Can someone explain this? Asymetrical waveforms . . .

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strangeandbouncy

Well-known member
Joined
Aug 8, 2004
Messages
2,112
Location
West Sussex. UK
Hi Guys,


    I often work with material recorded elsewhere. I often encounter waveforms that appear to be louder on one deflection than the other. I have an example attached, but it is not as bad as some I get! They always sound horrible, muddy distorted, esp vocals. I keep pointing it out, but cannot explain to clients how to rectify this. Is it an earthing issue, perhaps, or a faulty compressor, or what? it appears that one deflection id compressed, whilst the other isn't. i have had this from several sources.


    Any help most gratefully appreciated.


    ANdyP
 
It doesn't look like DC offset as waveform seems well centered around the 0 line...
Maybe the signal clipped, recorded at too hot level ? But it doesn't look like that either.. (only a few peak visible on your pic)
 
I have seen waveforms that were extremely lopsided. Most notably horns like a tuba or trombone. I really don't know what that is all about. I do know however that it was not an issue with recording technique or compression or distortion....I used a very clean solid state pre with no compressor.
I'd like to know what causes this too. ???
 
It is clipped(NOT my recording!), but only on the peaks. Definately seems to be not a DC offset as far as I understand .


   As I say, this is not the first time I have had this from more than 2 or 3 sources.


  kindes tregards,


  ANdyP
 
The human voice IS asymmetrical. -So is a violin, an oboe, a clarinet, a trumpet...

This is exactly why units such as the Urei BL-40 (and many others) have a polarity-flipper, to identify which way will result in maximum signal level to overcome issues such as noise versus transmission versus modulation depth in AM broadcast.

If you think of the mechanics of how a violin note is produced -dragging a bow across a string producing a "slip-stick-slip-stick" cycle at the string's resonant frequency- then it makes sense that the waveform will be asymmetrical. Likewise a clarinet reed is asymmetrically limited in travel.... as are lips in a brass mouthpiece.

It's also reputed to be why BL-40's go crazy when you play "don't go breakin' my heart" (by Elton John and Kiki Dee) through one. -Apparently the vocal take was recorded to a single track through a figure-of-eight microphone, with the two singers facing opposing sides of the mic. -Since the vocal is so dominant in the mix, and since the opposing asymmetricalities cause the waveform dominant pole to flip, the little LED in a BL-40 will apparently flash red for one singer and green for the other... as if it can "identify" the singer.

It may very well appear vary significantly with super-close singing versus off-mic placement...

Here's a simple idea to prove or disprove my hypothesis: set up a U87 or similar figure-8 mic, then record a voice identifying front and rear capsules, all in one take, moving to the front and rear, and back again a few times. -The look at the waveform.   -If the waveform flips polar weight-bias and back again (matching the front and back identifications) when absolutely NOTHING else in the chain has changed, then I'd say that's all it is.

As to a correlation between asymmetrical weight bias and 'muddiness', I can;t htink why there should be any, unless you're hearing absolute polarity... which may in fact be possible. -A polarity-flip would prove illuminating if this is the case.

Maybe try it and let me know?

Keith
 
I've encountered this before.  But mine was more extreme and the waveform distorted in shape without truly clipping.  I was able to track down the culprit both times.  Once was a dirty pad switch on my old 001, the other was a tired 421u5.  I suspect it had something to do with the bass roll off.

Do you have any notes on the signal chain for said tracks?
 
Here are a couple of good articles on audio asymmetry 
and the use of all-pass filters:

http://tinyurl.com/c5pphh
http://tinyurl.com/ctnsgp

Cheers.

ZAP
 
Hi Keith

   Thank you for your reply. I hadn't considered absolute polarity. Sadly, I am unable to try easily anything out, since it is not my recording. Without being patronizing, it is probably above the head of the songwriter who did record it, unless I attend the studio at some point. I will post 2 more images of the same vocal phrase sung by the same singer, one (AP vocal) recorded by myself, and the other by my good songwriter friend. You can see the difference! This degree of assymetry is less, even, than i do sometimes encounter, but I cannot lay my hands on any worse examples at the moment. I find it sometimes with DI'd bass and guitars too. As you can see, my (AP vocal) clip is NOT wildly assymentrical. The other thing of note is that a clipped waveform like this still sounds very quiet, and does not respond very well to much compression. Generally, they are hard to use in the mix! If I could find out what might cause this, I just might be able to work out a solution AFTER the fact. NO-one likes to have to try to re-do that great take . . . .

 Miko, Hi. I am not sure what exactly my friend uses, but I suspect that it it a Rode and a silver Focusrite box . . . . I've lent a couple of "proper" mikes for evaluation!


