Strange Looking Wave Form

higher amplitude on bottom than top

OK so here is a picture of what im talking about:



This is a picture i took from soundforge. Ignore the cursor in the middle of the wave. What i keep noticing about things that i record (with any mic) is that the wave is bigger in the down direction than it is in the up direction. Does anyone know why this happens, or how to correct it? Sorry if someone asked this before.

are you using a USB unit to record through - if so GOOGLE for DC offset - and/or does your unit have inbuilt effects - both can give the same result -

DC offset usually pushes waveform above the centre line and yours is shown below - if it is DC offset that is causing the problem then that track will never sit right in a mix untill it is removed -

i have same problem when recording through my Roland SP606 -

download trial version of GOLDWAVE, it has a DC offset remover - before putting track through remover you have to delete the .NPK file accociated with that track - if you dont N will always show the same waveform for track after offset has been removed - kiling original .NPK file forces N to redraw the new waveform -

Dr J

I’m pretty sure Soundforge has DC offset removal. Did you try it?

D

This wave is normal. Do not apply DC offset correction. It’s not at all unusual for a wave to look a little “lopsided.” As long as the baseline (where the wave crosses zero) is at -inf dB, everything’s okay. DC offset correction should only be used if a voltage is being introduced by your equipment. Zoom in close at the wave. If the very beginning starts at -inf dB, everything’s okay.

Check out this screenshot. These are two instances of the same wave file. The upper one has a DC offset making the baseline fall at -18dB instaed of -inf dB.

Though waves usually look pretty symmetrical, there is no reason is physics for waves to look symmetrical every time. Automatic DC offset correction, like that found in Soundforge, works by making the average value for the wave equal -infdB. And if some part of your equipment is applying a DC voltage to your input, this is better than all your waves having an inherant offset. But you would need to apply the same correction to every wave you record for everything to match up. If yopu apply automatic correction to a wave that did not have an offset introduced by your equipment, you actually wind up applying an offset to a normal wave just to make it look symmetrical.

As far as I can see at the zoom level of the screenshot you posted, the baseline of your wave looks like it falls on -infdB. You do not need to apply any correction.

That was a really good short explanation, Captain. Put that on the wiki. :)

physics - i have seen N draw a waveform that went back on itself forming a loop in the lower part of the wave ? -

comparing waveforms from the three units that i have used for recording on this PC -
M Audio Delta 4/10 pci soundcard (now uninstalled) symetric waveform -
Audigy NX2 USB symetric waveform -
Roland SP606 asymetric waveform -

tracks created with Delta and Audigy fitted correctly in the final mix - tracks sampled and then recorded into N through tha Roland USB did not -

recording a soft synth in N, showed symetric waveform - sampled synth to Roland and re-recorded it back to N (no effects used) showed asymetriic waveform - did this to try and isolate problem with Roland generated track that was causing problems in another project -

do not use SoundForge - i use Magix music editor 2 - ran problem track in other project through offset remover (before low in mix, after correct in mix) - offset removal start setting used was -95db - did nothing else to that track - also have had no adverse effects by running offset remover on all, tracks in project (79 in total) - did comparison betweem Magis and Goldwave on different Roland tracks and on non Roland tracks -

i came to the DC offset conclusion having read dozens of pages on that subject on the net, thanks to the information made availabile it worked for me - have yet to find that DC offset is applied to tracks not suffering from it though - maybe just lucky so far with programms used -

All praise to the Captain for his visual reply and the time and effort that he has spent in geting the picture in to this board -
Dr J -

Yep. That’s a good one for the wiki. However, one should be aware that poorly written plugin effects or even audio drivers can introduce offset. Not just hardware…

D

Just to try and clarify things a little bit:
DC offset is caused by a DC current (voltage) entering the signal path at some point. Immagine your original signal was a sine wave with peaks at +1 volt and -1 volt. If faulty equipment or software for some reason introduced a +1 volt current, then the resulting signal would be a sine wave with peaks at +2 volts and 0 volts - i.e., a DC offset of +1 volt. This is easy to correct manually in Soundforge if you know the ammount of offset correction to apply.

Now, what would cause a wave to not be symmetrical? Nature and physics. Imagine a slow motion movie of hitting a drumhead with a stick. The impact of the stick causes the drumhead to move down, tension causes it to spring back with enough force to overshoot it’s rest position. But since it’s lost energy to the air to make sound (and to friction) the upward movement (return stroke) is somewhat less than the downward movement. The resulting wave is slightly assymetrical.

Now put your hand on the drumhead and hit the head with your stick. The weight and softness of your hand will draw even more energy from the drumhead causing the return stroke to be very short compared to the impact stroke, resulting in a very assymetrical wave.

If you think about the ways sound enters the environment, it’s easy to see that assymetrical waves are normal and natural.

FYI: endorphin is one plug-in that I have seen personally to add a DC offset to the final mix, when used in the master insert.

Also, the Marshall MXL mics I have will flatten the tops of waves yet capture the full wave on the bottom. The results looks like clipping on the top only. When zoomed out wave looks like there is DC offset, but zooming in reveals the flattened top. Fortunately, it doesn’t sound like clipping and is some cases sounds good, like a built in hard limiter. While I consider this to be a defect in the mics (two of them do exactly the same thing) I almost want two more so I can use them as tom mics.

phoo, I have a couple Chinese made mics which are probably the same as your MXL’s with a different brand. They do EXACTLY the same thing.

D