Re: Strange Carrier Waves
Posted: Mon Nov 15, 2010 8:50 pm
Great, we've eliminated the AC adapter as the noise source. This leaves RF pickup.
Note that the bottom of the low frequency waves is pretty close to 0v. The wiggle could be rectification of an RF signal that's amplitude modulated at that low frequency, e.g. a signal from a moving vehicle being received directly and via one or more reflections. Or a signal that's received at a low level most of the time but occasionally a moving object reflects some of the same signal, which then mixes with the direct signal. Just like "airplane flutter" in analog TV. The small DC offset could even be due to rectification of the low-level direct signal, but I'm on a really thin limb here.
When you said "regardless of the input source" did you mean regardless of what's plugged into the computer's mic jack, or regardless of what input(s) are active in a mixer? If the latter, does the interference last long enough to try switching to a mic plugged directly into the computer for comparison?
One experiment I suggested in my last post was to try a patch cord plugged into the computer but left unconnected at the other end. I'd use the same patch cord you're using now. If/when you try this, try recording while moving the computer + patch cord around. See whether that DC offset varies with location as you move around. I chose my words carefully here - a small static charge may cause the DC level to shift during motion of the exposed connector at the end of the cable, but it would settle back to its previous value when the motion stopped. If the DC offset is location-dependent, that would support the RF rectification hypothesis.
There's an old trick people used on carbon-mic telephones when a strong AM radio signal was audible in the phone. We connected a small capacitor (like 1 nanofarad) across the carbon microphone. The carbon mic was rectifying the RF and making the telephone imitate a crystal radio. You could try making up a patch cord with a small capacitor across the plug at the computer end. Use a ceramic disc capacitor, 1 nF (.001 uF). But resort to this only after making sure you're using good quality braid-shielded audio cable. It might also be worth your trouble to add an attenuator in the same plug so you can set the mixer output higher above the noise floor. I'll post a drawing later if you wish. Or, you could use a commercial USB product like the one suggested in http://forum.audacityteam.org/viewtopic ... 73#p103564.
Note that the bottom of the low frequency waves is pretty close to 0v. The wiggle could be rectification of an RF signal that's amplitude modulated at that low frequency, e.g. a signal from a moving vehicle being received directly and via one or more reflections. Or a signal that's received at a low level most of the time but occasionally a moving object reflects some of the same signal, which then mixes with the direct signal. Just like "airplane flutter" in analog TV. The small DC offset could even be due to rectification of the low-level direct signal, but I'm on a really thin limb here.
When you said "regardless of the input source" did you mean regardless of what's plugged into the computer's mic jack, or regardless of what input(s) are active in a mixer? If the latter, does the interference last long enough to try switching to a mic plugged directly into the computer for comparison?
While you're at it, see if the small DC offset is still there. Be sure you have the same source connected to the input (preferably a microphone since that's what the input was designed for).kozikowski wrote:Next time you visit your mom out in the country, take the machine with you and see if you can make it break out there.
Koz
One experiment I suggested in my last post was to try a patch cord plugged into the computer but left unconnected at the other end. I'd use the same patch cord you're using now. If/when you try this, try recording while moving the computer + patch cord around. See whether that DC offset varies with location as you move around. I chose my words carefully here - a small static charge may cause the DC level to shift during motion of the exposed connector at the end of the cable, but it would settle back to its previous value when the motion stopped. If the DC offset is location-dependent, that would support the RF rectification hypothesis.
There's an old trick people used on carbon-mic telephones when a strong AM radio signal was audible in the phone. We connected a small capacitor (like 1 nanofarad) across the carbon microphone. The carbon mic was rectifying the RF and making the telephone imitate a crystal radio. You could try making up a patch cord with a small capacitor across the plug at the computer end. Use a ceramic disc capacitor, 1 nF (.001 uF). But resort to this only after making sure you're using good quality braid-shielded audio cable. It might also be worth your trouble to add an attenuator in the same plug so you can set the mixer output higher above the noise floor. I'll post a drawing later if you wish. Or, you could use a commercial USB product like the one suggested in http://forum.audacityteam.org/viewtopic ... 73#p103564.