Propellerhead's Rewire

Adding an audio file to your DP song as a Soundbite.

SOS's resident MOTU expert Robin Bigwood looks at ways of working with audio loops in Digital Performer, and hands out tips on using Propellerhead's Rewire to interface DP with software synths.

Not many home‑based musicians will even consider working with a sequencer these days unless it has some way of importing audio loops. They can serve as musician‑friendly 'click tracks' as you build up arrangements one pass at a time, or you can even use them as inspiration for your songs, provided you're careful about copyright. If you're a MOTU user (and if you're reading this page, there's a good chance you are!), you'll be pleased to know that DP (MOTU's seemingly favoured abbreviation for Digital Performer) has a number of features that make working with loops relatively painless.

Pulling Them In

Calculating the precise tempo of an audio loop in Digital Performer.

To start with, DP is able to import audio files in SoundDesigner II, AIFF and WAV formats, as well as others. It's also possible to drag a sound file directly from your Mac's desktop into DP's Tracks or Audio Graphic Editing windows. Alternatively, you can open the Soundbites window, and select 'Add soundbite...' from the mini‑menu (screenshot above). If you then navigate to your audio loop file and open it, DP will add it to the list of soundbites, converting it automatically into a usable format and resaving it if necessary. DP can't work with so‑called interleaved stereo files, preferring split stereo instead. However, if you do try to import an interleaved file, DP will make the format conversion for you.

Stereo files or soundbites can't be placed in mono tracks (and vice versa), so if a file doesn't seem to want to 'go', check you're not asking the impossible of DP before tearing out too much hair!

Once your rhythmic loop is in DP you'll probably want to figure out its tempo (even if the loop claims to already have been recorded at a specific tempo) so that DP's bars and beats counter will sync exactly to the loop. Fortunately, this is easily done.

First select the soundbite in the Tracks or Graphic Editing window, then select 'Set Soundbite Tempo...' from the Audio menu. A dialogue box will enquire how many beats long your rhythmic loop is (if you're not sure, you can always go back and count). If you know the answer, you just type it into the 'Length' text field, and DP will automatically calculate the soundbite's tempo, display it in the 'Tempo' box below, and show the resulting new 'End Time' for the soundbite. If you leave the soundbite selected, and choose 'Adjust Sequence to Soundbite tempo' from the Audio menu, you'll find that your sequence and soundbite will now be in perfect rhythmic sync.

One way to get your soundbite to play repeatedly is to select it, then choose 'Set Loop' from the Change menu. You can specify start and end times for the loop, and the number of times the loop will be repeated. Another way is to select your soundbite by dragging the cross‑hair pointer over it, and choosing 'Repeat' from the Edit menu (or hold down Command/Apple and hit the 'R' key). This will automatically place a row of repeated soundbites back to back in the sequence. The Repeat function works with most other types of data, too.

Looping the Loop

DP's powerful Change Tempo dialogue box, which permits time‑stretching of loops without changing pitch.

In some instances, you may want to import a rhythmic loop into a sequence whose tempo cannot be changed — for instance, one which is already based around a loop at a different tempo. In this situation, DP can do some nifty number‑crunching to either time‑compress or time‑stretch the new soundbite. In most cases, you should be pleasantly surprised by how little the sound quality is affected.

Here's how you do it. Import the new soundbite (the one at the 'wrong' tempo), place it in the sequence, and select it. From the Audio menu choose 'Set Soundbite Tempo...', and in the dialogue box shown below, type in the length in beats you want your soundbite to be (or specify an appropriate start and end time). Click 'OK', then from the Audio menu select 'Adjust Soundbites to Sequence Tempo'. The Background Processing window will appear at this point to give you an idea of how long it will take to process your audio file. When the number‑crunching is complete (the speed will depend on how fast your computer is, and the size of the time‑stretched file) your new soundbite will fit perfectly with your existing audio.

"Right Abouutt Nnnoooowwww..."

Muon's software TB303 emulator, Muon Tau Bassline.

While the 'Adjust Soundbites to Sequence Tempo' function is most commonly used for the task I've just described, it can, in conjunction with DP's Conductor Track, do something quite special; it can map tempo changes on to soundbites, so that the audio slows down (gets longer) or speeds up (gets shorter) as it plays. Just think of the crazy break in Fatboy Slim's Rockafeller Skank and you'll get the idea.

