Tech Tip:"Quantizing" Guitar Parts


Here's how to fix those pesky timing problems

By Craig Anderton

 

 

Ever play the perfect solo or riff except that one note was just a little bit early, or a  little bit late? If you never have, then stop reading this article immediately, move to Los Angeles, New York, or Nashville, and start doing session work. Otherwise, keep reading.

 

 

Digital Audio Quantizing Tools

 

The  first time I saw audio quantization was in Opcode's late, great  StudioVision program. The process required signals with strong  transients and fast attacks, like drums, guitar, bass, piano, etc.

 

Quantization  was a two-step process: First, you applied the equivalent of a noise  gate to strip out all the silences in a digital audio track. Thus, all  the notes with sharp transients started a new "block" of audio. The  beginning of each block could then snap to a grid, just like MIDI notes,  to quantize the parts.

 

 

Quantizing Guitar 1


Separating audio into smaller blocks allows quantizing them to a grid.

 

 

 

 

 

Beat  Detective for Pro Tools takes a more sophisticated approach. In a  nutshell, it too looks for transients, but works pretty much  automatically. Once it finds the transients, it moves them so they fall  on a grid, and extends or shortens the audio in front of the transient  to make up for any timing inconsistencies.

 

There  are several variations on these basic themes, but let's look at a  process that works with just about any DAW software. It's definitely not  an automatic process; you need to slice audio manually, and move the  pieces around yourself. While the downside is that this is less  convenient, the advantage is that you can be selective in what you do or  don't quantize.

 

This  is important, because it's okay if notes aren't quantized exactly to  the beat, as long as they sound right. Those little tempo variations  that lead or lag the beat are a crucial part of providing emotional  impact. I've analyzed guitar solos where notes would come in 30 or 50 ms  late compared to the beat, but that added a cool feel—quantizing them  to the beat took all the life out of the solo. So, always use quantization to fix mistakes based on what you hear with your ears, not what you see with your eyes.

 

Another  issue is that quantizing in isolation can lead to problems. For  example, suppose the drummer played the snare a bit late compared to the  beat, and all the players followed right along. If you quantize just  one part, then its timing will sound "off," so you'll have to quantize  the others too … and it all might be for nothing if you then decide that  the tune sounded better with the snare lagging anyway. If it ain't  broke, don't fix it.

 

 

But This One's Broke!

 

What  got me into writing this article was working on a new library of looped  guitar riffs called "Adrenalinn Guitars," using effects primarily based  on Roger Linn's Adrenalinn processor. Because loops like these are  generally used in conjunction with quantized drums and other loops, the  notes need to be right on the beat so they fit in properly. If someone  using the loops wants to add a timing variation, it's usually done by  inserting a small tempo change to lead or lag, not moving bits and  pieces of audio around.

 

 

The  upper part of the following illustration shows the original guitar riff  (only one channel is shown to save space). The four lines (three white,  one red) indicate eighth note divisions. Note that the first note is  right on an eighth note division, the second one is just a tiny bit  ahead, the third note lags by a bit, and so does the fourth.

 

 

Quantizing Guitar 2

 


A guitar riff before and after quantizing.

 

 

 

 

 

The  lower part shows the same riff after quantizing, using the technique  we're about to describe. The timing has been perfected, but without any  glitching, pops, or other indications that quantizing took place.

 

 

The Process

 

Let's  first show how to quantize a note that lags behind the beat; as an  example, we'll fix the last note in the above illustration (the one that  lags the red line). Here's the step-by-step:

 

  1. Zoom in on the waveform.
  2.  

  3. Turn off any snap-to-grid function.
  4.  

  5. Split the clip at the precise beginning of the lagging note. A yellow line  indicates the split in the top third of the following figure. Also split  at the end of the note, or before the next major transient. Basically,  we want to isolate the section that needs to move.
  6.  

 

 

 

 

 

Quantizing Guitar 3

 


Three steps involved in quantizing a note that lags the beat.

 

 

  1. Turn on the snap-to-grid function (in this case, it's set to eighth notes).
  2.  

  3. Slip-edit  the section of audio just before the lagging note, and move it back to  the point where the note should start. Because snap is on, the end of  the clip should snap right into position on the eighth note. This opens  up a space between where the note should start and its current location  (as shown in the middle third of the illustration).
  4.  

  5. Move the lagging note left to the proper start point, in the direction  indicated by the arrow in the middle third of the above screen shot.  Note that this will also open up a space at the end of the clip, which  is why we added a split there in step 3. Otherwise, when you moved the  clip to the left, it would move the entire rest of the track to the  left. However, you will likely need to go back and close up this space,  probably by crossfading in a manner similar to what's described in the  second step-by-step example.
  6.  

  7. Crossfade over the split point to eliminate any clicks or pops. Now the audio  looks like the lower third of the screen shot. Roll the crossfade out  starting from the note's attack, and move left so that it crossfades with the existing decay.
  8.  

  9. Audition what you've done to make sure all is well. Adjust the crossfade for the minimum amount of crossfade needed for a natural sound.
  10.  

 

If the note is ahead of the beat instead of behind, the process is somewhat similar.

 

  1. Zoom in on the waveform.
  2.  

  3. Turn off any snap-to-grid function.
  4.  

  5. Split the clip at the precise beginning of the leading note. A yellow line  indicates the split in the following illustration. Also split at the end  of the note, or the next note transient.
  6.  

 

 

 

 

 

Quantizing Guitar 4

 


Three steps involved in quantizing a note that leads the beat.

 

 

  1. Turn on the snap-to-grid function (e.g., eighth notes).
  2.  

  3. Move the leading note to the right so that it snaps into position on the  eighth note (middle third of the above screenshot). Do not slip-edit the  section of audio prior to the note attack to cover up the space, as  that will cause a double attack. (Note that because we added a split at  the end of the leading note, moving the clip to the right will cause its  end to overlap the start of the section after the split. You will need  to revisit this later and fix it, probably by slip-editing the clip so  it no longer overlaps the beginning of the next one, then crossfading to  smooth over the splice.)
  4.  

  5. Crossfade over the space and into the previous decay to eliminate any clicks or  pops (as shown in the lower third of the above screen shot). Roll the  crossfade out starting from the note's attack, and move left so  that it crossfades with the existing decay. Note: If the gap that needs  to be covered is large, prior to crossfading you may need to add a hard  fadeout to the end of the clip. Otherwise, even with crossfading, there  may be an abrupt cutoff.
  6.  

  7. Audition what you've done to make sure all is well. As before, use the minimum amount of crossfading needed for a natural sound.
  8.  

 

This  may seem time-consuming (and if you're trying to fix all the glitches  for a player with a timing problem, it is). But to fix the odd note here  and there, this technique provides a simple, transparent-sounding  solution.