Topic: What's up with this waveform?? (pic inside)

[travelinbeat]

Made a low-pro stack-tape tonight, stood about 4ft from the BLASTING speakers and came home with a tape that sounds pretty nice but the waveform is all screwy.  Any idea what caused this?  Is it a problem?  Should I change something about the technique to avoid a repeat in waveform?

EDIT:  It was CA-14 cards > CA ST-9100 > R09-HR at 24/96

TIA!

[willndmb]

I had one similar to that I posted last week but mine was high on the top.
http://taperssection.com/index.php?topic=160298.0

[DigiGal]

Understanding DC offset <<-- Source Link
DC(Direct or continuous current) Offset is good for changing the baseline of an audio file (e.g. wav) to compensate for electrical mismatches between your audio card and the input device, thus obtaining a louder amplitude after normalization. If DC off set exists, getting more loudness in the mix maybe a problem. 

DC offset occurs when hardware, such as a sound card, adds DC current to a recorded audio signal. This current results in a recorded waveform that is not centered around the baseline (-infinity). In some cases, when DC-Offset is present, glitches and other unexpected results can occur when sound effects are applied to the audio file. You can compensate for this DC offset by adding a constant value to the samples in the sound file.


In the example above, the red line represents the baseline. The lower waveform exhibits DC offset; note that the waveform is centered approximately 2 dB above the baseline.

Technical Explanation
DC offset is an offsetting of a signal from zero. The term originated in electronics, where it refers to a direct current voltage, but the concept has been extended to any representation of a waveform. DC offset is the mean amplitude of the waveform; if the mean amplitude is zero, there is no DC offset.

DC offset is usually undesirable. For example, in audio processing, a sound that has DC offset will not be at its loudest possible volume when normalized (because the offset consumes headroom), and this problem can possibly extend to the mix as a whole, since a sound with DC offset and a sound without DC offset will have DC offset when mixed. It may also cause other artifacts depending on what is being done with the signal.

DC offset can be reduced in real-time by a one-pole one-zero high-pass filter. When one already has the entire waveform, subtracting the mean amplitude from each sample will remove the offset. Often, very low frequencies are called "slowly changing DC". While not technically accurate, a highpass filter can remove such a "changing offset" better because its cutoff does not extend to as low a bandwidth as the above method.

When describing a periodic function in the frequency domain, the term DC coefficient or DC component refers to the mean value of the waveform (possibly scaled according to the norm of the corresponding basis function of the frequency analysis filter bank). The name comes from the middle 20th century design of electrical line codes for use with transmission channels unable to transmit a DC voltage or current. In such usage, this coefficient represents the useless DC, whilst the coefficients representing various other frequencies are analogous to superimposed AC voltages or currents.

A waveform with a zero DC component is known as a DC-balanced waveform. DC-balanced waveforms are useful in communications systems, since they can be used on AC-coupled electrical connections to avoid voltage imbalance problems between connected systems or components. For this reason, most line codes are designed to produce DC-balanced waveforms. The most common classes of DC balanced line codes are constant-weight codes and paired-disparity codes.

[willndmb]

great post digi

what music where you recording beat??
mine was clearly from the brass instruments (at least I believe)

[DigiGal]

great post digi

what music where you recording beat??
mine was clearly from the brass instruments (at least I believe)

Not my recording or info but thought it good to copy it here in case the source disappears, found it in a search for DC Offset, original article is linked above (Written by Edward Vinatea).   

[travelinbeat]

I was recording loud punk / hardcore music (straight forward, guitars, bass, drums, vox).   

That is a ton of good info but I'm a very simple person and it's kind of dense (even after four reads!)  .  I see that the article mentions a way to fix it by using a high pass filter-- can anyone translate the remedy? 

[dnsacks]

Similar waveforms developed when the batteries on my oade m148 would no longer hold a complete charge -- could you have been using defective batteries in your preamp?

