Strange Clipping

[Zave]

I run PTF on my iMac G4 800 15" through a Griffin iMic.

Today I was trying to record two channels in stereo simultaneously from my bass amp: one direct from the DI out, and one post EQ from the preamp out.

I tried various configurations of which signal went to which side through which cables/adapters, but no matter how I did it, one channel or the other always got this stange clipping on one side of the waveform, but not the other. It was quite clearly visible that there was a "line" of sorts that signal would not go beyond (sometimes on top, sometimes on bottom).

I tried testing it with an Audio-Technica AT-822 stereo mic, and it seemed to work fine, and both outputs on the amp worked fine with other gear.

Any ideas as to what might be causing this and/or how I might go about fixing it?

Thanks
Zave

[John Link]

You may be sending too hot a signal to the iMic, which doesn't have any way to reduce the level of the signal it receives. But it does seem strange that the clipping is only on one side of the zero line.

John Link

[Zave]

I had thought about that, but both signals were about the same level, and it only clipped one.
D'you suppose it's at all possible that the problem was with the adapters and/or stereo-dual mono y-cable I was using?
Thanks
Zave

[M.Brane]

Looks like a phase issue to me, since your pulling two direct signals from different points on the same circuit.

[Zave]

Unless I'm misunderstanding what you mean by "phase issue," I don't think so, because I'm recording the two outputs to two separate tracks. They're never mixed.
Any other possibilities?

Zave

[poncival]

You haven't mentioned whether or not you actually hear the distortion or whether you just saw it. It's quite possible that is the shape of the waveform that your bass amp is generating. I have seen this type of distortion several times- especially when using Protools in conjunction with classic gear like Pultec EQ's and Neve mic pre's. Sometimes one side of the waveform seems to clip more than the other- and "remove DC offset" doesn't seem to make any difference, sometimes it may be that the signal source itself is hotter on the "plus" side or the "minus" side and it causes this kind of clipping. I always say if it sounds the way we want it to sound, we can let it slide. If you don't have that same problem with other signal sources such as your AT mic, then the culprit may be the ouput from the bass amp. You might find that routing that "direct" output through an additional direct box may even things out and allow for a cleaner sound. If you are using several adapters and Y cables, etc. you should minimize that no matter what. Every connection is a weak link in your signal chain, so the fewer the better.

[poncival]

PS every major record producer I have ever worked with has tracked a DI bass track our of the bass itself (or out of the earliest possible point in the bass head: "pre EQ") and then tracked a microphone track in addition to that. Two DI's, pre EQ and post or not, probably isn't the most desirable blend. Maybe eliminating the direct out that was causing the weird clipping and replacing it with a mic track would eliminate your problem and make for a fatter sounding bass track. Don't forget to "visually" phase-align the two tracks once you're done so that they are as "in phase" as possible, since the mic track WILL be a certain number of samples later than the DI track. Don't forget to "select all" on the mic track you are moving so that you don't need to align all your edits separately. You will notice a difference in relative EQ for every sample you nudge the Mic track relative to the DI track which remains stationary, and you can come up with some really killer bass tones this way. Sometimes having them a little out of phase ends up with a more controlled sound than having them exactly in phase and then having to maybe EQ out some bottom end (thus messing with the phase). Speaking of which , it IS possible to end up with some weird phasing/ground loop type of stuff when running out of two separate outputs from the same amplifier. The way you had those hooked up, even though they were going onto two separate tracks, the actual inputs could have been trying to defeat each other somehow since they shared a common ground and basically had the same signal going through them, even though one was EQ'd and therefore a little out of phase with the other. I don't know the math behind it, I just know that when weird stuff like that happens in a situation like the one you described, usually it doesn't happen to either track if you disconnect the other- and it's always instrument amplifiers that you're trying to do stuff like that with. Anyway, good luck.

[Zave]

Poncival-
Thanks for your replies.
First, the clipping is definitely (very) audible (and unpleasant).
Secondly - the DI track I took was pretty much straight out - pre EQ, pre compressor, basically right out of the instrument. And the clipping wasn't consistantly on the DI or the post EQ signal; it happened to both at different times (never simultaneously). I'm pretty sure the problem had nothing to do with the amp - both outputs functioned just fine (no clipping) in conjunction with other gear.
I wasn't really having a problem with the sound itself (other than the clipping). In all cases, the tone of whichever track came through properly was great.
Also, what exactly does "Remove DC Offset" do? I think I have the rough idea, but could use a good explanation.
Thanks again,
Zave