CMX138 CML Microcircuits, CMX138 Datasheet - Page 26

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

Audio Scrambler and Sub-Audio Signalling Processor
7.5.1
The CMX138 is able to accurately detect valid CTCSS tones quickly to avoid losing the beginning of audio
or data transmissions, and is able to continuously monitor the detected tone with minimal probability of
falsely dropping out. The received signal is filtered in accordance with the template shown in Figure 13,
to prevent signals outside the sub-audio range from interfering with the sub-audio tone detection.
Once a valid CTCSS tone has been detected, Status register ($C6) b11 will be set and the host µC can
then route the audio band signal to the audio output. The audio band signal is extracted from the received
signal by bandpass filtering as shown in Figure 6.
To optimise the CTCSS tone decoder, adjustable decoder bandwidths and threshold levels allow the user
to trade-off decode certainty against signal-to-noise performance when congestion or range restrict the
system performance. The tone decoder bandwidth and threshold level are set in P2.1 of the Programming
register ($C8) and the desired tone is programmed in the Audio Control register ($C2). In systems which
make use of tones 41 to 50 or other “split” tones (tones in between the frequencies of tones 1 to 40), the
CTCSS decoder bandwidth should be reduced to avoid false detection of adjacent tones.
When enabled, an interrupt will be issued when an input signal matching a CTCSS tone in Table 2
changes state (ie: on, off or to or from a different tone). If a sub-audio tone is present, but it is not one of
the valid CTCSS tones (as shown in Table 2), then it will be reported as an unrecognised tone. If a tone
other than the programmed tone is detected, it will be reported as an Invalid tone, unless Tone Cloning is
enabled, in which case it will report the detected tone number. If enabled, an IRQ will be generated under
the following conditions:
Tone Cloning
Tone Cloning
to non-predictively detect any tone in this range. This mode is activated by programming CTCSS Tone
Number 00 (b0-7 of Audio Control register = 200 decimal). The received tone number will be reported in
the Tone Status register ($CC) and can then be programmed into the Audio Control register by the host
µC. The cloned tone will only be active when CTCSS is enabled in the Mode Control register ($C1). This
setting has no effect in Tx mode and the CTCSS generator will output no signal.
T Tone Cloning™ is a trademark of CML Microsystems Plc.
© 2008 CML Microsystems Plc
Receiving and Decoding CTCSS Tones
State change from:
No Tone
Own Tone
No Tone
Unrecognised Tone
No Tone
Invalid Tone
™
™
facilitates the detection of CTCSS tones 1 to 39 in receive mode which allows the device
Figure 13 Low Pass Sub-audio Band Filter for CTCSS and DCS
-10
-20
-30
-40
-50
-60
-70
10
0
0
To:
Own Tone
No Tone
Unrecognised Tone
No Tone
Invalid Tone
No Tone
200
400
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Frequency (Hz)
600
yes
yes
IRQ
yes
yes
yes
yes
800
Tone Status value b7-0
Own Tone
$00
$FF
$00
$FF or detected Tone
$00
1000
D/138_FI1.0/5
CMX138