To try this out, import a rhythmic soundbite into an empty sequence, then switch tempo control of the sequence over to the Conductor Track, by selecting this option in the 'Tempo control' popup menu on the left side of DP's Control Panel. Now select your soundbite and determine its tempo as described earlier. Choose 'Adjust Sequence to Soundbite Tempo' and you'll see that DP writes a single tempo change message into the Conductor Track at the beginning of the soundbite.

Now, make some copies of your soundbite and place them back to back, maybe using the Repeat function. In the Tracks window, drag over all of them to select them. From the Change menu choose 'Change Tempo...'. The complex dialogue box that appears allows you to very accurately define tempo changes in your sequence, even down to selecting different rate curves for the changes (see screenshot above right). The start and end times should be correct, matching up with the time‑range selection you've already made in the Tracks window. Clicking on the anchor symbol next to 'Start Tempo' enters the tempo at the start point of the sequence into the value box. You can type any value you like into the 'End Tempo' box, and if it differs from the start tempo, you'll notice that DP writes lots of individual tempo changes into the Conductor Track when you click 'OK'.

Finally, if you select 'Adjust Soundbites to Sequence Tempo' from the Audio menu, after the inevitable background processing has taken place, you should hear your soundbites changing in tempo but not in pitch, just as expected. This technique can be usefully employed for dance and experimental music, sound design, and for generally tweaking or repairing recordings. The tempo changes don't have to be gradual, either — try placing a number of wildly different tempos in the Conductor Track and repeating the process.

Using Rewire With DP

Any DP user with even the scantest knowledge of software synths will have heard of Propellerhead's Rebirth, the software TB303/TR808/TR909 emulation plug‑in, and Reason, the software studio. Whilst both Rebirth and Reason can be used perfectly well by themselves, Propellerhead's Rewire protocol allows them to work in tandem with sequencers which offer Rewire support, such as Digital Performer. You could, for example, record the output of Rebirth into DP, or use a MOTU Audio System (MAS) reverb plug‑in to treat the output of Reason's sampler. Rewire also syncs DP with the onboard sequencers of Rebirth or Reason, or both.

Although Digital Performer's Rewire implementation is pretty good, Propellerhead's promise of sample‑accurate synchronisation between compatible applications is still a little way off — but there are ways round that.

When you install Rebirth (for example) a 'Rewire' library is put in your Extensions folder, along with an accompanying 'Rebirth Engine' file. DP is aware of these files and adds Rebirth's audio outputs to its list of available inputs on each audio channel.

But in order to get DP working in tandem with Rebirth (and this applies equally to Reason), the two programs must be launched in a specific order. Boot up DP first, and create at least one new Aux track (not a Voice track) with one of Rebirth's outputs as its input — then start Rebirth.

When the two programs are up and running, you should notice that Rebirth's sequencer starts playing when you hit Play on DP's transport, and vice versa. DP's Stop, Rewind, locate and Memory Cycle functions will control the equivalents in Rebirth, and you should be able to hear Rebirth audio via that new Aux in DP.

To record this audio, you need to route the output of the Aux via a buss (or buss pair) to a record‑enabled audio Voice track, and start recording — it's as easy as that. However, on my own system, I find Rewire audio reaches DP 16mS before it should! To remedy this, I inserted DP's Echo MAS plug‑in on the Aux, specified a 16mS delay with one of the four delay taps and set the Mix control to 100 percent to stop the 'early' signal passing through.

I've heard of DP users who suffer Rewire timing problems of more and less than 16mS, so the problem may be processor‑dependent. To determine what it is for your own setup, try recording rhythmic audio from Rebirth or Reason into DP, and then examine the audio at high magnification in the Audio Graphic Editing window. You should see the audio's transient peaks coming ahead of the bar/beat grid. Try different time values with DP's Echo plug‑in, rerecording audio after each change. When you find a delay setting which puts the audio transients exactly on the beat, that's the one to go with. You might also find you need to leave a couple of empty bars at the beginning of a DP sequence and the accompanying Rebirth song, to allow the two sequencers to synchronise properly.

Interestingly, some software synths, such as Unity DS1 and those from Koblo can also use Rewire for audio output, which might suit some users.

At present Rewire doesn't carry MIDI data, so to control a Rewire instrument (like, say, Reason's sampler) from DP, you have to treat it as if it were a conventional software synth. For starters, you'll have to use FreeMIDI in OMS compatibility mode, because Propellerhead don't yet offer FreeMIDI support. You'll also have to set up inter‑application communication busses in OMS and make the necessary settings in Rebirth or Reason — see last month's Performer Notes for details.