Audio drift in Cool Edit 2.0

Scott Pollak · Posted: Thu Jan 04, 2007, 17:34 (GMT) Post subject: Audio drift in Cool Edit 2.0

Hey folks, over the past few months something weird has been happening when I record any type of audio in CE 2.0. I call it audio 'drift' for lack of a better term. Maybe audio 'creep' would be more accurate. I'm attaching a screenshot of what happens to my file after I record. I've noticed that the whole time I'm recording, the horizontal midline is up above the 0 db level. I've tried to get answers via the Cool Edit/Audition help forums but haven't seen any reference to this type of problem. I've uninstalled and reinstalled the program and it's still doing this. It's not a huge problem, but it's an annoyance. Because of the 'upwards creep', it causes a click at the beginning and end of any audio that I edit, and then I have to do a fade-in and fade-out at the beginning and end of every piece of edited audio to eliminate the click or pop on the track, which IS a real pain.

Anyone who can solve this for me will be my savior. Thanks!

Todd Ellis · Posted: Fri Jan 05, 2007, 02:20 (GMT) Post subject:

Scott - try this:

Effects > Amplitude > Amplify > Center Wave

You may not be having the same problem - but this fixed it for me.

Good Luck

Scott Nilsen · **Voice Talent** Joined: 27 Apr 2006 Posts: 231

Hey Todd- the new photo looks really good.

Todd Ellis · Posted: Fri Jan 05, 2007, 03:03 (GMT) Post subject:

Not to get too far off Scott's topic - but it's amazing what a bottle of Glenfiddich will get you when you live across the street from a photographer.

Scott Pollak · Posted: Fri Jan 05, 2007, 06:11 (GMT) Post subject:

Thanks for the suggestion, Todd. That DOES correct the 'creep' or 'drift' and will help quite a bit in my editing as I work on a gazillion little audio clips for a mammoth e-learning project. I'm just curious as to what might have caused the system to start doing this in the first place though.

Todd Ellis · Posted: Fri Jan 05, 2007, 06:47 (GMT) Post subject:

Thomas C. Gass · **Voice Talent** Joined: 11 May 2006 Posts: 179

Scott

your problem is related to a so called "dc offset" which is the change in input voltage required to produce a zero output voltage when no signal is applied to an amplifier. By some reason your recording shows a dc offset of some -xxx db.

Inaccuracies in the A/D converter or conversion subsystem can produce a slight output offset voltage. As with analog circuits, a processor that provides lots of gain (like a distortion plug-in) can turn a small amount of offset into something major. In either case, offset appears as a signal baseline that doesn't match up with the "true" 0 volt baseline.

Digital technology has also brought about a new type of offset issue that's technically more of a subsonic problem than "genuine" DC offset, but nonetheless causes some of the same negative effects.

In addition to reduced headroom, there are two other major problems associated with DC offset in digitally-based systems.

When transitioning between two pieces of digital audio, one with an offset and one without (or with a different amount of offset), there will be a pop or click at the transition point.

Effects or processes requiring a signal that's symmetrical about ground will not work as effectively. For example, a distortion plug-in that clips positive and negative peaks will clip them unevenly if there's a DC offset. More seriously, a noise gate or "strip silence" function will need a higher (or lower) threshold than normal in order to be higher than not just the noise, but the noise plus the offset value.

There are three main ways to solve DC offset problems with software-based digital audio editing programs.

Most pro-level digital audio editing software includes a DC offset correction function, generally found under a "processing" menu along with functions like change gain, reverse, flip phase, etc. This function analyzes the signal, and adds or subtracts the required amount of correction to make sure that 0 really is 0.

Apply a steep high-pass filter that cuts off everything below 20Hz or so. (Even with a comparatively gentle 12dB/octave filter, a signal at 0.5Hz will still be down more than 60dB). In practice, it's not a bad idea anyway to nuke the subsonic part of the spectrum, as some processing can interact with a signal to produce modulation in the below 20Hz zone. Your speakers can't reproduce signals this low and they just use up bandwidth, so nuke 'em.

Select a 2—10 millisecond or so region at the beginning and end of the file or segment with the offset, and apply a fadein and fadeout. This will create an envelope that starts and ends at 0, respectively. It won't get rid of the DC offset component within the file (so you still have the restricted headroom problem), but at least you won't hear a pop at transitions.

Cheers

Thomas

Scott Pollak · Posted: Sun Jan 07, 2007, 08:52 (GMT) Post subject:

Terrific answers, Thomas! Man, you know your techie stuff. I had been using your third suggestion for a while now to compensate, the 'fade-in' and 'fade-out' at the end of segments, but needless to say, it was time consuming and a band-aid patch for finding the real problem. Since then, I've applied Todd's suggestion of selecting the wave and then using CE 2.0's "Amplify-Center Wave" to correct it. Nonetheless, I want to find and eliminate the cause, which I suspect may be in my Behringer mixer, based on what you said. I'm going to run a few tests and see, but you've completely illuminated my question and I can't thank you enough!

Thomas C. Gass · **Voice Talent** Joined: 11 May 2006 Posts: 179

You're welcome, Scott Wink

I don't know anything about your equipment but as mentioned before, units that change gain in some kind (like a gate, compressor, expander, distortion plug-in) can turn a small amount of offset into something major.

In case you use an external mic pre-amp, try to record a line signal over your mixer. If the recording is o.k. the mixer is most likely not the cause of the dc offset problem. Now make a linear recording via mic and mic pre-amp (whether external unit or built-in unit in the mixer). Then add one pre-amp option after the other (gate, expander, compressor a.s.o.) to be able to find the "bad guy".

Cheers

Thomas[/list]

Jeremy Ryan · **Voice Talent** Joined: 07 Dec 2005 Posts: 17

Hey Scott,

I am a fellow Cool Edit/Adobe Audition user with a suggestion. Rather than having to manually 'Center' each wave, try the following:

> Open Cool Edit
> Options
> Device Properties
> Wave In
> Check the box "adjust to zero-DC when recording"

This will solve the problem within Cool Edit/Audition but you should certainly heed the advice of Thomas C. Gass and attempt to locate the source of your "bad guy". If you haven't already, you may want to consider a power conditioner for your equipment (Monster, Furman, Alesis). Perhaps your problem is the result of 'dirty power'. I ran into that same DC-offset problem at my last studio as a direct result of the power supply. A power conditioner completely solved the problem. Clean power is a beautiful thing... and so is Glenfiddich! Good luck.

Jeremy Ryan
www.jeremyryan.biz

Scott Pollak · Posted: Sun Jan 07, 2007, 19:48 (GMT) Post subject:

Thanks, Jeremy. Just went in and set it and will give it a shot. Many thanks!

Jeremy Ryan · **Voice Talent** Joined: 07 Dec 2005 Posts: 17

You're welcome. Let us know if it works!

Jeremy Ryan
www.jeremyryan.biz

Todd Ellis · Posted: Mon Jan 08, 2007, 02:02 (GMT) Post subject:

Jeremy Ryan · **Voice Talent** Joined: 07 Dec 2005 Posts: 17

Glad to help. I'm just excited to know that there are actually others out there that still use Audition/Cool Edit. I was beginning to feel like the last producer on Syntrillium Island (inside joke for the Old School CEP users in the house). You never see or hear it mentioned anywhere nowadays. I think it's a phenomenal program!

Jeremy Ryan
www.jeremyryan.biz