Using Buses in SuperCollider

Control Rate vs Audio Rate

SC works with two internal signal types or rates. When something is used with the extension .ar, this refers to audio signals (audio rate), whereas .kr uses the control rate. For both rates, buses can be created.

Creating Buses

An audio bus with a single channel is created on the default server s with the following command:

~aBus = Bus.audio(s,1);

A control bus with a single channel is created on the default server s with the following command:

~cBus = Bus.control(s,1);

Bus Indices

The variable ~aBus is the client-side representation of the Bus. The server only knows it by its bus index. Bus indices are counted upwards and can be queried with the following command:

~aBus.index
~cBus.index

The indices of user-defined audio buses start counting after all output an input buses. The number of input and output buses can be defined before booting a server. The default setting uses 2 input and 2 output buses.

Audio buses

Indices

Audio Buses

0...1

Outputs

2...3

Inputs

4

First user-defined bus

The number of input and output buses can be queried after boot:

s.options.numOutputBusChannels;
s.options.numInputBusChannels;

Audio Input

The SoundIn UGen makes it convenient to access the audio input buses without keeping track of the outputs. This node simply passes the first input to the firs output:

{ Out.ar(0,SoundIn.ar(0))}.play

Note that this is equivalent to using the proper offset with a regular audio input:

{ Out.ar(0,In.ar(s.options.numOutputBusChannels))}.play

Monitoring Buses

Any bus can be monitored with the builtin scope with the following command. The first argument defines the number of buses to be shown, the second the index of the first buses:

s.scope(1,~aBus.index,rate:'audio')

There is a short version, which has limitations and does not specify the bus type:

~aBus.scope()

Frequency Scope

Any bus can also be monitored with a frequency scope. The first arguments define the size. The third argument defines the bus to analyze, in this case the first output bus:

FreqScope.new(400, 200, 0, server: s);

Control Buses

This simple sawtooth node will be used for showing how to use control buses. It has one argument freq, which affects the fundamental frequency and uses the first hardware output:

~osc  = {arg freq=100; Out.ar(0,Saw.ar(freq))}.play;

Mapping a Control Bus

The map() function of a node can connect a control bus, identified by its index, with a node parameter:

~osc.map(\freq,~cBus.index);

Setting a Control Bus

After mapping the bus, the synth stops its sound., since the control bus is still set to the default value 0. This can be visualized with the scope command. A simple and quick way for changing the control bus to a different value is the set() function of a node. It can be used for all arguments of the node which are internally used for control rates:

~cBus.set(50);

Multichannel Buses

Both control and audio rate buses can be created as multi channel buses. A scope will automatically show all channels. Individual channels can be mapped with an offset in relation to the index of the first channel. The setAt() function can be used for changing individual channel values:

~mBus = Bus.control(s,8);

~mBus.scope;

~osc.map(\freq,~mBus.index+3);

~mBus.setAt(3,150);