Stereo signal techie question

**Chris Fitzgerald** · #1 05-26-2010, 12:45 PM

Lately I've been using a small mixing board (Yamaha MG 10/2) in between my computer and monitor speakers. In addition to volume and EQ control, it allows me to also run a keyboard, module, and CD player through the same monitor speakers without swapping cables every 2 seconds. So far, so good.

The issue is this: when running music from iTunes, I've been assuming the I should run the left side of the signal to one channel and the right to another, and to pan each fully to the side they represent on the mixer. Sometimes this sounds fine. Other times, the vocals on the recordings sound completely out of phase and ambient, and when I change the pan on the two stereo channels to the center, the vocals sound solid again, but I seem to be getting a mono signal.

What could be going on here? I know some of these recordings weren't made with these phase-y ambient "vocal artifacts panned to the far outsides with nothing in the middle" techniques, but don't understand why some recordings do this and others don't. Interestingly, it seems to happen the most with downloaded recordings rather than stuff I ripped from CD.

EDIT: I just found a "greatest hits" recording that I downloaded this morning from emusic that contains a track that I had previously ripped from an actual CD. Listening to the tracks back to back proves that the pan issues in the emusic version are not a figment of my imagination. Nobody with ears would record vocals that way, and in fact they didn't. WTH?

johnz · #2 05-26-2010, 05:09 PM

Interesting.

I think I'm missing one piece here.

What I'm hearing is that, given one of those "out of phasey" recordings: If you pan the Left channel all the way left and the right channel all the way right, it sounds, well, out of phase. And, if you pan both channels to the middle, it sounds OK, but mono (as you'd expect).

So, what happens when you pan the channels to their opposite positions - i.e., pan the Left channel to the Right and the Right channel to the Left? Is it still out of phase or does it sound right?

**Chris Fitzgerald** · #3 05-26-2010, 05:29 PM

Quote:

Originally Posted by johnz

So, what happens when you pan the channels to their opposite positions - i.e., pan the Left channel to the Right and the Right channel to the Left? Is it still out of phase or does it sound right?

Still out of phase, or at least the vocals are reverberating against the outside of the stereo field without a definite center as in the original (ripped) version. The only way to make the downloaded version sound sort of normal is to set the pan controls on the mixer to 9:00 and 3:00, which brings the center of the vocal back into the center.

onlyclave · #4 05-26-2010, 05:36 PM

It sounds like the absolute polarity of the original signal is wrong and when you hard pan the channels L and R what you are hearing is the out-of-polarity information. That washy, ambient sound is what creates the air and spaciousness of a signal, and if you blend that signal back to the center it sounds very dry and "hard".

The problem is most likely an artifact of lossy compression algorithms.

johnz · #5 05-26-2010, 05:58 PM

I figured as much.

I took a quick look at the manual. That's a cute little mixer!

I suspect that onlyclave is correct. A quick search does reveal that iTunes uses Advanced Audio Coding (AAC), which is indeed a loosy compression algorithm.

http://en.wikipedia.org/wiki/Advanced_Audio_Coding

**Chris Fitzgerald** · #6 05-26-2010, 06:07 PM

Here's the funny thing: both of the versions I'm comparing are on iTunes, both using the same compression scheme (MPEG-1, Layer 3). The one that sounds normal is actually recorded at a lower resolution (192 kpbs) than the one that sounds screwy (256 kpbs). The screwy one was encoded with LAME 3.98, while the good sounding one was encoded with iTunes 9.1. The only other difference is that the good sounding one is marked "Stereo" and the screwy one "Joint Stereo". I honestly don't know what to make of it, but it's weird.

Uncletoad · #7 05-26-2010, 07:47 PM

Quote:

Originally Posted by Chris Fitzgerald

The only other difference is that the good sounding one is marked "Stereo" and the screwy one "Joint Stereo".

Pot smoking engineers, whaddya expect from Joint Stereo. I'd go for the Scotch Stereo mix myself.

**Chris Fitzgerald** · #8 05-26-2010, 10:53 PM

Problem is, you don't know what they're smokin'/drinkin' until you've already bought the file and clicked "get info".

Oh well. I guess I'll just adjust the pan on those cuts to where it sounds as good as it can and remind myself I've learned a lesson about how downloaded music isn't fully the equal of what it's replacing just yet.