0 Replies Latest reply on Dec 28, 2010 2:01 PM by the_wine_snob

    Good Audio Develops Pops, When Cut and Butted?

    the_wine_snob Level 9

      Occasionally, one will Cut muxed Clips (both Audio & Video combined into one file), move things around a bit, and then end up with “pops,” “ticks” or “clicks,” right at the junction of those Clips.

       

      One possible problem could be the exact spot of the Cuts.

       

      If one does not Cut right at a "zero-crossing," then the wave will be "broken," and a pop will result. Sometimes, if one is close to a zero-crossing, the pop will be fairly small, and might not be heard at all. However, and especially where one has two Clips, where the zero-crossings were missed and the resultant Cut was at points of high amplitude,  the pop will be greatly pronounced. So loud in fact, that it could blow out speakers, if the monitoring volume is turned up.

       

      One most often encounters these pops, when using a Video-editing program, like PrE, PrPro, Vegas, FinalCut Pro, as most good Audio-editing programs will allow one to limit all Cuts to zero-crossings. Most Video-editors do not have that capability.

       

      Now, you might ask what are "zero-crossings?" If you look at an Audio wave, you will see that it crosses the center line  The number of crossings in a set Duration will be based on the Frequency of the Audio signal. Those zero-crossings are the point of silence. If one has cut the wave at its high-point, only part of that signal will be included at that cut, and it will be fairly loud. That will create a pop. If one butts up to another Clip, and it was cut at a low-point, the pop will be much more pronounced.

       

      There are a couple of ways to address the non-zero-crossings issue:

       

      The application of a Cross-Dissolve will often "hide" the lack of zero-crossings, and mask the pop, or at least greatly diminish it.

       

      As most NLE's (Non Linear Editors for Video) are Frame-Accurate, the Audio is pretty much ignored. Some, like PrPro, will allow one to work on Audio, displayed in Audio Units, though the Video can still ONLY be Cut at the Frame Level. Still, one can independently adjust the Audio portion of the Clip at the Audio Units level, and can just shorten that Audio to get to a zero-crossing. Almost ALL Audio-editing programs, from Adobe Audition, down to the free Audacity, will allow one to edit to Audio Units, so it's much easier to find and Cut on the zero-crossings. Audition even has a setting to ONLY edit on zero-crossings. By editing the Audio portions (files) in such a program will almost always clear up the pops. Audition also has some neat Effects to eliminate almost all transients (basically what one is building, when the Cuts do not fall on zero-crossings), such as pops from the mic, or near the mic. These Effects will do a great job of maintaining the non-Transient frequencies, so that one keeps the majority of the Audio, and ONLY addresses the Transients. Note: PrE does not allow one to change the TimeCode from Frames to Audio Units, like its "big-brother."

       

      If one has many Clips, that miss the zero-crossings badly, one workflow would be to Export the Audio to a PCM/WAV file, bring it into Audition, and address the Transients, then Save that edited/restored file, replacing the various Audio Clips with this new file.

       

      There is also a great little Audio utility, Audio Cleaning Lab, from Magix, that does a surprisingly good job of cleaning up ticks and pops. Though I use Audition for most of my Audio work, I have to admit that I reach for ACL first in many cases - amazingly good little program!

       

      Now, pops, clicks, ticks and the like, are not ALWAYS because of a lack of zero-crossings. Other factors can contribute to their introduction into the Audio, when editing Video. Still, the can cause all sorts of Transient noises, and should be considered.

       

       

      Here are some illustrations:

       

      Zero-Crossings_01.png

      A Waveform with the Zero-Crossing highlighted

       

      Zero-Crossings_02.png

      Examples of how a Cut not on a Zero-Crossing can cause a pop

       

      Hunt