Many people are shocked to hear that Stewart Copeland, the virtuoso drummer for the Police, created many of his amazing rhythms by playing through synchronized echo effects. “With our first bit of money in the Police, we went to Manny’s Music in New York City and bought all kinds of crap,” he wrote in the foreword to The Art of Digital Music. “I got a Fender Stratocaster, a cool Roland amp, and a Roland Space Echo tape delay. At the sound check that night, I plugged a microphone into the amp and got this echo response going with the snare drum where I could get these cool rhythms bouncing back off my amp. One of the techniques I discovered was the dotted-eighth-note delay, which creates that swinging ‘dugga-dugga-dugga’ sound.” Copeland later switched to digital delay processors, which let him set a precise, repeatable tempo for the rhythmic dialog. (In this article, we’ll use the terms “echo” and “delay” interchangeably.)

When you synchronize echoes and other electronic effects to your music, it truly makes the sound come alive. For instance, compare this straight drum pattern: this one, in which I processed the snare drum with the dotted-eighth-note delay Copeland mentioned:

Now listen to what happens when we add feedback to the echo so that it repeats:

Guitarists love this effect. Listen to these excerpts from U2 and Pink Floyd:

Theory 101

What is a dotted-eighth delay, exactly, and how do you create one? Here’s a quick music-theory refresher. Typical songs have four beats per bar. Consequently, each beat is called a quarter note. Play two notes in the space of one beat, and you get eighth notes, because there are eight divisions per bar. Four notes in one beat are called sixteenth notes (16 divisions per bar). A dotted eighth note lasts for an eighth plus a half of an eighth, or three sixteenth notes.

In the first drum-beat pattern, we had two quarter notes (two beats), two eighth notes (one beat), and then another quarter note (one beat), making one complete bar:


An easier way to visualize note divisions is with a drum-machine grid. I made the pattern above with a free online drum machine called GrooveLab. In this screenshot, you can see the 16 divisions per bar:

Note how the checkmarks for kick and snare match the musical notation above.

Calculating Delays

To create a tempo-synchronized effect, you simply divide 60 by the tempo of the music in beats per minute. For instance, if the tempo is 120 beats per minute (bpm), the duration of one quarter note (that is, one beat) is 60/120, or 0.5 seconds. Engineers commonly refer to that as 500 milliseconds (ms).

An eighth note at 120 bpm lasts half as long, or 0.25 seconds (250ms). And a sixteenth note lasts 125ms. Because a dotted eighth note is equivalent to three sixteenths, it lasts 3 x 125 or 375ms.

Try it with this JavaScript calculator. Enter a tempo and click the button to see the duration of one beat.

Tempo in bpm:  milliseconds

Calculating Tempo

Most modern grooveboxes and groove-oriented software can calculate the tempo of a section of audio for you and automatically set delay effects for synchronized echoes. But it’s extremely helpful to know the theory, as we’ll see in a moment. Here are several ways to determine a tempo.

The Casio Method. This method is handy if you’re stranded on a desert island or in a dive club with only a digital wristwatch. Using its stopwatch function, play some music and time ten beats. (That is, click Start when you hear the first beat and Stop when you hear the eleventh.) Shift the decimal point two places, and the readout will tell you the duration of one beat in milliseconds. For example, ten beats at 130 bpm will last about 4.62 seconds, so 462 milliseconds is the number you want for a quarter-note echo.

The Microscope Method. In an audio editor, zoom in on the waveform and determine the length of one beat in samples. Divide that number into 60 times the sampling rate. For example, say one beat lasts 20,000 samples, and the audio file is at 44.1kHz resolution. The tempo will be 132.3 bpm because (60 x 44,100) / 20,000 = 132.3. To account for human timing variations between beats, it's best to measure more than one beat. To do that, multiply the numerator of the fraction by the number of beats you're measuring. A two-bar (eight-beat) phrase that lasts 170,000 samples has a tempo of 124.52 bpm, because (60 x 44,100 x 8) / 170,000 = 124.52.

Here’s a JavaScript calculator for that:

Length of audio in samples:
Sample rate in Hertz:
Number of beats:

The Tap Tempo Method. Most music software and grooveboxes contain a “tap tempo” feature that determines the tempo as you tap the spacebar along with the beat. There are standalone tap-tempo programs, too, such as AnalogX TapTempo (Windows), Music Math (Mac) , Pocket Beat (Palm), and Pulse Meter (Nokia phones).

Derek Chilcote-Batto created this elegant JavaScript tap-tempo calculator that runs from a web page. Click your mouse repeatedly in this window to try it.


Free JavaScript provided by The JavaScript Source.

Video Sync

Tempo synchronization adds an elegant effect to video productions as well. For the The Art of Digital Music, I wanted to showcase sound bites from the 56 interviews that comprise the book. First I created some background music in Ableton Live to move the sound bites along. Then I used LQ Graphics Photo to Movie to animate some press photos the interviewees provided, aligning the photo transitions to the beat.

Typically, I made each photo last for four bars (16 beats). Because the background music was running at 105 bpm, four bars lasted 9.1 seconds (60 x 16 / 105 = 9.1). Adding one second to cover the crossfade transition, I wound up with a total duration of 10.1 seconds for each photo.

Photo to Movie
If you know the tempo, it’s easy to sync transitions to the beat in video editors like Photo to Movie.

The timeline in Photo to Movie is calibrated in seconds, so it’s easy to plug in musical durations such as 10.1 seconds. If you’re using a video editor that displays time in frames, use the formula (fps x 60) / bpm = fpb to calculate the number of frames per beat. This online calculator does the math for you.

Here’s one of the final movies (crunched down for the Web): an interview with trumpeter and film composer Mark Isham. Notice how the titles are synchronized to the beat as well. If you listen closely, you can also hear some tempo-synced instrumental parts in the background.

Click to View Movie

(Click the image to view the movie.)

Is There an Echo in Here?

We’ve only scratched the surface of creative tempo sync here. Using a stereo delay with a different echo time in each channel creates some fantastic swirling effects. But don’t stop with echoes; try syncing filter sweeps, flanging, phasing, panning, and other effects. For more ideas, check out Joe Bennett’s delay article or the inspiring AdrenaLinn demos at Roger Linn Design. The AdrenaLinn is a digital guitar processor that takes beat-syncing to extremes.