Elastic Audio — Better Than Beat Detective?

Digidesign (Avid) Pro Tools Tips & Techniques

Published in SOS January 2010
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Technique : Pro Tools Notes

Many long‑term Pro Tools users still rely on Beat Detective, but the newer Elastic Audio often produces better results in half the time.

Mike Thornton

Two years ago, back in January 2008, I wrote about Elastic Audio when it was bright and shiny new. In that article I outlined what Elastic Audio could do, so if you need an introduction, look up the article in your old issues or surf to /sos/jan08/articles/protools74.htm. In this month's workshop, we're going to look in more detail at some of the applications for this technology.

Building A Session From Loops

If you work a lot with loops, Elastic Audio is really useful. It enables you to import loops into a Session and allow Pro Tools to adjust their tempos to match the Session tempo.

Create a new Session but don't add any tracks. Go to the Processing tab in the Preferences window and tick 'Enable Elastic Audio on New Tracks', and in the Editing tab make sure 'New Tracks Default to Tick Timebase'.For loop‑based work, you'll want your tracks to use the tick‑based timebase and the Polyphonic Elastic Audio algorithm. Now go into your Workspace Browser and find your first loop. (Don't forget you can adjust the audition volume in the Workspace Browser, so that you don't end up with your speaker cones splattered against the back wall!) Once you have found your first loop, drag it into the Edit window where the tracks would normally be and let go. Because this is the first file to be brought into the Session, Pro Tools will ask if you want to import the tempo from the file as well. Click on Import and now you will have your first loop in place and the Session tempo will match its tempo. Now you can bring in more loops and Pro Tools will use Elastic Audio to conform the new loops to the Session tempo irrespective of the loops' original tempo. You can also audition the loops in the Browser in time with the session if the 'Audio Files Conform To Session Tempo' icon is highlighted at the top of the Browser window. Also check the other settings at the bottom of the Workspace Browser drop‑down menu, which is accessed in Pro Tools 8 using the arrow icon in the top right‑hand corner. Loop Preview, Spacebar Toggles File Preview and the Conform option should all be ticked. Now, with the Session in Loop Playback mode, select a loop in the Browser and hit the space bar. Once Pro Tools has analysed the loop, it will play it at Session tempo and in time. Once you are happy, drag the new loop into the Edit window and Pro Tools will automatically create a new track for you. You can then build up your track as I have in the screenshot below.Here, Elastic Audio is conforming three different loops to the same Session tempo — which itself is varying.

You can then vary the tempo of the Session afterwards to suit. In the screen, I have drawn a manual tempo change with the Pencil tool so that the tempo increases over the last two bars, quite severely, from 85 up to 197 bpm! Notice how the beats get closer together as Pro Tools speeds up the loops; all this is done in real time, non‑destructively, so one click of the Undo button puts everything back to normal.

Fixing Drum Timing

There was a lot of discussion in the early days about how good, or not, Elastic Audio was for handling multitrack drums, and whether it could keep everything in sync with no flamming. There are a number of threads in the Digidesign User Conference about it (see, for instance, http://duc.digidesign.com/showthread.php?t=211034 and http://duc.digidesign.com/showthread.php?t=212488). In v7.4cs2, Digidesign tweaked the way in which Elastic Audio works, so that it puts the first transient across a set of grouped tracks onto the grid.

Let's look at how we might use Elastic Audio to tidy up a drum session where, although the drummer played to a click, the timing in places was not as tight as it should have been, and he and the bassist weren't always together. A real‑world situation if ever there was one!

First, make sure you set up an Edit & Mix group for all your drum kit tracks. This is essential if you don't want to end up with phasing or, worse still, flamming in your corrected drum parts.

Then make sure that all your tracks are in tick‑based mode, by clicking on the clock icon in one of the tracks' title sections and changing it to the metronome icon (doing this to one will change all the tracks in the group to tick‑based). Now click on the Elastic Audio plug‑in box and select the Rhythmic algorithm, as these are rhythm parts we will be working on. (Monophonic is best for instruments that play single notes, like bass guitars and vocals, while Polyphonic is best for pre-mixed loops, piano and keyboard tracks, and so on). Don't worry if your tracks go grey when you do this for the first time: Pro Tools takes the regions off‑line to analyse them ready for the real‑time time compression and expansion that is Elastic Audio.

