tones (tones between
the frequencies of tones 1 to 40) may be received. The all-call tone 40 can still be used after Tone
Cloning™ has been performed. The CTCSS decoder detection bandwidth should be set to its lowest
value (in P2.1 of the Programming Register) to ensure accurate detection.
CTCSS Tones
Table 2 lists the CTCSS tones available, the tone numbers and the equivalent (decimal) values that need
to be programmed into b7-0 of the Audio Control register ($C2) and which will be reported back in the
Tone Status register ($CC).
7.5.2
DCS code is in NRZ format and transmitted at 134.4±0.4bps. The CMX138 is able to decode any 23- or
24-bit pattern in either of the two DCS modulation modes defined by TIA/EIA-603 and described in Table
3. The CMX138 can detect a valid DCS code quickly enough to avoid losing the beginning of audio
transmissions.
The CMX138 detects the DCS code that matches the programmed code defined in the Audio Control
register ($C2) in either its true or inverted form. Register values 1 to 83 correspond to modulation type A
(“true”) and register values 101 to 183 correspond to modulation type B (“inverted”). A facility for a user-
defined code is available via Program Block 2 – CTCSS and DCS Setup. The signal inversion caused by
the input amplifier is automatically compensated for in the device, so that a true DCS signal applied at its
input will be decoded as a true code in the Tone Status register ($CC). Note that monitoring this signal at
the DISCFB pin will show an inverted waveform.
To detect the pre-programmed DCS code, the signal is low-pass filtered to suppress all but the sub-audio
band, using the filter shown in Figure 13. Further equalisation filtering, signal slicing and level detection
are performed to extract the code being received. The extracted code is then matched with the
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1. Register value 00 in b0-7 of the Tone Status register ($CC) indicates that none of the above sub-
2. Tone number 40 (240 decimal) provides an all-user CTCSS tone option; regardless of the sub-
3. Tone number 55 (255 decimal) is reported in the Tone Status register ($CC), when CTCSS
4. Tones 40 to 51 (241 to 251 decimal) are not in the TIA-603 standard.
5. Tone number 52 (252 decimal) will select the User Programmable Tone value in Program Block 2
6. Tone number 53 (253 decimal) will select the XTCSS call maintenance tone, 64.7Hz.
7. Tone number 54 (254 decimal) will select the DCS turn-off tone, 134.4Hz.
8. Tone Clone, register value 200, is a write-only value to the Audio Control register ($C2). It will not
Notes:
audio tones is being detected. If register value 00 is programmed into the Audio Control register
($C2) and CTCSS enabled in the Mode Control register ($C1), only CTCSS tone 40 (240
decimal) will be scanned for. If CTCSS transmit is selected, this tone setting will cause the
CTCSS generator to output no signal.
audio tones set, the CMX138 will report the presence of this tone whenever the CTCSS detector
is enabled. This feature is useful for implementing emergency type calls e.g. All-Call.
receive is enabled and a sub-audio tone is detected that does not correspond to the selected tone
or the all-call tone (tone number 40). This could be a tone in the sub-audio band which is not in
the table or a tone in the table which is not the selected tone or All-Call tone.
– CTCSS and DCS Setup.
be reported back in the Tone Status register ($CC). Instead, the received tone number is reported
back in this register.
Receiving and Decoding DCS Codes
Modulation Type:
A
B
Table 3 DCS Modulation Modes
Data Bit:
0
1
0
1
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Negative frequency shift
Negative frequency shift
FM Frequency Change:
Positive frequency shift
Positive frequency shift
D/138_FI1.0/5
CMX138