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Logic X After flexing, drum tracks later in song start misaligning, why?

Discussion in 'Logic Pro X' started by Fusion Head, Jun 26, 2015.

  1. Fusion Head

    Fusion Head New Member

    Hi everyone, I have a problem with Flex Time that has happened a couple of times now. I am attempting to adjust the tempo of a song with a live drum kit recorded in 'free time' (i.e. not to a click). After applying Flex Time (see my process detailed below) I am finding that the first part of the song sounds perfectly synchronized, but in the last part of the song the tracks start to audibly misalign. Can you help me determine what is going wrong?

    This is the process I am using:
    1. I first apply to all tracks the function 'Remove all tempo information from track'
    2. Set the desired tempo for the song.
    3. Add all tracks to the same Group, with Phase-Locked Audio turned on.
    4. Turn on Flex Time for all tracks, choosing 'Slicing' mode.
    5. Make sure no Flex markers are present in the tracks. (I have found that Flex sometimes adds unwanted flex markers even before you begin to do any manual edits.)
    6. Begin aligning sections in the tracks to the Bar markers of the song. Since I do not want to make the performance sound too mechanical, I do not want to use Quantize to align every beat to the bars and quarter note markers. So I typically drag the downbeat (kick drum track) transient of each 2 or 4 bars to the appropriate Bar marker.

    After taking these steps, the drums sound good throughout most of the song. But at about 75% of the way through, the tracks start to audibly misalign slightly. This makes it sound like there is a very short slapback delay on the hi hat and snare beats. The transient markers look okay. I tried adding or removing some of the Flex markers, and also tried changing the Flex mode from 'Slicing' to 'Rhythmic'. Neither of these seem to entirely fix the problem.

    Any ideas why this is happening? Any help would be very much appreciated! FH
  3. Eli

    Eli Senior member

    Are you sure that the regions on all of your phase locked tracks are EXACTLY the same length? If not, trim them slightly so that they are all the same.
  4. Fusion Head

    Fusion Head New Member

    Hi Eli, thanks for the response. Yes, all my tracks are exactly the same length. This was assured because, prior to using Flex, I had added an extra region of silence to the end of each track, then used 'Join per track' to create new audio files (and regions) with exactly the same end points. Any other ideas to solve this? Should I still try trimming off a bit of the end of all tracks to see if that does anything?
  5. Eli

    Eli Senior member

    I guess it couldn't hurt to try?
  6. Markdvc

    Markdvc Administrator Staff Member

  7. coolcat

    coolcat New Member

    Thanks for those tips Mark - good to know
  8. kinsax

    kinsax New Member

    Correcting timing on a freely recorded track is always inviting problems. When you move an audio region, flex markers are automatically added to the sound byte at various transients. Certain parts are expanded and compressed. The same parts of beats(related to downbeats of following material later in the song) won't change in the same proportions. Hence timing issues. Also, the more you stretch audio, artifacts start to creep in, also changing the timing. In short, you are trying to correct your drums to a grid that doesn't relate to the timing of you drum tracks. When you flex to slicing or rhythmic, what are you slicing to? Some kind of grid. Also, bleed on tracks (especially drums), doesn't always shift on each track the same way, hence slap back effect.
    You say the drums sound good for most of the song. There is a "drift" problem when you do long processes. Try just using material from the 75% that works.

    Just outlining the many things that go wrong in the type of maneuver. In some ways your asking the software to do things it's not designed to do. My advice? Take small portions of your drum track that really work. Flex them into nice manageable loops in two, four or maybe 8 bar segments. Fit them into the grid by finding their tempo. You can effectively quantize small portions. Try by percentage, or using the Groove track function. Sometimes it's just easier to re-record the drums, after you hear what's wrong.
    Just my 2 cents. Hope this helps and good luck!
  9. Fusion Head

    Fusion Head New Member

    Eli: I tried trimming off some of the end of all regions, but it did not fix the problem.
    Mark: Thanks for those links. I have, in fact, used all the steps outlined in those tips, where appropriate.
    I think my next step will be to start from scratch again, by removing all Flex edits, and removing all tempo information from all audio files. I'll start adding flex markers from the start of the song to see how far I get before things start going out of sync again, and maybe this way I can find the source of the problem. I'll report back with results. In the meantime, if you have any more ideas for me to try, let me know. Thanks everyone!
  10. Fusion Head

    Fusion Head New Member

    kinsax: Thanks for your explanation on the limitations of using Flex. I can appreciate what you are saying regarding artifacts causing problems if audio is stretched too much. But in this case I am only stretching and compressing a very small % of tempo. I am using Flex mainly to make the performance more consistent in tempo, rather than change the overall tempo.

    Regarding your comment that "... bleed on tracks (especially drums), doesn't always shift on each track the same way...", if I move a flex marker only slightly, I would expect all audio on all phase-locked tracks to stretched or compress by exactly the same amount, thus preserving the phase synchronization between them. I have successfully done this for other songs I have edited, adjusting the timing over an entire 4 or 5 minute song with no problems. Yet, for some reason, this particular song is giving me problems. Are you saying that in your experience Logic is not able to reliably keep track-to-track transients in alignment over anything more than a few bars? Have I just been lucky in the past?

    If this is the case, then I may be forced to use the method you recommend: breaking up the song into smaller regions, converting each to new audio files, then flexing each section of the song separately.

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