Audacity Forum

I'm trying to record from a PA mixer and I keep getting this weird oscillation in the recordings. It seems to work fine for a simple spoken vocal track, but if I try to record the whole band everything goes haywire. I'm using an Acer laptop as the Audacity host. I've tried recording from the headphone output and the tape out port on the mixer to the line in on the laptop. Not sure what's going on here. I'll try anything, so no suggestion is too out there.

I also have an Acer laptop. I'm very pleased with it, but the on-board sound card is rubbish. I connect the tape out of my mixer to the input of a Behringer UCA 202 USB sound card and the sound quality is amazingly good for a $30 sound card.

I have an HP laptop and routinely record from the mixing board at church. I find I have to use battery power for the laptop to get a clean recording. This is at line level, not mic level - the laptop has a setting for "mic boost on/off" to select input level range. If the laptop is AC powered I get something that fits the general description of a "weird oscillation". I don't get the noise with an ungrounded source like a microphone. I think it's the switching power supply aliasing with the sample rate, probably getting in via a ground loop.

<<<"mic boost on/off" >>>

'Mic Boost' converts the laptop input between loud and soft microphones. Not convert from Microphone to Line level. So you're still beating up your laptop with the powerful signal from the church sound system.

It's also mono, by the way. If you have a stereo signal from your mixer, you're feeding powerful sound signals (Right Audio) into the battery system of the laptop.

So you're complaining about odd sounds in a sound system badly wired and under electrical stress.

Thought of another way, you're connecting the very sensitive Mic-In of your laptop to a long wire out to the board. Ground the laptop anywhere and you have a perfect, textbook radio antenna.

The UCA202 might be a very good option for you.

http://www.behringer.com/EN/Products/UCA202.aspx

Now we can go into gray magic. Can you plug the laptop into the same power -- exactly the same wall socket -- as the mixer? Can you do that temporarily and see if the buzz goes away?

Can you flip the power socket over when you plug the laptop in? My laptop supply will plug in either direction.

Koz

Koz -
Actually, I wasn't looking for help with a problem - I was pondering whether ljsilver might be having the same symptom I was having. I mentioned running the laptop on battery because it fixed the problem for me.

Now re: your comment. I've been using the laptop to digitize sermons because we're going to start podcasting some time soon. As you pointed out, I can only record in mono on the laptop. I'm using the MON 2 output from the board and a mono connecting cable (1/4" to 1/8"), and copying from (I hear the gasp coming) cassette tape, at double-speed, no less. I put a broken cassette in the copy side of the dual cassette deck and set the deck to double-speed copy. That gets a whole cassette onto the laptop before the battery craps out. The noise (when running on AC) is within the audible range before I apply the speed correction. And no, I'm not overloading the input - there's no clipping unless I crank the MON 2 output. The LED level meter is peaking right around 0 for the MON2 output, so it's not a matter of dropping the output down to the noise floor either. I think the "0 dB" level of the output is actually -20 dBm, but I'll have to check the manual Sunday when I'm back there. Anyway, the noise is present with no signal, so overloading can't be the cause.

The computer's AC adapter has a 3 prong plug, so can't reverse it. It's already plugged into the same line as the board, which is why I strongly suspect the AC adapter as the noise source. I could try a ground-lift cheater on the AC plug too. The AC Hot & Neutral really should be pretty well isolated internally from the DC output of the adapter, but if there's a heatsink grounded to Earth that could be the noise injection point.

I could also try the attenuator cable I made when we had a guy with a camcorder videographing the whole services. He was set up on the other side of the room, so he used a different AC socket for his wall wart. He had only a mic input. I gave him an Effects Send output so it would be post-fader and put it through an isolation transformer at my end and a homemade 30 dB attenuator assembled right inside the connector shell at the camcorder end, with signal common tied to the shield at that end. Camcorder was thus grounded to the board via the shield to minimize electrostatic coupling due to capacitance in the isolation transformer, but no ground loop via the signal common. Worked great. But I'm rambling now. I haven't tried that for the laptop only because I didn't expect to improve a noise problem by reducing the signal level, but having an isolation transformer in the line might be the solution. Will try it.

What I have tried (back on topic) is a common-mode choke (AKA split ferrite bead, the kind used on computer power supply cabling and similar to the ones on all sorts of computer cables). I tried it on the AC adapter cable and on the audio cable. It made no difference. So I don't think the antenna effect is what's happening. What's getting in is apparently not high enough frequency to be attenuated by the choke, or else the impedance of the ground loop is not low enough for the additional impedance of the choke to make much difference. No doubt the AC adapter has its own common-mode choke at the AC end.

Now your other point, about the ring terminal on the laptop's mic jack. Am I shorting out a power supply when I use a mono plug? I note some DC voltage on both Tip and Ring, and never knew why it was on Ring. I figured it was on Tip just to bias condenser mics. The voltages are different, however, and there's no Right input. Seems to me I tried loading it once with a resistor and determined it was current-limited. Maybe I should use a TRS plug and only connect Tip.

Just looked at that UCA202. Wow, that's a lot of features for $30!

It's without many knobs or whistles but for a straight line level in/out sound card it's great value for money. It's important that whatever you use to feed the signal into the UCA-202 has an adjustable output (for example a mixing desk) as the UCA-202 does not have a volume control and it is possible to overload its inputs if fed directly from a CD/cassette deck.

Posting update here because I think it's still potentially useful to ljsilver:

Since we have a wall switch in the sound room that kills power to the whole system, I tried the laptop power adapter plugged into the power strip along with the mixer but with the wall switch turned off. Any ground loop would still be there, but the AC Hot side not powered and thus the AC adapter, mixer etc. not running. No noise. Left the wall switch off so still no signal from the mixer, but moved the Laptop AC adapter to an unswitched outlet: Noise present. Tried a ground-lift cheater (AKA 2-prong to 3-prong adapter) on the AC adapter's line plug. Noise gone! Turned the sound system power back on, still no noise. I can record from the mixer on the laptop using AC power as long as there's no Earth connection to the AC adapter. Turns out there's DC continuity through the AC adapter between Earth ground and the DC output. The noise is generated by the AC adapter, and the noise voltage appears between the Earth ground and the DC output. Open the Earth ground and there's no path for the noise current.