Polyphony Tutorial 1: Using the poly~ Object

A different approach to polyphony

In an earlier MSP tutorial on using MIDI with MSP , we demonstrated how to use the poly object to make polyphonic voice assignments in a simple case. This chapter will describe a more elegant and efficient way to handle polyphonic voice allocation - the poly~ object.

In the example in the previous chapter, we created multiple copies of our synthesizer subpatch and used the poly object's voice numbering to route messages to different copies of the subpatch. Our example could just as easily have used any kind of sound-producing subpatch.

• Take a look at the tutorial patcher. Open the patcher object labeled 'thehardway'. The example inside uses the subpatch littlesynth~ to implement a simple four-voice polyphonic synthesizer:

While this method works, it has two disadvantages. First, there's a lot of housekeeping necessary to duplicate and patch the multiple copies of littlesynth~ together. But there is also a problem in terms of CPU usage. All four copies of the littlesynth~ subpatcher are always on, processing their audio even when there is no sound being produced.

There is a way to solve this problem - the poly~ object allows you to create and manage multiple copies of the same MSP subpatch all within one object. You can also control the signal processing activity within each copy of the subpatch to conserve CPU resources.

The poly~ object

• Close the 'thehardway' subpatch and open the patcher object labeled 'simple_poly'. Turn on the audio in the subpatch by clicking the startwindow message to the dac~. Click the toggle object at the top of the patcher and turn up the volume on the gain~ slider.

The poly~ object takes as its argument the name of a patcher file, followed by a number that specifies the number of copies (or instances) of the patch to be created. You'll want to specify the same number of copies as you would have had to duplicate manually when implementing polyphony the old-fashioned way.

• Double-click on the poly~ object. This opens up the subpatcher to show you the inside of the littlebeep~ object.

Let's look at the littlebeep~ patch for a minute. While you haven't seen the in, out~, or thispoly~ objects before, the rest of the patcher is pretty straightforward; it takes an incoming MIDI note number, converts it to a frequency value using the mtof object, and outputs a sine wave at that frequency with a duration of 140 milliseconds and an amplitude envelope supplied by the line~ object for 140 ms with an envelope on it.

But what about the in and out~ objects? Subpatches created for use in the poly~ object use special objects for inlets and outlets. The objects in and out create control inlets and outlets, and the in~ and out~ objects create signal inlets and outlets. You specify which inlet is assigned to which object by adding a number argument to the object - the in 1 object corresponds to the leftmost inlet on the poly~ object, and so on. The poly~ object keeps track of the number of inlets and outlets it needs to create when you tell it which subpatch to load.

Messages sent to a poly~ object are directed to different instances of the subpatch dynamically using the note and midinote messages, and manually using the target message.

When poly~ receives a note message in its left inlet, it scans through the copies of the subpatch it has in memory until it finds one that is currently not busy, and then passes the message to it. A subpatch instance can tell its parent poly~ object that it is busy using the thispoly~ object. The thispoly~ object accepts either a signal or number in its inlet to set its busy state. A zero signal or a value of 0 sent to its inlet tells the parent poly~ that this instance is available for note or midinote messages. A non-zero signal or value sent to its inlet tells the parent poly~ that the instance is busy; no note or midinote messages will be sent to the object until it is no longer busy. The busy state was intended to correspond to the duration of a note being played by the subpatcher instance, but it could be used to mean anything. In the example above, when the audio level out of the *~ is nonzero -- that iteration of the subpatch is currently busy. Once the amplitude envelope out of line~ reaches zero and the sound stops, that subpatch's copy of thispoly~ tells poly~ that it is ready for more input.

Muting voices

• Close the 'simple_poly' subpatch and open the patcher object named 'poly_using_mute'. Start it running as you did the last patcher, and double-click the poly~ object.

The thispoly~ object can also control the activity of signal processing within each copy of the subpatch. When the mute message is sent to thispoly~ followed by a 1, all signal processing in that subpatch stops. When a mute 0message is received, signal processing starts again.

In this patcher, we've rewriten the littlebeep~ subpatcher to take advantage of this by turning off signal processing when a note is finished and turning it on again when a new event is received. While this doesn't change the function of the patch, it would be more efficient, since the amount of CPU allocated is always based on the number of notes currently sounding.

