1st of all let me say Welcome to the world of Amateur Radio and the Digital Modes. As some have called using an AIR INTERFACE or the correct term would be Acoustical Coupling us both using Microphones and Speakers from PC and the Radio.
This will atleast get you started. Lots of great programs out there, everything from A - Z:
DigiPAN - Great for new users but very limited in capabilities but easy to setup.
(It's Free Software and NO support or upgrades) Windows Only!
FLdigi - Also great for new users and it also a MultiMODE, MultiOS supporting MS Windows, Apple MacOS as well as Linux. Being used for Emcomm work!
(Also Free software with very good support)
HRD/DM780 - Another MultiMode (Windows only) program. Very good user support.
(Was Free Software for the previous versions but since has been sold) Updates?
MixW - MultiMode (Windows only) program. It is not free ($50.00) and has not been very well supported buy the Author and does not meet the needs of the users today in terms of Features and Capabilities.
MultiPSK - Another MultiMode (Windows Only) program. Not very user friendly and is free for the most part.
Since I too have a Icom 718 and also run 3 Signalink USB Interfaces I have some recommendation. With the exception of using Acoustical Coupling I highly recommend never to use the Mic Jack. In this case using the DataPort you cant go wrong. When using the DataPort you will need to set the Mic Gain on the Radio to Zero. During TX on the DataPort the Mic Circut does not get disabled so the MIC is HOT during this time. Also make sure the Comp (Speech compressor) is turned off as well as the DSP function and ANF.
Yes you can build your own Interface and that would be great if you had a PC that was dedicated just for this operation. If you don't then I recommend getting a Interface that has it's own built in Sound Card. Lets keep the Rats Nest of wires to a minimum. The Signalink offers the following:
Built in Sound Card.
Built in VOX Relay for PTT Control
Power off the USB Port of the PC.
Only requires 2 cables to operate, 1-USB and 1-Radio Data Cable (13pin)
If you wish to have CAT control of the Radio, you can find on Ebay USB to CI-V cables or run down to your local Radio Shack (Now called the Shack) and purchase their USB Scanner Programming Cable.
Well I cant think of anything else. That was my Nickles worth.
Again welcome to the Hobby.
73 de Steve KB6HOH