Low Voltage CTCSS Encoder/Decoder
7
MX165C
General Description
Description
Voice on shared radio channels are multiplexed with a subaudible CTCSS (Continuous Tone Controlled Subaudible
Squelch) tone as a means of directing messages among user groups sharing the same RF frequencies. CTCSS
modulates the transmitter with a discrete tone, from 39 standard CTCSS tones in the range (67.0Hz to 250.0Hz)
according to TIA/EIA-603. There are an additional eight CTCSS tones not specified in TIA/EIA-603 that the MX165C will
encode and decode. They are : 159.8Hz, 183.5Hz, 189.9Hz, 196.6Hz, 199.5Hz, 206.5Hz, 229.1Hz, and 254.1Hz, for a
total of 47 CTCSS tones plus Notone.
The MX165C also incorporates TX/RX on chip speech filters. In early CTCSS designs, TX speech was not filtered from
the CTCSS tone, rather the filtering was dependent upon the host transmitter’s pre-emphasis network. At only
6dB/octave, the attenuation of speech components at higher CTCSS tones was only a few dB which resulted in talk-off
(low frequency voice components unsquelching the receiver audio).
I/O Conditions
D0-D5
Input Pins Condition
Tone
0
0
X
0
1
No Tone
0
X
X
0
1
Tone
1
0
0
0
1
Tone
1
1
0
0
1
Tone
1
X
1
1
0
No Tone
1
X
X
X
0
RX / TX
PTL
Decode Comparator In
Output Pins Condition
RX TONE DETECT
RX TONE DECODE
Result / Function
Tone Transmitter Enabled
TX Audio Path Enabled
Tone Decode Enabled
RX Audio Path Enabled
Notes
Notes:
1. Normal tone transmit condition.
Yes
Yes
No
No (Bias)
1
No (Bias)
Yes
No
No (Bias)
2
No (Bias)
No
Yes
No (Bias)
3
No (Bias)
No
Yes
Yes
4
No (Bias)
No
Yes
Yes
5
No (Bias)
No
Yes
Yes
6
2. Notone programmed in TX mode, tone transmit output set to V
DD
/2. TX audio path enabled.
3. Normal decode standby.
4. Normal decode standby with PTL used to enable audio.
5. Normal decode of correct CTCSS tone condition, PTL has no effect.
6. Notone programmed in RX mode, tone transmit output (Bias). RX audio path enabled.
7. X = don’t care
Table 3: Combinations of Input / Output conditions
© 1997 MX•COM Inc.
www.mxcom.com Tele: 800 638-5577 910 744-5050
Fax: 910 744-5054
Doc. # 20480102.008
4800 Bethania Station Road, Winston-Salem, NC 27105-1201 USA
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