Why do we care about efficiency? Well, even though modern computers perform at a level not imaginable fifteen years ago, they still have limits. Audio processing constantly pushes those limits in a way little else does. (After all, the internet does not mind a short pause during an upload.) When the limit is reached, it is audible as clicks and distortion. That is why there is a non-realtime output option. (See Audio I/O - Audio input and output with MSP ) Now consider this patch:

A silent MSP patcher

As shown, the patch is making no noise. It will, as soon as it receives a note number, but for now it is on standby. However, it still requires computation. The signal from the cycle~ object is multiplied by 0, so nothing is heard, but multiplies are happening 44,100 times a second, not to mention whatever else cycle~ does to produce a signal. Once enough patches, plug-ins and so forth are open, something like this can be the "straw to break the camels back", so disabling audio that is not currently needed is a good idea. The littlebeep2~ subpatcher demonstrates the use of the mute message to thispoly~. We can disable individual voices in poly~ with a mute message to the parent poly~ object. This takes the form of mute n x where n is the voice number and x is 1 for mute and 0 for not muted.

If we send a poly~ the message mute 0 1 all audio processing in the poly~ stops. This is a powerful tool for managing DSP resources, even if you only need one copy of some process. There are certain objects, such as pfft~, (discussed in Signal Processing with pfft~) that can be quite computationally expensive. Yet the uses of pfft~ are varied enough (pitch changing, precise filtering, and vocoding, among others) that a patch could easily wind up with a dozen or so. This is enough to account for 30% of the CPU on a medium powered machine. Encasing each pfft~ routine in its own poly~ will allow you to disable all that are not currently in use. (Note that once you have muted the entire poly~ with mute 0 1 you cannot start a single voice with something like mute 1 0 until you have sent a mute 0 0.)

Targeting individual voices

• Close the 'poly_using_mute' subpatch and open the one named 'poly_using_target'. Start the program and examine the patcher logic both inside and outside the poly~ object.

Another way to allocate events using poly~ is through the target message. Sending a target message followed by an integer in the left inlet of a poly~ subpatch tells poly~ to send all subsequent messages to that instance of the subpatch. You can then use poly~ in conjunction with the poly object to create a MIDI synthesizer.

In this example patcher, pairs of incoming MIDI pitches and velocities are used to synthesize a sine tone. When a list is received, the subpatcher sends a bang to thispoly~, causing it to output the instance or voice number. In our tutorial patcher, the voice number is sent out an outlet so you can watch it from the parent patch.

In the parent patch the poly object assigns voice numbers to MIDI pitch/velocity pairs output by makenote. The voice number from the poly object is sent to poly~ with the target message prepended to it, telling poly~ to send subsequent data to the instance of the targetbeep~ subpatcher specified by poly~. When a new note is generated, the target will change. Since poly keeps track of note-offs, it should recycle voices properly. The second outlet of poly~ reports the voice that last received a message -- it should be the same as the voice number output by poly, since we're using poly to specify a specific target.

Using poly~ for audio processing.

• Close the 'poly_using_target' patcher and open the one named 'poly_using_signal_input'. Turn on the audio, raise the gain~ slider, and start the metro by clicking the toggle object. Change the value in the rightmost inlet from 100. to 50. and listen to the result.

The floating-point number box can be used to specify parameters to specific instances of a poly~ subpatcher. By connecting a loadbang object to thispoly~, we can use the voice number to control the center frequency of a filter.

• Open the littlefilter~ subpatch by double-clicking the poly~ object.

The littlefilter~ abstraction uses the voice number from thispoly~ and multiplies it by the base frequency received in the second inlet. The incoming signal is filtered by all sixteen instances simultaneously, with the output amplitude of each instance being controlled by an integer coming into the first inlet.

The metro object in the main patcher is hooked up to both a counter and a random. The counter, which feeds the target message, cycles through the 16 voices of littlefilter~ loaded into the poly~ object, supplying each with a random number which is used to control the amplitude of that voice.

A signal connected to an inlet of poly~ will be sent to the corresponding in~ objects of all subpatcher instances, so the noise~ object in the example above feeds noise to all the subpatchers inside the poly~. The second inlet (which corresponds to the in 2 box in the subpatcher) controls the base frequency of the filters. Note that for the frequency to get sent to all poly~ iterations it is preceded by a target 0 message. You can open a specific instance of a poly~ subpatch by giving the object the open message, followed by the voice you want to look at.

• Open voice 15 of the littlefilter~ abstraction by typing that number in to the number box attached to the open message. The patcher assigned to voice number 15 should look like this:

As you can see, the base frequency of this particular iteration of littlefilter~ is 1500. Hz, which is the multiple of the voice number (15) with the most recently entered base frequency into the second inlet (100. Hz).

Summary

poly~ is a powerful way to manage multiple copies of the same subpatch for polyphonic voice allocation. The thispoly~ object works inside a subpatch to control its busy state and turn signal processing on and off. The objects in, in~, out, and out~ create special control and signal inputs and outputs that work with the inlets and outlets of the poly~ object. The ability to disable audio processing with mute messages makes poly~ avaluable tool for managing the CPU load caused by standby audio processes.

MSP Polyphony Tutorial 1: Using the poly~ Object