Getting strange distortions while recording

[lubi]

Hi everybody,

I'm not a great expert in this, but i wondered if somebody could help me out.
I'm using Audacity to record myself singing. I use a condensator microphone which is plugged into a CEntrance MicPort Pro, which then goes into my MacBook via USB.
I've been successfully using this setup for half a year now. Then some days ago, I keep getting distortions, when I record new tracks. Oddly, recording the first track in the Audacity file is never a problem, but as soon as I start recording a second track parallel to an existing one (I'm overdubbing), I get really strange distortions some seconds into the recording, which sound almost like clipping. Also, sometimes, I get a series of pops in the recording.
The microphone is obviously working fine, as the first track always ends up perfect. I also changed the USB port, but that didn't help. When I tried recording with the crappy internal mic built into the MacBook, I got the same pops, but only occasionally. Today, for example, it works alright.

Strange thing is, I didn't change any settings or hardware and the next day it doesn't work any more.

What could be the source of this?

Thanks already for your answers.

Lubi

[kozikowski]

It could be something you don't normally associate with live recording. Did you recently install Skype? Did you recently decide to record internet shows and install SoundFlower or other internet program or software?

If one of those events happened, you may be recording yourself twice. The first recording is just you. The second recording is you plus the you from the first track -- double -- usually enough to throw you into serious distortion.

When you watch the red recording meters, do they go very high up during the second show? Do they ever smack the right edge -- all the way up?

This is a normal recording (your window sizes may be different).

http://kozco.com/tech/audacity/Audacity1_record.jpg

Koz

[kozikowski]

<<<CEntrance MicPort Pro>>>

That's a cousin to the Shure X2U that I use. It's rough to mess that up. They're pretty simple.

Koz

[lubi]

As far as I know, I didn't install any programs like that. Also, the meters look pretty normal when I record. Instead, they vibrate when I play back the distortion part, but they never go all the way up.
Another strange thing I observed today is that this always starts at exactly 6 seconds into the recorded track.
I tried reinstalling Audacity, but that didn't work either.

As the meters look as they should when I record, but don't when I listen to my recording, I reckon it must have something to do with the software. Maybe some settings are messed up? I tried going up with the latency, but that didn't change anything.

I attached a part of a recording. I recorded this track secondly, and the file starts at 5 sec into the original recording. There's a screenshot of the same file how it looks in Audacity.

Hope that helps clarifying things.

Thanks for your answers.

Lubi

[kozikowski]

Don't mess with the latency delays. That causes problems if you have a sick machine.

Audacity > About Audacity and give me the version number, plz.
Audacity > Preferences > Recording. Which selections do you have checked?

This recording had tone on it, right? Can you make another short recording? With your clear single voice, count "One, Two, Three" and leave empty time for "Four, Five, Six." Repeat that a couple of times.

Go into overdub and speak "Four, Five, Six" in the right places.

If this was a normal recording, we could play the show and it would be you counting to six.

Koz

[kozikowski]

I'm hoping Billw is going to jump in here. I'm just data gathering. I don't have a clear idea (yet) what could cause this. I know how to mess this up on a Windows machine, but you're on a Mac.

Apple > System Preferences > Hardware > Sound > Input. Read what you have selected.

Apple > System Preferences > Hardware > Sound > Output. Read that.

Koz

[billw58]

I'm hoping Billw is going to jump in here. I'm just data gathering. I don't have a clear idea (yet) what could cause this. I know how to mess this up on a Windows machine, but you're on a Mac.
Apple > System Preferences > Hardware > Sound > Input. Read what you have selected.
Apple > System Preferences > Hardware > Sound > Output. Read that.

Thanks for the invite, Koz

And thanks for doing the data gathering. This one has me stumped too.

@Lubi:
I notice you are recording at 96000/24-bit. Try changing the project sample rate and format to 44100/16-bit. http://manual.audacityteam.org/index.ph ... references

When all else fails, reset the preferences ...

