wiki.radioreference.com
See Notes: Use the link in my first reply to see the radio shack isolation transformer on RS webpage.
"Notes
The typical radio output will be an 1/8 inch mono jack. Always use the Line Out, Record Out, or Tape Out when available on the scanner. This provides a constant audio level to the streaming computer and allows you to listen to the scanner's local speaker whenever you wish. Failing that, use the Headphone jack, not the speaker output. You can monitor your scanner via the streaming computer's speakers.
The typical computer sound card input will be an 1/8 inch stereo jack. The Line Input is desirable and is usually color-coded blue. Some computers, particularly laptops, only provide a Mic Input and this is usually color coded red.
Line output of the radio to line input on the computer is the best connection as this gives the cleanest audio and still allows you to listen to the scanner on its local speaker. Once you set the audio level, you don't have to touch it again since the volume control does not affect the line output.
Uniden scanners must be programmed properly so that each channel is set to Record = ON to allow audio to flow to the Rec or Tape Out jack. If you skip this step, the feed will not get audio from the scanner on those channels.
Newer Uniden scanners label the line output “Rec” (for Record out) and is located on the rear of the radio and is color coded green. The Record output needs to be turned ON via the software to activate it for use.
If you need to connect the headphone output of the radio to Mic in on the computer you will need to watch your audio levels on both the radio and computer. It is very easy to overdrive the audio.
Avoid using the speaker output of the radio because the output level maybe too high for the computer input and cause distortion. If this is all you have, keep the volume level low, between 1/4 to 1/2 volume.
The use of an audio isolator can help to eliminate hum between radio and computer due to different a.c. power grounds. Radio Shack has an isolator listed in #Related Links. It is an audio transformer that blocks d.c. continuity between the scanner housing and the computer case.
Eliminate the electrical noise and hum caused by ground loops with this ground loop isolator. Gold plated connectors enhance performance and resist corrosion. Includes Y-adapter with 1/8" mini plug for use with portable device. Parts Warranty: 90 Days
www.radioshack.com
Monitor the desired frequencies to ensure that the scanner does not receive radio interference from computers and computerized equipment nearby. If necessary, move the scanner around or re-orient the power, audio, and data cables to avoid picking up stray radio energy from devices in your shack. You'd be surprised how dirty some devices are."