One of the songs on a recent recording project didn't include a drum part, because the writer figured I could add sampled drums later. Sounds simple if you have a hard disk recorder, right? Well, maybe.

Whether adding drums after the fact is easy or hard depends on several factors. If there's even the slightest chance that drums will need to be added, record any basic tracks while playing along with a click track, which should be recorded on its own track. The click will help keep the players in time while recording. Later on, it will provide a visual reference against which you can line up drum hits to make sure they're in time with the tune. If the recording doesn't have an existing click (or a dummy drum track that you're planning to replace), adding drums will be quite a hassle.

This particular project was all-acoustic, so I needed the realism of human-generated drums. I didn't want to use MIDI to drive drum module samples, since drums recorded this way generally don't "mesh" well with acoustic instruments. So instead of using a sequencer and trying to do a "tap tempo" operation to set the timing, I took a more complex approach, pulling acoustic drum loops off of sample CDs.

Adding a drum track in Ensoniq PARIS. The blue line shows the duration of 13 beats on the click track; the red line shows the same number of beats for the imported drum part. Placing the red-and-yellow cursor at a specific location provides a time read-out in the lower left-hand corner. By subracting the beginning time for the region from the ending time, you can derive the duration. (In this example, the click track is 148 bpm, and the CD drum part is 146 bpm.

The CD that worked for me, Double Platinum Drums (distributed by Ilio Entertainments), includes patterns, intros, and fills for a number of styles, which makes it easy to assemble complete parts. When choosing drum loops, the primary issue is tempo-matching. The loop should come as close as possible to the song's tempo, because stretching or compressing too much compromises sound quality. The song tempo was around 148 bpm; several rock styles on the CD were at 146 bpm -- in the ballpark.

To bring the loops into your project, do a digital transfer from CD player to computer, and simply record all the loops back-to-back onto an audio track as a single long file. You can then copy and paste from this file into the tune as needed. The transfer process is easy if your CD player has a digital out and the computer includes a digital in. However, several programs (including BIAS Peak, Steinberg WaveLab, and a Windows shareware program called Audiograbber, which can be found at www.audiograbber.com-us.net) let you import audio directly from audio CDs placed in the computer's CD drive. If all else fails, there's always the old standby, analog-out-to-analog-in transfer.

A QUESTION OF TIMING
To match tempos, figure out the ratio between the song tempo and the loop tempo. In the example above, the 146 bpm drum track had to be sped up to match the song's 148 bpm. The formula is simple:

loop tempo / song tempo = ratio needed to change loop tempo

Plugging in the numbers (146/148), we get a ratio of 0.98486. If you're using a time-stretch algorithm, you would multiply the loop length by 0.98486 to bring it up to 148 bpm -- at least, in theory. However, there's no guarantee that the click track recorded on the hard disk was at exactly the right tempo, or that the drum loops on a given CD are perfectly calibrated. Even the slightest difference will cause the loop to drift out of sync.

Rather than accept the tempo indications at face value, visually line up the loop with the click track, as shown in Figure 1. The peaks should be obvious enough that you can tell whether the loop is faster (peaks closer together than the click) or slower (peaks further apart). Measure the time between an equal number of beats for the CD track and the click track (your software should allow you to do this fairly easily), then get out the calculator. Divide the measured loop length by the measured click length to derive the ratio.

WHICH STRETCH METHOD?
There are several ways to change the length of a digital audio signal. Personally, I find time compression/expansion algorithms less than satisfactory, especially with stereo loops. Even with a mono loop, these algorithms tend to mess with the groove.

To time-stretch, I prefer using an old-fashioned pitch-shift function that changes the pitch without readjusting the duration back to its original value. (In other words, when you shift pitch up, the file becomes shorter, just as if you were playing the sample from a MIDI keyboard.) I prefer these algorithms' fidelity. The ratio required to shift is the same, but now we're applying it to frequency instead of time. If too large a shift is required, though, pitch-shifting will change the timbre of the drums. This method works best when you're shifting by only a few bpm.

Another excellent option is Steinberg's ReCycle, which I talked about in the June 1998 column. It allows a sound to be stretched with minimal sonic artifacts, even over a fairly wide tempo range, although you need to incorporate a MIDI sampler and sequencer into the act. Yet another option is to load the drum loop into Sonic Foundry's Acid, change the tempo as required, and use Acid's output to create a file you can import into your hard disk editing program. Either of these programs will give better results than standard time-stretch algorithms, because they use a drum sound's percussive transients as beat markers to intelligently shorten or lengthen the file.

CONSTRUCTION TIME
Next, piece together the part using the various available phrases. Here are a few tips:

• Use loops that don't include ambience. Dry sounds glide more easily into each other, and adding a bit of reverb afterward can smooth over any rough transitions.
  
  
• Place the fills and main patterns on different tracks if possible. There will often be overlaps between hits on the two files (e.g., the intro ends on the downbeat with a cymbal crash, whereas the pattern it leads into starts with a kick without a cymbal crash), and it helps to be able to change the levels of the two drum clips independently.
  
  
• Add individual drum hits for variety. Throwing in the occasional off-beat hi-hat or an extra snare flourish can make the loop seem more real. (Place the individual hits on the track containing the fills.)

CLOSING DOWN
Once you've cut, copied, and pasted the various pieces to make a coherent drum part, don't forget to work the drum faders during mixdown. For example, you can bring up a fill to make it more prominent.

So did the drum part work in my particular project? Judge for yourself -- there's a short clip at the top of this page. Hopefully, your efforts will elicit the response: "Who did you get to play drums?"

Craig Anderton is the author of Home Recording for Musicians and Multieffects for Musicians, both published by AMSCO. He recently returned from gigging in Germany at the Cologne Battery Park music festival.