If statements in a SynthDef
It’s only a matter of time before a user tries to write something like this in a SynthDef:
… with the disturbing result:
ERROR: Non Boolean in test.
“Non Boolean in test”? But x > 0 is a comparison, and surely should produce a Boolean, right?
This should be the first clue that Boolean logic in the server is a very different animal from the so-called “normal” use of conditionals on the client side (in the language).
What is a Boolean in the server?
In fact, there is no such thing. The server handles floating-point numbers. It doesn’t have true or false entities.
Since everything in the server is a number, the result of the comparison must also be a number. The server follows the same convention as other DSP environments (Max/MSP, pd etc.):
- True is represented by 1.0
- False is represented by 0.0
Why is x > 0 “non-Boolean” in the “test”?
This goes back to the general issue of handling operators in the server. Math operators in a SynthDef are not calculations to do right now. They describe calculations that will be done in the future, many thousands of times.
x: an OutputProxy
(x > 0): a BinaryOpUGen
The precise value of x is unknown at the time you execute the SynthDef code. x actually represents an unlimited number of values, which will be provided to Synths using argument lists. So, it’s meaningless to determine, once and for all, whether x > 0 or not. x may be > 0 now and < 1 a split second later. So, instead of producing a Boolean, x > 0 produces a Binary Operator UGen that repeatedly executes the comparison.
Going back to this:
To do this, the language must know which function (true or false) to execute. But there is no way to know which one the BinaryOpUGen will be. So, SuperCollider throws an error.
If you can’t branch, what good is a comparison in the server?
Comparisons have a lot of uses, actually.
- Choosing one of two signals: This is the closest we can get to
if-then-else in the server. Both then and else must be running
continuously. That’s a requirement of how the server works: the
number and arrangement of unit generators within a single Synth
cannot change. Instead, you can choose which of those signals makes
it downstream. One will be used and the other ignored.
Since true is 1 and false is 0, you can use a conditional to index into an array using Select.
- Generating triggers: A trigger occurs whenever a signal is <= 0, and then becomes > 0. Extending this to comparisons, it means that a trigger occurs when a comparison is false for a while, and then becomes true. Comparing a signal to a threshold may then be used anywhere that a trigger is valid. For a simple example, take the case of sending a message to the language when the microphone input’s amplitude crosses a threshold.
- Passing or suppressing triggers: You might need to generate
triggers continuously, but permit the triggers to take effect only
when a condition is met. Multiplication handles this nicely:
condition * trigger. Since the condition evaluates as 0 when
false, the trigger will be replaced by 0 and nothing happens, as
For a simple case, let’s refine the mic amplitude example by suppressing triggers that occur within 1/4 second after the previous.
Logical operators: And, Or, Not, Xor
Logical operators have simple arithmetic equivalents.
- And = multiplication: (x > 0) * (y > 0) means both conditions must be true (nonzero) for the result to be nonzero.
- Or = addition: (x > 0) + (y > 0) means nonzero in either
condition is enough to make the result nonzero.
NOTE: If both are true, then the result will be 2, not 1. In some cases, the 2 may not be acceptable. That can be fixed by wrapping the Or in another comparison – ((x > 0) + (y > 0)) > 0 – because 2 > 0 evaluates to 1!
- Not: I prefer to negate a condition by comparing it to zero: condition <= 0. 0 <= 0 is 1 (i.e., not 0), and 1 <= 0 is 0 (not 1). If you’re certain the logical expression will only ever be 0 or 1 exactly, you can also negate by subtraction: 1 - condition.
- Xor: Exclusive-or is true if one or the other condition is true, but not both. We can add the two conditions and compare it to 1. The syntax is a little bit tricky because ‘==’ doesn’t turn into a BinaryOpUGen automatically. We have to create the BinaryOpUGen by hand.