Sudden and severe digital distortion/crackle

I work in live audio, don’t record often, but lately I’ve had a couple artists ask me to record and have discovered a frustrating problem…

MacMini 5,2 (2011)
OS X 10.6 (Snow Leopard)
Audacity 2.0.6 (I know, just realized it was out of date tonight.)
Audio interface is a MOTU 828mkII
Audio source is a Yamaha M7, line level analog

I’ve been doing live mono mix-downs and recording them in Audacity as mono single tracks. 44.1KHz on both MOTU and Audacity.
I don’t run any other programs in the background, this is strictly a production machine.
There is no clipping anywhere in the signal path.
The mac is set to never sleep / hibernate / powersave.

Sometimes this set up works just fine. I’ve recorded tracks more than 45 minutes long without incident. Sometimes… not so much. Suddenly, and seemingly without warning, the audio starts to “crackle”, and continues to do so until the end of the track. (File and screen shot of a couple seconds around the start of the crackle attached.) Crackle persists when exported as WAV and played elsewhere.

One time it started about 5 minutes into a recording, tonight it started about 27 minutes in. I don’t recall the circumstances of previous incidents, but I do know that tonight I recorded the soundcheck (45min, crystal clear), saved and closed the project, then started a new project and started the new recording.

I brought the project files home with me and loaded them up on my personal machine (Macmini 7,1; 10.10.3, 2.1.0) and had the same problem in the same place at playback. After scouring the forums for a while, I started experimenting with the “Audio to Buffer” setting, trying it from 0 all the way up to 150 in increments of 10 with no positive effect on the playback. (0 made it way worse, everything else sounded pretty much the same.)

Being so intermittent has made it hard for me to troubleshoot, especially since I can’t stop, playback and listen during a live event. Any fix suggestions welcome.

I realize this is probably a shot in the dark, but is there a chance I could salvage the recording? It was a fantastic concert and I would love to be able to give the artists something useful.
Screen Shot 2015-06-12 at 2.18.18 AM.png

You can’t repair crackling once recorded, but setting Audio to Buffer to 10 or 20 ms should help avoid crackly recordings in the first place.

You should definitely use Audacity 2.1.0. It has a fix for an uninitialised audio buffer. Without the fix you may receive this type of crackling when playing or recording unless you reboot before every use of Audacity.

In case you have not done this yet, make sure MOTU playback and recording is set to 44100 Hz 1 channel in /Applications/Utilities/Audio MIDI Setup.


I believe the buffer settings affect recording. You can do an experiment where you try to make it worse. Change the buffer settings and set the Mini to record anything continuous like a radio. You only have one sample point. If you get multiple points you may discover much worse performance is you go one way and much better if you go the other.

I assume you have lots of memory and drivespace? When was the last time you did a Verify Disk and Repair Permissions?

It is a little disconcerting you have blank entries in the Device Toolbar. If you have nothing plugged in, you should see “Built-In Audio” for both input and output.

What does Apple > System Preferences > Sound say for input and output. Screen caps (attached)?

Screen Shot 2015-06-12 at 7.11.13 AM.png

Screen Shot 2015-06-12 at 7.14.24 AM.png

They affect both playback and recording.

I agree that is odd, but that is the Yosemite machine, not the one that has the recording issue.

Does connecting the AudioEngine 2 USB speakers cause the recording device to disappear from Device Toolbar?


Are you sure that Mac’s setup is OK?

From memory, a 2011 Mini 5,2 needs Lion as minimum system version. It’s a Sandy Bridge i5, I believe. I don’t think Snow Leopard knows how to address Sandy Bridge CPU’s.

2011 Mac Mini came with Lion but perhaps blueeyesdesigns downgraded to 10.6.6 out of dislike of Lion?

It appears from that this would largely work since the original Sandy Bridge supported 10.6.6.


That’s what I was afraid of. I was hoping that it might have been some sort of data corruption that I could fix or snip and recover some of the audio. Alas.

Noted. I will do that next time I get a chance.

The screen cap is from my home machine, so no interface, just a pair of speakers and the onboard speaker. Sorry for the confusion. Everything shows up as it should on the production machine.

I think I typo’d that. Sorry. Late night and all. I didn’t build that machine, but I don’t recall my predecessor having rolled it back for any particular reason. I mostly just included it so folks would know that it was several versions behind and that I wasn’t trying to live on the bleeding edge.