[DigiGal]

I've never encountered the problem myself but Audiofile Engineering's Wave Editor has a remove DC Offset feature.  Seems other editors would offer the same functionality.

edit to add:
youtube "Wave Editor TIP: Using Remove DC Offset as you might not expect"

[stevetoney]

I've seen that waveform oftentimes when recording loud horns.  Sound is good, but the waveform isn't symmetric.

I'm reading that definition of DC offset as being that the waveform isn't centered.  These waveforms seem to be centered, but they're just not symmetrical. 

Since you say it sounds good, I think I'd chalk it up as being 4 feet into the PA blast zone, but not sure about that.

[ashevillain]

These waveforms seem to be centered, but they're just not symmetrical. 

I too am curious about what causes the asymmetrical waveforms. It does appear to be at least tangentially related to brickwalling but I'm pretty sure I've also had brickwalling with perfectly symmetrical waveforms.

I've got one right now with the asymmetrical waveforms where most of the asymmetry occurs with loud percussion/drums...it's only present on the SBD signal though...not on the AUD signal.

[DigiGal]

These waveforms seem to be centered, but they're just not symmetrical. 

They do appear centered from [travelinbeat]'s screenshot, what do they look like if zoomed in?  If I magnify his screen shot they don't look centered but a zoom from within the editor would show it more clearly.

[flipp]

Samplitude and iirc Audacity both have a way to remove DC offset.

To be a contrarian I don't think this is an example of DC offset since the base line is centered. In the OPs pic during the quieter parts the entire wave is centered and when music is playing that baseline doesn't move even though the wave is now no longer symmetrical about that line. But that is also the case in the pic in DigiGal's post. In the upper picture, which Edward Vinatea indicates does NOT show DC offset, there is the same non-symmetrical characteristic shown and in the lower pic the entire wave form is now shifted above the baseline. Applying his fix to the DC offset will not make the waveform symmetrical above and below the baseline.

Try applying the DC offset fix and I doubt any difference will be apparent in the waveform - even when highly zoomed, there will still be a significant amount of nonsymmetry.

Just a guess but what I think the OP's capture is showing, and willdmb's capture in the thread he linked, is a bias error/misadjustment - most likely in the PA amp but definitely not in the recorder's equipment chain.

The only actual experience I've had with bias misadjustment is with an old Heil Omega power amp I used to power a PA. Used to split the signal after the amp and before the speaker cabs and ran one side to the speakers and the other side to a reel-to-reel deck. Had a strange waveform when playing the tape back. An army trained service tech at a local music store who is also a very good bass player looked over the amp, saw some components had been replaced and asked if he could put the amp on his service bench. Came back in about five minutes and said the bias hadn't been adjusted after the components were replaced. He made a few tweaks and we made a test recording later with the usual setup. No more strange waveform.

The above is pure speculation on my part. Perhaps one of the local electronic wizzes can offer some knowlegeable input (and tell me how full of shit I am).

< I see three new posts appeared while I was typing. I still think someone like Jon, Chris Church or one o fthe posters with an EE dgree could shed more light on the real causes of the assymetrical waveform. >

[bombdiggity]

This may or may not be DC offset. 

Audition or Cool Edit allow you to check DC offset in the wave form statistics panel.  If the reading there isn't zero then go to amplitude and there is a preset to "center wave".

Digital sources rarely have DC offset.  Even when present it is usually an infinitesimal percentage. Adjusting it will change the wav but not much. 

I have gotten things appearing a little like this even on unamplified shows - usually from brass.  Certain instruments/mixes seem to ride mostly above or below infinity. Brass seems to be above in what I saw of my own. 

[travelinbeat]

The plot thickens!   What an interesting conversation we have going, thanks to everyone!  I've captured a screenshot of a magnified portion of the waveform along with a muted section I inserted for reference.  I've also converted 1 song to v0 mp3 and posted it to Soundcloud so that you could take a listen: https://soundcloud.com/travelinbeat/sample-waveform-song

[Church-Audio]

not sure what caused this. it could be a weak battery. That is my only guess here. I recommend 9.6v maha batteries for my preamps. Because you can charge them every time you go out and have a good voltage. My preamp will run fine down to 7.6 volts. Thats a pretty dead 9v.