Performance Parameters

Performance Parameters are a set of eight Audio Unit parameter values (1000-1007) which can be configured to modify any of the Sampler’s subcomponents. These parameters are discoverable via the kAudioUnitProperty_ParameterList property, and they default to an “unmapped” state in which they control nothing, and by default are named “Performance Parameter 1” through “Performance Parameter 8". The input range for Performance Parameters is always normalized to -1.0 to 1.0, or 0.0 to 1.0, depending on what input is being controlled.

The advantage of these parameters over the other types of input connections is that they are discoverable (via the AUParameterInfo mechanism) by host applications such as Logic Studio, so they can take full advantage of the parameter automation features of such applications. They can be created and configured using the PERFORMANCE PARAMETERS panel in AU Lab’s editor for the Sampler.

In Figure 2, two Performance Parameters have been created and given them custom names. The first of these is selected to show that the parameter is mapped to the Pan input for the Layer.

MIDI Controller Messages

In order for the default Sampler Audio Unit (i.e., with no user preset loaded) to behave in a reasonable fashion in response to MIDI input, it is set up to use several of the standard General MIDI controller connections. Among these are controller #7 and #11 modifying gain, controller #10 modifying pan, and so forth. The input range of these messages conforms to the MIDI specification’s range of 0 to 127. The vast majority of the 128 total MIDI controller numbers are not set up by default to modify anything, and are available to use as control inputs for various Sampler subcomponents, which is useful if you have a complex preset.

Once a MIDI controller connection is configured and loaded as part of the preset, each particular subcomponent can be modified independently by sending MIDI control messages with the specific controller numbers (and values) to the Sampler via MusicDeviceMIDIEvent. Each of these connections may be created, modified, and examined using the INSTRUMENT CONNECTIONS panel in AU Lab’s editor for the Sampler.

In Figure 3 an additional connection has been created, mapping MIDI controller #55 to the Attack Time of the Layer's envelope.

Group Scope Parameters

The Sampler Audio Unit also supports a way to send control information to any MIDI controller connection (described above) using AudioUnitSetParameter() calls rather than sending MIDI events. When parameter values are set on the Sampler using the Audio Unit Scope kAudioUnitScope_Group, the parameter change is interpreted as though the parameter ID was a MIDI controller number. For example, the code in Listing 1 will produce the same effect on a Sampler which has a connection set up for MIDI controller #55:

Listing 1  

UInt8 channel = 0;

UInt8 controllerNumber = 55;

UInt8 controllerValue = 127; /* set it to max */

MusicDeviceMIDIEvent(theSampler,

                     kMidiMessage_ControlChange|channel, /* status combines the message type and channel */

                     controllerNumber,

                     controllerValue,

                     0);

AudioUnitSetParameter(theSampler,

                      controllerNumber,

                      kAudioUnitScope_Group,

                      (AudioUnitElement )channel,

                      (AudioUnitParameterValue) controllerValue,

                      0);

These parameters are not externally discoverable like the Performance Parameters, but using MIDI events you can still create MIDI control data which will, when played back via the Sampler, automate the changes to the subcomponents controlled by those connections.

What Settings Can I Control in this Fashion?

Here is a list of the subcomponents and their inputs that can be controlled using connections:

Envelope

Table 5  

Parameter	Units
Delay Time	Time Octaves**
Attack Time	Time Octaves**
Hold Time	Time Octaves**
Decay Time	Time Octaves**
Release Time	Time Octaves**
Sustain Level	Percent (1.0 == 100%)

It is important to remember that within any Sampler preset, the filters, oscillators, low frequency oscillators, and envelopes are defined at the Layer level, so presets with multiple layers have multiple independent sets of subcomponents, each of which can be controlled independently. If you open a preset with multiple layers in AU Lab’s INSTRUMENT CONNECTIONS editor, you will see the “destination” for each connection specifies both the layer number and the particular subcomponent input.

In Figure 4, a new connection is being created which will control a subcomponent in Layer 2.