Extreme DC Offset

[janB]

Am I the only one who has this problem? I have it every time I try to record sound on any computer, with the on-board audio or with a Sound Blaster USB audio adaptor. But I can't find anything in this forum or anywhere else on the web about it. It's making me paranoid.

Because of this problem, there's only one volume control setting that works. Play-volume has to be no more than about 50%, and record-volume has to be exactly 10 percent. Raising record-volume just a hair above that results in distortion, and dropping it down a hair results in no sound at all. This can't be the way it's intended to work. And of course every recording has to be normalized afterward to be usable.

I have no problems playing any kind of sound, but I always have this problem with recording. I have it on every app I've tried (Audacity, Acoustica, mp3DirectCut, Total Recorder). And I have it on my new computer, my old one, and my laptop (an MSI board, a Soyo, and an IBM Thinkpad), on Windows 2000 and XT. Does anyone know what's causing it?

[kozikowski]

<<<Does anyone know what's causing it?>>>

Do you live near any tall radio towers? Take the ThinkPad to your mom's house in the next town and try it there.

Koz

[janB]

Thanks, Koz. No TALL radio towers around here, but there's a small one at the town hall, about 800 feet away, for calling the town's 3 police cars.

Maybe one of my neighbors has some kind of radio project that I don't know about. Is there any way to test for the presence of radio waves? If they can throw off the sound cards like that, I hate to think what they'd be doing to my brain. Hmm. It would explain a lot, though.

I'll take the Thinkpad over to the library and try it with their wireless internet connection. That's a couple of miles away.

[kozikowski]

The ThinkPad should be able to support a plain microphone plugged into the side. I would do that before connecting the machine to the internet. Once you start to involve the internet and recording on-line presentations, you throw a lot of additional complications into the mix, not the least of which is cranking around the Windows Sound Control Panels. Let's get the basics down first.

A common condition on all your machines is that they're all Windows and they all have been set to record from the internet, right? There are a number of really messy ways to do that incorrectly that can create damage--although I've never seen what you have in a computer. I've seen a lot of problems like that in radio stations that happen to be co-located near their towers. What's called "Near Field" to a radio transmitter can give you seriously magic problems.

But may not be a health risk. Every transmitter engineer I ever met (and there have been a few, including me) all retired in their 70s to a little farm in Maryland.

Not the same farm. You know, different ones.

Koz

[kozikowski]

<<<Is there any way to test for the presence of radio waves?>>>

Yes, most computers go crazy in the presence of intense radio waves.

Just kidding. Yes, there are radio detectors and you can get a Non-Ionizing Radiation Hazard team into your house to check it out. All very expensive.

Let's see how your field trip comes out before we go any further. No internet. Sit in your car several miles away, open up the ThinkPad, plug a simple microphone into the side and record it. The ThinkPad may even have a built-in microphone.

<<<there's only one volume control setting that works. >>>

Yes, you didn't have to go much further than that. Clearly if some of your recording capacity is taken up with battery voltage (DC), that leaves little for the show.

I can't wait to figure this one out.

Koz

[janB]

Oops. I guess I mis-stated the problem. This is an example of what I meant when I said that radio waves to the brain would explain a lot. I can still retire to a farm, but I get confused sometimes. Might milk the chickens instead of the cows.

When I tried recording with Mic as the only source, I didn't get any DC offset! Then I realized I hadn't actually tried that since discovering the DC offset problem (since I had a hard time finding the microphone). So I played with the control panel and tried other sources, just to compare, and I found I couldn't get the DC offset with any source!!!

After much experimentation, I now think this is the correct statement of the problem: Whenever Stereo Mixer is selected as the recording source, and Phone playback volume is not muted, a large DC voltage is introduced into the recording signal, which is added to any sound that's also going into the stereo mixer -- i.e., the problem only occurs when Phone is one of the sound sources. Same on all machines.

AudScrShot2.jpg (30.07 KiB) Viewed 860 times

I guess it must be that Phone wasn't muted before, and I didn't notice it because I didn't have it selected to appear on the volume control panel. And I guess in my previous experiments I was always using Stereo Mixer as the record source, and isolating the actual sound source in the mixer panel. Doh!

I don't know why Phone wasn't muted. Maybe the result of some other experiment, a long time ago, now forgotten (with the radio-fried brain problem, that could mean last month). But with Phone muted, everything works the way it should. If I ever want to use a phone with a computer, maybe I'll have to study this again and find out why it happens. But for now, problem solved! Thanks for the nudge that sent me onto the right track.

[kozikowski]

<<<Thanks for the nudge that sent me onto the right track.>>>

Glad to help. When we can't solve a problem immediately, sometimes we'll try to get you to change the system around a little in order to discover additional data, or to isolate the problem. In your case, the road trip wasn't only to get away from radio interference--although it would have done that, too. To record a microphone or an internal microphone, you would have had to turn off all your fancy settings and oddball preferences and recursive audio pathways. That's most often what the problem is.

Most times in the Audacity Windows forum, we solve Windows problems, not Audacity problems.

Enjoy.

Koz