With the groundwork done, we can begin. Take a look at the screen to the right: the snare drum should be on the second beat of the bar, but it is a little late.The snare hit in the centre of this screen is so late that Pro Tools attempts to move it to the wrong place entirely! Using Elastic Audio we can quantise these drum tracks just as if they were MIDI. Highlight the tracks, go into the Event menu and choose Event Operations / Quantise… In the Quantise window, make sure you have Elastic Audio Events selected, choose appropriate settings for the style and timing of the music you are working on, then hit Apply. You will see the waveforms move as Pro Tools quantises your audio for you, just as if it was MIDI. Now play the track against the click and check to make sure Pro Tools has got it right.

In this case, the snare was so far off the beat that Pro Tools quantised it the wrong way! When this happens, you will need to go in and give Pro Tools a helping hand. Select Warp view (one up from Waveform in the track drop‑down menu) and you will see where Pro Tools has put the warp markers on the transients.By creating our own warp markers, we can move the snare to the right place manually. You will notice that there are two sets of warp markers, one on the beat and one on the snare hit. Delete the warp marker on the beat by double‑clicking on it. Once it is gone, drag the warp marker on the snare and drop it on the correct beat (you can, of course, choose to do this with Grid edit mode engaged if you want it bang on the beat). Work your way through the track fixing the other markers that Pro Tools doesn't get right and you are done. If you don't want a strict metronomic drummer, you can chose to re‑quantise the kit using a groove, by selecting one from the Quantise Grid menu in the Quantise window. Remember, this is all done in real time and non‑destructively, so you can always go back and have another go, which is really great when you're learning how to do this. I managed to sort this particular track out in a few minutes, where previously, in Beat Detective, it took several hours, working along in four‑bar sections and then manually fixing the bits that didn't work properly.

The Ultimate Test

In the December 2005 issue of SOS (/sos/dec05/articles/protoolsnotes.htm) I mentioned a problem that, at the time, required advanced timing correction in Beat Detective. I had a track that had been recorded without a click, in which the client decided they wanted to replace the keyboard part with a piano part. The pianist understandably found it hard to play exactly in time with the original, but we worked hard to get the best match possible, and then I set about using Beat Detective to analyse the track, produce a tempo map of the piece and cut up the new piano part to match the song's tempo map. It took hours and hours, but the result was pretty amazing: although there were loads of edits in the piano part, in context you couldn't hear them.

I wondered what Elastic Audio would make of the same challenge, so I imported the 'non‑Beat Detectived' piano track to a tick‑based stereo audio track and activated the Polyphonic Elastic Audio algorithm. In Warp view, when you slide a warp marker, the two either side of it act as anchors; because they were all so far out, I found I was pulling the piano part all over the place. To reduce this, I went through and removed most of the automatically generated warp markers, then placed new ones in strategic positions. As I created them, I slid each one to the correct position to bring the next section of the piano part into time. In this fashion, I worked my way through the piano part in about 30 minutes, creating and moving warp markers where I wanted them, until the piano and keyboard parts looked very similar in the waveform view.

Using the Polyphonic Elastic Audio plug‑in, the sound of the piano was significantly compromised in places. Once I was happy that I had got the new piano part in time with the piece, I changed from Polyphonic to X‑Form (Rendered Only). Pro Tools then went away and used the excellent X‑Form time compression and expansion plug‑in to create a rendered version of my piano part. Hey presto: one excellent piano part that, when soloed, sounds so much better than the Beat Detective version. Chalk another one up to Elastic Audio. (The screen below shows the Beat Detective track on top and the completed Elastic Audio track below.)A piano part, corrected using laborious work in Beat Detective (top lane) and much more quickly in Elastic Audio (bottom).

I hope this has been helpful and encouraged you to have a go with Elastic Audio. Once you have got to grips with it, you won't be disappointed!    .


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