Instance Property


Expresses valid combinations of input and output channels.


var channelCapabilities: [NSNumber]? { get }


Array elements are NSNumber values containing integers.

The array index alternates between input and output counts, in ascending order of input/output channels—for example: [0] = first input count, [1] = first output count, [2] = second input count, [3] = second output count, etc.

Positive array values specify the number of input and/or output channels supported.

Negative array values have particular meanings. An input/output value pair of (-1, -1) (i.e. [0] = -1, [1] = -1) indicates that any number of channels are supported, as long as they are the same number for both input and output. An input/output value pair combination of -1 and -2 (e.g. [0] = -1, [1] = -2) also indicates that any number of channels are supported, but without the requirement that the input and output counts are the same. A negative value less than -2 (e.g. [0] = -16) specifies a total number of channels across every bus in that scope, regardless of how many channels are set on any particular bus.

An array value of 0 (e.g. [0] = 0) specifies that the input/output channel is not applicable (though typically only used for input channels).

Table 1 shows a sample selection of valid input and output channel combinations:

Table 1

Example Input/Output Channels

Array Index

Integer Value






First input/output count.

Any number of input and output channels, requiring that the numbers match. This is the default case.





Second input/output count.

Any number of input and output channels, without requiring that the numbers match.





Third input/output count.

Exactly two input channels, exactly six output channels.





Fourth input/output count.

Any number of input channels, exactly two output channels.





Fifth input/output count.

No input channels, exactly one output channel.





Sixth input/output count.

Up to four input channels, up to eight output channels.

This version 3 property is bridged to the version 2 kAudioUnitProperty_SupportedNumChannels API.

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