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Audio Connections

Copyright © 1999 by H. Paul Shuch, Ph.D.
Executive Director, The SETI League, Inc.
PO Box 555, Little Ferry NJ 07643
email n6tx @

Except for those specialized computer-controlled receivers which couple all signals directly to a personal computer through its serial or parallel port, an amateur SETI station requires some kind of audio interface between the receiver and the computer's sound card. Some experimenting is usually required, since a receiver may have one of three kinds of audio output port, and a sound card may have two different types of audio input port.

The audio output options on a microwave receiver are Line Out, Speaker, and Headphones. The Line Out jack provides a low-level, high-impedance audio signal, which (when available) is usually the best option for interfacing the receiver to a computer sound card. Speaker and Headphone are generally both low impedance interfaces, with the former usually providing a higher amplitude signal than the latter. Since sound cards are prone to overload and distortion from high-amplitude signals, the Headphone output is generally preferred.

Computer sound cards typically sport Microphone and Line audio input connections. The Line In port is almost always the right choice for interfacing a receiver, as the audio levels present will usually overload the sound card if the Microphone connector is used. If the receiver has a Line Out connection, that will probably best match the sound card's Line In port.

Especially if the Microphone input on the sound card must be used, the receiver's audio level may be sufficient to overdrive the sound card, causing distortion. This problem can be alleviated by placing an audio attenuator circuit in the audio line between receiver and computer. Although you can purchase attenuators at a stereo shop, or build them yourself out of resistors, probably the simplest solution is to obtain an attenuating audio cable from your local stereo shop. These cables, which contain resistive attenuators built in, usually employ shielded wire and high quality plugs, and are used to patch the high-level outputs of turntables, tape decks and CD players directly into the low-level inputs of stereo preamplifiers. I have had good luck with the Radio Shack catalog number 42-2152, an attenuating audio cable six feet long with 1/8 inch phono plugs at both ends, which sells in the US for $3.49 each.

If you choose not to use an attenuating audio cable, be sure to connect your receiver to your computer via a well shielded audio patch cord. Lamp cord, telephone wire, and speaker wire should be avoided, as these may make your system susceptible to electromagnetic interference and hum.

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