  Kindest regards,

   ANdyP
 
DI's Bass guitars can produce asymmetrical waveforms because -while the string itself may vibrate symmetrically- the pickup will likely have a tendency to see the 'nearer' pole of the wave strongly than the 'further' pole, as a result of diminishing magnetic field strength with distance. -Pickups closer to the strings would be expected to exhibit this characteristic rather more than pickups mounted lower (and thus further away from) the strings.

Keith
 
Hi Keith,


  Absolutely, they are assymetrical, but I often find them wildly so, with hardly any swing on one pole. Not "normal" looking at all. They always sound pony to me, weak and hard to sit, since I find compressors don't like 'em any more than me!

  KIndest regards,


    ANdyP
 
The only problematic situation I know of is in broadcast where dynamic range is very limited. With a human voice, clipping will occur much faster than on symmetric sources, . In regular studio situation I don't think this should be a problem. Remember that on 2" machines asymmetric waveforms would have gone unnoticed :p
 
I actually don't know if the crappy sound correlates to assymetry or not, and i cannot be any more scientific at this stage! I have noticed that when I get crappy recordings to work with, I have often found the waveforms to be far from what I have come to expect after so many years at the coalface.

  Maybe I have touched on something new, or maybe I am trying to find a scapegoat for what I am hearing. I would love to have the time to build a "Phase Rotating Asymmetry Eliminator" link posted by Zapnspark earlier on this topic.

  If there is a simple way of eliminating this before or after recording, I would love to know, since, if it is this assymetry that is causing a problem, it would make my life easier when dealing with poor recordings.


    Just my observations from the coalface . . .




    KIndest regards,


    ANdyP
 
Good reading those links and other comments too! Reminds me of the fact that because FM processors use all pass filtering, a heavily clipped mix will sound awful because the distortion products "move" to different places, and our ears react somewhat to waveforms, not only to spectral dimensions. So, the right thing is to leave clipping to the processor which will then make it happen_after_ the all pass and sound less harsh.

It is quite rare to see completely symmetrical waveforms, but usually they are close enough to go unnoticed.

I hope I don't ridicule myself too much, but I have to tell how I got kicks when hearing the reason for the extreme asymmetry for extremely loudly played brass.
The reason at extreme volumes is no more only lip movement asymmetry, but also the fact that SPL _inside the mouthpiece_ where the diameter is small is so high that the air "runs out of linear pressure displacement" which is, if I remember correctly something like 160dB-180 dB SPL. At this point the sound gets that "metallic" timbre which has not much to do with the material of the instrument :)

Violin string movement on the other hand is fairly symmetrical. The slip-stick mechanism, when (over)simplified, does produce a fairly close sawtooth waveform _in the string_ which does of course mean that sawtooth would come out of the violin body, but it will anyhow be quite symmetrical.

Sorry and thanks,

Jonte

 
Some good examples of very unsymmetrical waveforms can be found on some of the minimally-processed Sheffield recordings, ideally one of the direct-to-disc LPs of Harry James.  However the asymmetry of James' trumpet and one of the trombone solos survives nicely on the CD.

These are, as noted, hell on wheels for potentially clipping things throughout the signal chain and of course present terrible problems for broadcasting if not alleviated.  But oh my goodness do they add to the realism if accurately reproduced!

These sorts of material are likely to allow absolute polarity detection, although what that perception is due to is questionable---is it the effect the signal is having on the system, on your own ears/brain (the so-called Wood* effect), or both?

A little story I may have related elsewhere:

When the UCLA Electronic Music Studio director decided he had to have Dolby B noise reduction on a tight budget (this was circa 1969), he purchased some consumer boxes (branded Advent IIRC?).  They worked o.k. for some program material and horribly for others.  It turned out that the polarity of specific waveforms common to the synthesizers made an enormous difference.  Internal to the boxes were amplitude detection circuits that, to save a few nickels were strictly half-wave.  They and the level control circuitry they served were faked out almost completely if the sawtooth had a slow rise and rapid fall versus the opposite.  You get what you pay for.  And for most heavily processed consumer audio the boxes worked fine.  For amateur recordists I wonder though.

To save the day, in collaboration with a physics grad student who was an amateur musician and hung around the studio, we devised an add-on that would not per se require surgery on the boards, but would merely be tacked on in parallel to make the detector circuits symmetrical.  It worked.  The downside was that the edge connectors for the stock boards were unbelieveably cheap and good for at most a few mating cycles, so the result was a plague of intermittencies.

As so often was the case, the engineers were initially praised and ultimately reviled.  After dealing with the prima donnas and getting completely fed up, I quit to work full-time for the Astronomy Dept., where I remained until 1985.


*no relation that I know of
 
I know that "phase correction" is one of the things that mastering engineers often spend their time doing.  Does this often involve trying to "correct" some of these naturally occurring anomalies? 

For instance, that Harry James reference-would a modern Mastering engineer try and "fix" that?
 

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