MSP Polyphony Tutorial 1: Turning Signals On and Off

Click here to open the tutorial patch: Media:01hTurningSignalsOn-Off.maxpat

Introduction

In the next group of tutorials we'll look at ways to manage the signal chain resources in MSP patchers. We'll start by looking at ways to selectively turn on and off signal processing within patcher logic; this allows us to effectively 'mute' parts of our patchers so that they don't process any samples when we don't want them to.

DSP Resources in MSP

Consider the following piece of patcher logic:

A silent MSP patcher

As we can see, this patcher, as illustrated, makes no sound - the volume of the line~ has been set to 0 by the number box. However, the signal processing chain computing the samples is still going: the cycle~ object is still sending signal vectors that resemble a full-amplitude cosine wave, the line~ object is sending a constant signal value of 0, and the *~ object is multiplying the two together, generating values of 0 as well. The overhead of this patcher on our computer would be minimal, obviously, but more in situations where we create complex signal processing networks that use large amounts of CPU, it's useful to be able to disable the parts we're not listening to.

When an MSP patcher is created and the audio is turned on, the Max program builds a function graph of all the signal (MSP) objects within the patch. This graph is then organized so that the objects pass their samples to one another in a logical way, much in the way that Max objects pass messages based on their visual layout within the patch. MSP objects communicate by passing packets of audio samples called signal vectors to one another. When an audio object is silenced by passing it through a volume control (e.g. a *~ object), it is still passing values. If we want to actually turn of signal processing within part of a signal network in MSP, we first have to design the patcher so that parts of it can be declared as 'mutable'.

Take a look at the tutorial patcher. The main (lower) part of the patcher consists of an MSP signal network with a selector~ object in-line before the final volume scaling. The selector~ object is the opposite of an MSP gate~ object, and is roughly analogous to the Max switch object; the left-hand inlet allows us to choose which signal input to listen to when sending signals to the output. The argument to selector~ determines how many selectable inlets there are in addition to the leftmost ('chooser') inlet.

• Start the audio in the patcher by clicking the message box

labeled startwindow at the top of the tutorial patcher. Using the umenu labeled 'Wave Type' at the top of the patcher, select the different waveforms and listen to their results. Each selection enables a different inlet in the selector~ object.

The startwindow message to a dac~ or ezdac~ object turns on the audio in one patcher only (as opposed to all open patchers in Max). If other audio-generating Max documents are open, it mutes them.

Begin the begin~

Under normal circumstances, the selector~ object would switch what part of the signal processing chain you were listening to, but wouldn't turn off any of the signal chain, i.e. all of the objects connected to all of the inlets of the selector~ would still be generating their signal vectors, regardless of whether they were actually sending any audio past the selector~ or not.

However, in our tutorial patcher different parts of the signal processing chain are being turned off. The begin~ object declares the start of a signal-processing chain; other than that, it does nothing (it outputs a constant value of 0 no matter what happens). When a selector~ or other signal-switching MSP object cuts off a signal-processing path with a begin~ at the top, the begin~ object mutes everything downstream, freeing their DSP resources.

• Double-click the patcher object named triangle.

Notice that there is a begin~ object inside. Notice also that the triangle~ object needs a sig~ object to set its frequency when used in conjunction with begin~. In our tutorial patcher, the sawtooth and triangle waveforms (and any accompanying MSP objects connected to them) are disabled automatically when their inputs are not being selected by the selector~ object. The cycle~ object at the left is always processing samples. The patcher on the right (labeled square) is as well; we have a different strategy for muting it.

Two ways to mute a subpatcher

• Enable the 'Square' option from the umenu so that you

hear the square wave. Click the toggle object connected to the mute~ object in the patcher. Notice that, when checked, the square wave is silenced. Click it again to hear the square wave. The mute~ object in MSP allows you to disable signal processing in a patcher or abstraction. This lets you use subpatchers of either type as self-contained signal processing networks that you can turn on and off through patcher logic. In order for the mute~ object to work properly, however, one more object is needed, within the patcher being muted.

• Double-click the patcher named 'square'. Connected to

the rect~ object is an object called pass~. The pass~ object performs a very important function when muting and unmuting parts of the signal processing chain - it zeroes out the signal vector coming out of the patcher or abstraction. In our example, we are using mute~ to switch off a patcher containing a rect~ object, which outputs -1 or 1. When we mute it, we want it to output a 0, in case we have its output mixed in with other elements of our signal chain. Otherwise, it will put out its last (un-muted) value as a constant signal, leaving us with DC bias in our audio network. The pass~ object guarantees this won't ever happen.

• Switch the sound off in the main patcher by selecting 'Off'

from the umenu. In the area of the tutorial patcher labeled '2' (in the upper-right), click the toggle object connected to the message box containing the message enable $1. Double-click the patcher object named 'harmonics'. Inside, turn on and off the selector~ object controlling the output for the cycle~ object on the right. Back in the main patcher, click the toggle again to mute the subpatch. A second way to turn on and off signal processing in a patcher or abstraction is by using the pcontrol object. The enable message to pcontrol turns off all activity within a subpatcher, both MSP signals and Max scheduling. Notice that within the subpatch, we have a number of cycle~ objects computing different harmonics of the fundamental frequency used throughout the tutorial patcher. In the subpatch, the selector~ object allows us to disable the fundamental frequency. Even with the fundamental missing, our ears still resolve the same pitch.

Summary

There are a number of strategies for turning on and off parts of a signal chain in MSP. The startwindow message to a dac~ turns on audio only in one open Max document. The begin~ and selector~ objects allows us to declare and selectively disable parts of an MSP signal processing network within a Max patcher. MSP signal objects within patcher objects and abstractions can be disabled as well, either by using a mute~ object or the enable message to a pcontrol object. The pass~ object, when used within a mutable subpatch, zeroes the audio coming out of the subpatch to prevent clicks or DC noise.

See Also

selector~ - Assign one of several inputs to an outlet

pcontrol - Open and close subwindows within a patcher