CTCSS

Continuous Tone-Coded Squelch System

Initially the purpose of this system - also known as tone squelch was to be able to "tune out" of conversations in a shared communications channel. Only stations sharing the same tone could communicate with each other. It works by the transmitting station transmitting a continuous tone wich the receiving station locks on to and hence lets the voice signal in to be decoded.

In ham radio the system is fequently used in repeater locations that are shared with other spectrum users such as pagers. Close physical proximity and in some cases poor filtering can cause interference to repeater operations.

CTCSS is used to "open" the repeater for use. Any signal that does not contain the required CTCSS signal is effectively filtered out.

CTCSS tones and codes

#	1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
Freq Hz	67.0	71.9	74.4	77	79.7	82.5	85.4	88.5	91.5	94.8	97.4	100.0	103.5	107.2	110.9	114.8
EIA code	XZ	XA	WA	XB	SP	YZ	YA	YB	ZZ	ZA	ZB	1Z	1A	1B	2Z	2A
#	17	18	19	20	21	22	23	24	25	26	27	28	29	30	31	32
Freq Hz	118.8	123.0	127.3	131.8	136.5	141.3	146.2	151.4	156.7	162.2	167.9	173.8	179.9	186.2	192.8	203.5
EIA code	2B	3Z	3A	3B	4Z	4A	4B	5Z	5A	5B	6Z	6A	6B	7Z	7A	M1