August 7, 2008 9:43 AM   Subscribe

I seem to be getting a regular ground hum-type noise when I record onto my Macbook.

I have two older analog synths, both plugged into a mixer, which is then hooked up to the macbook.

Clearly, the noise is coming from the one synth: If it isn't plugged into the mixer, I get no noise; if it is, I get noise. However, if I plug headphones into either the synth or into the mixer, I don't get any noise; I can only hear it when I'm listening to the final result through the macbook.

Why is this? This doesn't seem to make much sense to me.

As an aside, it wouldn't be quite so bothersome if Audacity would be able to remove this noise; it's clearly a very regular ground-hum type noise; but if I get a noise profile from before recording, and use that to eliminate the noise, it doesn't do anything noticeable to the resulting audio.

Any ideas? I've tried using different cables, I've tried pluggin the synth directly into the macbook to no avail. It'd be nice to be able to record from the keyboard, but as it stands, I seem to be out of luck.
posted by vernondalhart to Technology (16 answers total)
Are the synth and the mixer plugged into the same extension cord? If yes, try changing that.
posted by preparat at 9:56 AM on August 7, 2008

Or try running everything on batteries, if that is an option.
posted by jeffamaphone at 10:29 AM on August 7, 2008

What's the connection from the mixer to the Mac? What's the connection from synths to mixer (1/4" balanced/unbalanced/RCA?)

I used an iMic USB for a few years with various frustrating results, but recently started using a Behringer FCA202 (FireWire) which has been a huge improvement.
posted by Paid In Full at 11:08 AM on August 7, 2008

Get an inexpensive DI box and lift the ground. You'll also need a short XLR cable to go from the DI box to your mixer.
posted by jdfan at 11:24 AM on August 7, 2008

I believe the phenomenon is called a ground loop. That's all I got.
posted by GleepGlop at 11:44 AM on August 7, 2008

Response by poster: Synth to mixer: Mono 1/4", what I presume to be unbalanced cables.

Mixer to mac: two mono 1/4" outputs with a cable that combines them as a single stereo miniplug input.

It's worth noting that there is an XLR input on the mixer, but no XLR output on the synth. This, I presume, is what the DI box would be for?
posted by vernondalhart at 11:46 AM on August 7, 2008

yeah, it's a ground loop.

I bought a three to two prong adapter from radioshack for $3 and it fixed it, easily.

I don't leave it plugged in when not using it for musical purposes though. I have no idea if it is completely safe for your equipment.
posted by Espoo2 at 12:31 PM on August 7, 2008

Response by poster: Interesting to note... it doesn't stop if I unplug the laptop power... but it does if I unplug the laptop from the external LCD monitor that I use.
posted by vernondalhart at 3:23 PM on August 7, 2008

For whatever it's worth, my housemate's macbook pro emits a lot of noise on the line out (not ground hum; sounds like RFI leakage, lots of sizzly ticking noises) when plugged into our projector via DVI. Mine doesn't. I've heard other stories of weird line-out noise problems that only happen when macbooks are hooked up to a second display. If it's a humming sound, though, that really sounds like a ground loop -- is the monitor plugged into the same outlet as everything else? If not, try that.
posted by runehog at 4:49 PM on August 7, 2008

but it does if I unplug the laptop from the external LCD monitor that I use.

That's because your external LCD monitor is tying the laptop ground to its signal &/or DC ground (and, from there, to mains ground). If you think about it, there's (in most cases) 3 'types' of ground - signal ground, DC ground, and mains ground. These can be tied together in different ways; differences in the actual 'ground-ness' of these can lead to so-called 'ground loops' when two or more are connected between different pieces of equipment. Ground ain't ground ain't ground...

In your case, you have a signal ground between your synth and your laptop, which is fine. But you also have a ground path from the synth's signal ground tied internally to mains ground in some fashion, around your AC mains, through the LCD monitor where mains ground is tied internally to signal and DC ground in some fashion, then via both signal and DC ground back to your laptop through the video cable. Hence, you get slight differences in ground potential (because of the different path lengths and conditions) which results in 'ground hum'.

The solution: only ground at one point. Easier said than done, but DI boxes, signal isolation transformers, and mains isolation transformers (all of which break various ground connections) are the usual solutions.
posted by Pinback at 4:51 PM on August 7, 2008

Response by poster: I also might just do the lazy thing and unplug the monitor when I need to record from the one synth. Sure, it's not exactly ideal, but it would at least work.
posted by vernondalhart at 5:10 PM on August 7, 2008

Response by poster: Also: How exactly do I ensure there is only one ground? They're all plugged into the same power bar which is plugged into an outlet...
posted by vernondalhart at 5:11 PM on August 7, 2008

I'd still suspect the 2 into 1 cable from the mixer to the mac as not helping the situtaion.

At some point the analog signal from the mixer is going to be converted into digital for Audacity, and the miniplug input on the macbook is not great quality.

If you spring for something like the Behringer FCA202 I mentioned (about $80), that should eliminate the ground loop, and will certainly greatly boost the bit rate and sampling frequency.

A DI box might add more noise to the signal chain, and doesn't really have a role between the mixer and the mac. Instead you'd use one between the synth and the mixer, but you have hooked the synth to the mixer directly with no hum already.
posted by Paid In Full at 7:18 AM on August 8, 2008

Response by poster: How does this relate to the fact that if I unplug the mini-DV out on the macbook going to the spare monitor, that the noise stops though? It seems funny to assume that it's a noisy input on the mac when the noise only comes from the one synth, not the other, and unplugging a monitor eliminates the noise.
posted by vernondalhart at 7:21 AM on August 8, 2008

I'm not assumuing the input is noisy, I'm assuming the cable is the problem.

My guess is that the 2 into 1 cable has limited shielding, and that one of the synths and the extra monitor are putting out enough electrical interference to overpower that shielding.
posted by Paid In Full at 11:17 AM on August 8, 2008

In my case, with both my MacBook and my HP whatever laptop, the noise comes when they're running off the AC supply.

Regardless, when I connect either laptop through either my Lexicon Lambda or my Echo Indigo IO (either equivalent to the Behringer unit mentioned above), there's noise on the ground that's not 60hz hum. It's, as runehog mentions, sizzly noise. Running off batteries, I don't get the noise.

My solution was two (Behringer, as it turns out) DI units between my converter and my mixer. Lifting the ground with those entirely eliminates the noise on the ground.

Frankly, even inexpensive DI units shouldn't add appreciable noise to one's signal. In the case of the Behringers, the only thing in the box is a transformer.
posted by jdfan at 1:16 PM on August 13, 2008

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