CMX138 CML Microcircuits, CMX138 Datasheet - Page 29

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

Audio Scrambler and Sub-Audio Signalling Processor
7.5.3
The sub-audio CTCSS tone generated is defined in the Audio Control register ($C2). Table 2 lists the
CTCSS tones and the corresponding decimal values for programming b0-7 of the register.
7.5.4
A 23- or 24-bit sub-audio DCS code can be generated, as defined by the Audio Control register ($C2).
The same DCS code pattern is used for detection and transmission. The DCS code is NRZ encoded at
134.4±0.4 bps, low-pass filtered and added to the audio band signal, before being passed to the
modulator output stages. Valid 23-bit DCS codes and the corresponding settings for the Audio Control
Register are shown in Table 4, and include a user-defined facility. The least significant bit of the DCS
code is transmitted first and the most significant bit is transmitted last. The CMX138 is able to encode and
transmit either of the two DCS modulation modes defined by TIA/EIA-603 (true and inverted) described in
Table 3. If 24-bit mode is required, bit 11 of Programming register P2.1 should be set. The MOD output
inverts the signal from the device, so, depending on the detailed design of the following modulator
sections, it may be necessary to select an inverted DCS code in the Audio Control register ($C2) in order
to produce a true DCS code "on-air".
To signal the end of the DCS transmission, the host should set the Audio Control register ($C2) to the
DCS turn off tone (register value b0-7 = 254 decimal) for 150ms to 200ms. After this time period has
elapsed the host should then disable DCS in the Mode Control register ($C1). Note that if a CTCSS tone
is to be transmitted following the DCS turn-off tone (in a subsequent transmission) the new CTCSS value
will need to be written to the Audio Control register ($C2) immediately after the selection of Tx mode.
7.6
The CMX138 supports a user-programmable in-band tone between 288Hz and 3000Hz. Note that if a
tone below 400Hz is used, sub-audio signalling should be disabled and the 300Hz HPF disabled.
By default, the CMX138 will use a 1750Hz tone, however this may be changed by the host to any valid
tone within its operational range by use of the Programming register. This ensures that the device can
remain compatible with all available tone systems in use. The CMX138 does not implement automatic
repeat tone insertion or deletion: it is up to the host to correctly implement the appropriate protocol.
Selection of the In-band signalling mode is performed by bits 10-9 of the Mode register ($C1). Detection
of the selected In-band signalling mode can be performed in parallel with audio or data reception.
See:
7.6.1
In-band tones can be used to flag the start of a call or to confirm the end of a call. If they occur during a
call the tone may be audible at the receiver. When a valid input signal is detected, it will be reported in the
Tone Status register, $CC. If the input signal matches the In-band tone value then b15 will be set (tone
detected), otherwise b14-11 will be set (unrecognised tone) – see Table 5. If enabled, an IRQ will be
generated as shown below:
The frequency of the tone is defined in Programming register P1.2.
Adjustable decoder bandwidths and threshold levels are programmable via the Programming register.
These allow certainty of detection to be traded against signal to noise performance when congestion or
range limits the system performance. The in-band signal is derived from the received input signal after
the bandpass filtering shown in Figure 6.
© 2008 CML Microsystems Plc
o
o
o
In-band Signalling – User Tones
Transmit CTCSS Tone
Transmit DCS Code
Mode Control – $C1 write
Tx In-band Tone - $C3 write
Tone Status - $CC read
Receiving and Decoding In-band Tone
State change from:
No Tone
Own Tone
No Tone
Own Tone
Unrecognised Tone
To:
Own Tone
No Tone
Unrecognised Tone
Unrecognised Tone
No Tone
Page 29
IRQ
yes
yes
no
yes
no
Tone Status value b15-11
10000
00000
00000
01111
00000
D/138_FI1.0/5
CMX138