Metronomes

Metronomes play a fundamental role in coordinating the timing of Afterglow’s effects, and helping them relate to musical structure. They keep track of musical time relative to some starting point, in terms of beats, bars (by default four beats to a bar), and phrases (usually eight bars to a phrase). In addition to keeping track of the current beat, bar, and phrase, metronomes can also tell you the phase of that beat, bar, or phrase, which is a measurement of progress through the beat, bar, or phrase. The phase starts out at 0.0 at the very beginning of the interval, and grows towards, but never quite reaches, 1.0, because at that point you have moved on to phase 0.0 of the following interval.

Metronomes are generally combined with Oscillators to generate wave shapes which drive the parameters sent to lights, creating color shifts, brightness pulses, or movements.

Since they are so important, there is always a metronome associated with a show in Afterglow. It gets created when the show is created, and is used for the show’s timing unless you create a separate metronome and explicitly tell an effect to use it (which you might want to do if that effect is to run at a very different speed than the music). The main show metronome can be synchronized to external timing sources, such as DJ controllers and mixers, to keep the light show tightly in time with the music being played. You can access the the main show’s metronome through the :metronome keyword in the show map, for example:

To create a separate metronome, you call:

The bpm parameter tells the metronome what tempo to use, that is, how many beats happen per minute. You can also adjust the number of beats which make up a bar (or measure), and how many bars make up a phrase:

The metronome starts counting from the moment it was created, and you can ask it for all the counts as a string by calling:

For that metronome, the current count was phrase 15, bar 1, beat 4. You can determine those values as individual numbers as well, but you might end up with confusing results because time is moving on while you are calling the individual functions to ask for information, and you might be on a different beat, bar, or phrase for a later call than an earlier one. So the way to start is by asking the Metronome for a snapshot of its state at an instant in time, and then ask the snapshot for the individual pieces of information you want. Afterglow does this when it starts generating a frame of lighting effects, to make sure that all the effects share the same notion of the point at which they are being rendered, and create a consistent look:

Although this captures all of the information, it is not necessarily in the format you want it, for example the beat number and bar number are counted since the start of the metronome, and you usually want them since the start of the bar or phrase. The snapshot has ways of providing this information:

Since the notion of a “down beat” (the first beat in a bar) is important in music and thus potentially to lighting effects, there is a snapshot function for checking that, too:

The metronome can be restarted at a particular beat by calling metro-start with a beat number. (Calling it with no argument returns the time at which the metronome was started.) This can be useful to resynchronize the metronome to the music if it has drifted, either because there is no BPM synchronization in place, or if MIDI clock sync is being used, and it’s been a long time, since that can drift, unlike Pro DJ Link sync:

If the beats are landing in the right place but the notion of the down beat (bar start) or phrase start is wrong, you can reset those by calling metro-bar-start or metro-phrase-start with the desired bar or phrase number (usually 1). These adjust the metronome so the larger intervals are resynchronized without changing the individual beats themselves.

The metronome’s tempo (beats per minute) can be determined or changed by calling metro-bpm. Without an argument it returns the current BPM, and an argument is used as the new BPM value. The same can be done with the beats per bar (metro-bpb) and beats per phrase (metro-bpp).

There are a number of other functions which can be called on metronomes and snapshots. The IMetronome and ISnapshot protocols in afterglow.rhythm specify and describe them.