Computer sound card weirdness

[Scott]

Computer sound card weirdness

Greetings:

It appears that one of the following combinations makes a very noisy, “spread spectrum” radio receiver:

[list:0ffe5713c0]a. computer sound card output connected to external transmitter
b. computer sound card output connected to long wire[/list:u:0ffe5713c0]

I was working on simulcasting my audio through Icecast when I noticed that the signal was very noisy. I have two sound card in my Linux workstation, and one of them does the sound for my radio station. The sound output splits into a feed for the transmitter, plus a feed that goes back into the card’s input.

When I removed the audio source, I noticed that I could hear several of the area’s more powerful radio stations, all at once, on the input of my souncard. When I disconnected the line-out to my transmitter, the radio reception ceased.

I am not sure if this means the transmitter also feeds back back through the line-out, or if the line-out cable serves as an antenna and the sound card is doing the receiving.

In any case, I was able to avoid it by using a monitor radio to feed the received signal into the sound card’s input. This means that my Icecast netcast will actually be the received AM signal, complete with hum and nighttime interference. I suppose it’s nice to be able to monitor the signal remotely though the Internet, but if I wanted to offer the best sound possible on the netstream, I’d be out of luck, unless I can find a way to get rid of the weird multi-station AM interference. Since my netstream will be low bandwidth (16 – 24 kbps), perhaps it’s not so important.

I still haven’t published my netstream address on my website; I want to first polish up my programming automation, but if any of you want to test it out, you can point your browser’s to: http://robinvalley.2y.net:8000/

Try it out and let me know if you have any comments ideas for improving the sound. (Listen at the top of the hour and you’ll hear the time and weather)

Regards,

Scott
Robin Valley Community Radio
Omaha, Ne
http://robinvalley.org/

[radio8z]

Hi Scott,

I tried to follow your technical description, but got a bit lost. That probably doesn’t matter since my comments apply generally to audio equiipment.

Any nonlinear device will act as an AM detector. This is how the old crystal radios worked. The galena crstal is non linear. So if a strong AM signal gets into a circuit, such as an audio circuit, it can be detected by the non linearities in transistors, etc. and appear on the audio signal. This can happen even if the audio lines are shielded.

I tackled this problem at my church when the PA system began receiving CB transmissions. I installed a small capacitor from center to ground on the input line and put a small inductor in series with the center line. Problem was solved. Just be aware that too much C or L will limit the audio high frequency response.

You might also try the clip on ferrites around the audio cable, though I have found they aren’t very effective at AM frequencies.

Make sure your computer case is closed properly and that the panel screws are installed. This shields the sound card. You might try moving the sound card to another slot because the signal could be coming in through another card connected to the outside (modem, game port, etc.).
Try unplugging the phone line at your modem. Phone lines are effective AM receiving antennas.

Also make sure that your connecting cables are good. I have found some brand new ones that had an open shield. Check with an ohmmeter.

Anything that you can do to keep the offending signals out of your equipment by grounding, filtering or shielding is the approach to follow.

Neil

[techpuppy]

Hi Scott,

I checked out your website and audio stream.

First of all…GREAT programming schedule! I’m curious as to any feedback you receive about it.

At home I have only a connection speed of 26.4K, but I had no trouble listening to your station online. I think the audio quality is very good and I had no dropouts. I did not notice any extraneous noise or echo.

[Author]

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