Understanding Cycles

The concept of cycles is very central to be able to understand how Strudel works. Strudel’s mother language, TidalCycles, even has it in its name.

Cycles and BPM

In most music software, the unit BPM (beats per minute) is used to set the tempo. Strudel expresses tempo as CPS (cycles per second), with a default of 0.5 CPS:

s("bd")

Here we can hear the 0.5CPS in action: The kick repeats once every two seconds. Let’s make it 4 kicks:

s("bd bd bd bd")

Now we have 4 kicks per cycle, but the whole pattern still plays at 0.5CPS. In terms of BPM, most musicians would tell you this is playing at 120bpm. What about this one:

s("bd hh bd hh")

Because the second sound is now a hihat, the tempo feels slower again. This brings us to an important realization:

Setting CPM

If you’re familiar with BPM, you can use the cpm method to set the tempo in cycles per minute:

s("bd hh").cpm(110)

If you want to add more beats per cycle, you might want to divide the cpm:

s("bd sd bd rim, hh*8").cpm(110/4)

Or using 2 beats per cycle:

s("bd sd, hh*4").cpm(110/2)

Cycles and Bars

Also in most music software, multiple beats form a bar (or measure). The so called time signature specifies how many beats are in each bar. In many types of music, it is common to use 4 beats per bar, also known as 4/4 time. Many music programs use it as a default.

Strudel does not a have concept of bars or measures, there are only cycles. How you use them is up to you. Above, we’ve had this example:

s("bd sd bd rim, hh*8").cpm(110/4)

This could be interpreted as 4/4 time with a tempo of 110bpm. We could write out multiple bars like this:

s(`<
[bd sd bd rim, hh*8] 
[bd sd bd rim*2, hh*8]
>`).cpm(110/4)

Instead of writing out each bar separately, we could express this much shorter:

s("bd <sd rim*<1 2>>,hh*4").cpm(110/2)

Here we can see that thinking in cycles rather than bars simplifies things a lot! These types of simplifications work because of the repetitive nature of rhythm. In computational terms, you could say the former notation has a lot of redundancy.

Time Signatures

To get a time signature, just change the number of elements per bar. Here is a rhythm with 7 beats:

s("bd ~ rim bd bd rim ~")

or with 5:

s("bd hh hh bd hh hh bd rim bd hh")

We could also write multiple bars with different time signatures:

s(`<
[bd hh rim]@3
[bd hh rim sd]@4
>`).cpm(110*2)

Here we switch between 3/4 and 4/4, keeping the same tempo.

If we don’t specify the length, we get what’s called a metric modulation:

s(`<
[bd hh rim]
[bd hh rim sd]
>`).cpm(110/2)

Now the 3 elements get the same time as the 4 elements, which is why the tempo changes.