Go to <your home>/Library/Application Support/audacity/
Inside the "audacity" folder you will find the file "audacity.cfg"
Option-drag a copy of that file to your desktop (as a backup and to look at later if this happens to fix the problem)
Right-click on the audacity.cfg file and from the pop-out menu choose "Open With", then from the fly-out menu choose "TextEdit"
[If TextEdit is not in the list, choose "Other..." from the fly-out menu and choose TextEdit from the list in the dialog the appears.]
In TextEdit, delete everything except the line "NewPrefsInitialized=1".
Save your changes - make sure that TextEdit does not try to stick ".txt" on the end of the file name - make sure you save it as "audacity.cfg"

Now start Audacity and try again.

If none of this helps, it would help to hear a sample. Could you select 5 seconds of a bad recording (as shown in your shot - without the original track but only the overdubbed track), and export it as a 16-bit 44100 AIF. That should be short enough that you could upload the sample to the forum.

-- Bill

[lubi]

Wow, really a big thank you for your extensive answers. So, here's what I did.

So when I reinstalled Audacity I downloaded the newest version, hoping that would solve the problem. So my version is the beta of 1.3.12. I think the preferences must have been set on default, when I installed it, but I'll try resetting them anyway later this day, as billw suggested.

My recording preferences in Audacity are as follows:
Overdub is checked, hard/software playthrough aren't. Audio to buffer is 100 ms, latency correction -340 ms. Sound Activated Recording not checked, Activation Level is on -36 dB.
I don't hear myself played back over the computer, I can just plug my headphones into the MicPort and the sound from the mic goes directly into the phones, hardwired.

I also tried recording in 44.100 Hz and 16-bit, but without any effect. Also, I really would like to record in 96 kHz, because that's what comes out of the MicPort (96 kHz at 24-bit). At least it says that at their website.

Ok, so next thing: my system preferences.
They're German, so I'm just gonna try and translate it.
On Output, the MicPort is selected as output device. Settings for that aren't available, because "the selected device has no output control". Well, of course it has, but that's just a knob on the device itself.
On Input, the MicPort is selected, too. And same thing here, "the selected device has no input control". Again, a knob for that is on the thing itself.

And if you'd like to know, my OS is Mac OS X 10.6.4.

So, then, I tried the 1,2,3/4,5,6 routine. Same thing again. Starting at exactly 6 sec to about 7.5 seconds, it's there again, of course only in the overdubbed track.
I also tried generating silence as the first track, then overdubbing some tone. Same thing.

Once this morning I got those pops again (they're really rare now), so I thought, I would attach a short sample of those. Also, there's a longer .wav (.aiff is not allowed on this forum) with a distortion sung over a generated silence track. For that last one I changed the settings to 44.100/16-bit, so the file wouldn't get that big.

Thank you again for the effort you put in this

Lubi

[billw58]

I think the preferences must have been set on default, when I installed it, but I'll try resetting them anyway later this day, as billw suggested.

Preferences are not reset to default when you re-install Audacity, so editing the audacity.cfg file is the surest way of resetting them.

The audio files were helpful. It is not clipping, and is not, apparently, the first track leaking into the second. Doing Analyze > Plot Spectrum it looks like broadband noise is being added.

Let's eliminate the MicPort as the culprit. Go to system preferences and set the input and output to "Built-in Audio". Then select those same inputs and outputs in Audacity preferences, and select the MacBook's mic as the input. Plug your headphones into the Mac's headphone jack. Now try your overdubbing experiment.

-- Bill

[lubi]

Ok, so it is definitely the MicPort that is causing the trouble. I conducted the following experiment:
Setting the MicPort as both the input and output device caused distortion. With this setup the headphones are plugged into the MicPort.
Then I only changed the output device to built-in output, plugged the headphones into the Macbook' jack. And voila, no distortions at all. So, what is causing the trouble, I guess, is that the playback sent to the MicPort is somehow interfering with what is recorded.

The only thing I did to the MicPort, though, as far as I remember, is unplugging it once because I had to move the mic.

Damn, I could've found that out, when I tried recording with the internal mic. But somehow I probably only imagined hearing pops in the crappy recording.

Well, gotta look up the guarantee of that MicPort. My temporary solution now is just recording 7 secs of nothing and then starting with what I actually want to record.

Sorry for all that trouble, I could've eliminated that error source right at the beginning.

But thanks anyway for your support

Lubi