If you are programming repeaters into your radio or working certain satellites, you likely encountered the above. This is not a full background or technical paper on either CTCSS or CDS but it has enough to get you to understand them and get going.
Let's start with CTCSS which stands for Continuous Tone Coded Squelch System and is used in the "Analog" repeater world at times to squelch unwanted traffic. This signal, while technically audible, is below the normal analog speech as is generally received by radios. It is often called sub-audible, but this is not technically correct. If you were to listen with full spectrum or low frequency capable speakers, such as a sub-woofer, you would be able to hear it unless you radio pre-filters it. Many do this already as do many repeaters before re sending the signal. In the repeaters case, it is sometimes done for added security. Generally this will be a tone between 67 and 254.1Hz. Generally speaking, CTCSS is a little more forgiving then DCS and has much wider implementation as well as compatible with older radios.
Was originally developed by Motorola, who of course, is known for massive contributions to radio technology over the years. Below is a list of commonly used CTCSS as well as the corresponding Motorola Equivalents used in some (mu understanding is, mainly older) systems.
Please note: There are some ADDITIONAL codes used, but mainly by some specific manufacturers such as Kenwood and are cleanly not cross compatible. Avoid using these if possible unless you have a specific reason to.
DCS stands for Digital Coded Squelch and is what you will encounter in the "digital" world...sort of. It is a digital signal being sent along side the audio (which can be Analog) and usually sits for the most part, as CTCSS in the first 300Hz of the signal being transmitted. DCS does take more bandwidth and it is more important for the radio to be on frequency due to the fact that much of the signal resides on the very bottom portion of the signal being sent. Also note, it is also known as Motorola Digital Private Line and again, the name Motorola comes up for a good reason. Below is the, much larger, table of used DCS codes.
Besides CTCSS and DCS there are some other tone signals out there. If you are really interested in these you can find a very nice PDF on the subject here: http://www.repeater-builder.com/tech-info/pl+dpl/tone_signaling.pdf
If you find something incorrect, have an update, or if there is something you would like to see added, drop me a line here. I like to keep things as up-to-date and as accurate as possible. Thank you! - Lucas / W6AER