Sound Card Interfacing
Contents
1. General info
In some cases, interfacing radio equipment audio lines to a computer sound card can be as simple as buying cables or adaptors with the appropriate connectors. However, there can be many problems which can appear due to impedance mismatching, lack of shielding, ground loops, etc. which can lead to audio that may be weak, noisy, or distorted. A common problem is RF “feedback”, where an audio line acts as an antenna feeding RF into the computer. If the level is high enough, it is possible to damage the sound card or other major components in the computer.
One of the simplest cures is to install isolation transformers. ComTekk offers sound card isolators with sound card connectors already installed. Snap-on ferrite filters are also good for solving difficult RFI problems, and should be attached to audio cables very close to the end of the connectors plugged into the computer.
2. Sound Card Connections
Most PC’s have at least three audio connections available:
1. Output (for speakers, usually black or blue)
2. Microphone Input (pink/red)
3. Line/Aux. Input (green)
4. Line/Aux. Output (for speakers or amplifier – green)
There are usually tiny symbols next to the jacks, which may be very hard to see! Most include an arrow pointing toward the jack (input), or away from the jack (output).
Here are some more examples – click on the images to zoom in:
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If there is no Line/Aux Input jack on your computer, the Microphone jack may be used instead. Here’s an example, with the symbols highlighted:
A stereo connecting cable such as this one, is suitable for connection to most receivers. Notice the 3 metal rings – the two near the tip are the left/right audio, and the 3rd is a common or ground ring:
For best results, plug a mono-to-stereo adaptor into the receiver first. 1/8″ ‘phone’ plugs are standard on all computers, and most radios:
This will split the mono signal from the radio into stereo, feeding audio to both channels of the sound card.
* Protecting your computer *
Direct connections can be made between sound card and radio equipment, but there can be a risk involved. It is our recommendation that a sufficient isolation and protection circuit be installed between the computer and any radio equipment. A pair of 600-ohm 1:1 isolators for sound cards is available for only $29 (order) to protect input and output ports. This will block DC current, protect against a hot or floating ground, and has the added benefits of reducing AC hum and RF interference (RFI).
If connecting sound card to a very long line, such as a twisted pair, a quality telephone-type surge protector should be installed to guard against spikes, surges and lightning.
When connecting a low-impedance radio output (like the external speaker jack) to the MIC input on a computer, the volume control on the radio must be set very, very low, to avoid over-driving the sensitive Mic input on the sound card. Don’t worry – in most cases, it would take a LOT of power to actually damage your sound card. It’s best to use the LINE or AUX input if one is available.