GPO and other Multi-Instruments in Logic Tutorial
by Victor Eijkhout

Setting up multi-timbral multi-channel instruments such Garritan Personal Orchestra, SampleTank, or Logic's own EXS24, is not entirely trivial. It takes a couple of tricks both in the environment and the arrange window. In this tutorial I will show you how to set up GPO, using a string section (2 violins, viola, cello, bass) as example.

Environment

Go to the environment window, Audio layer, and do the following. In this tutorial I have started with an empty environment.

Create a Multi Instrument.
Give it a descriptive name, and uncheck the first 8 channels.
Next you need the Audio Object that will actually hold the multi-instrument

and set it to be an Audio Instrument.

Load GPO into the Audio Instrument (Important! Load it as Multi-Instrument, not as Stereo.)
and cable the Multi Instrument to the Audio Instrument. (Ignore the strange dialog, Just click "Remove".)
(Not pictured but important) Set the Audio Instrument (not the Multi) to All channels.

We can now start loading instruments in our multi-channel instrument.

Load the first violin in the first slot of GPO. (The story is similar for other instruments.)
Similarly with second violins in the second slot.

You see that in GPO the midi channel for slot 2 is automatically set to 2. In other instruments (for instance SampleTank) you need to assign the slot explicitly to the midi channel.
The first two midi channels are set for output channels 1 and 2, which go to the Audio Instrument you have created. This means that they use the Pan settings that you set up in GPO. For all the other instruments, the story is more complicated.
Next you load slot 3, channel 3, with the violas.

You need to send this to the next stereo output of GPO, which is 3/4. However, this instrument will be handled by a mono object, so you need to Pan it all the way left.
Similarly, the next instrument, the cellos on midi channel 4, goes to the same 3/4 output but now panned all the way right.
The remaining instrument, the bass, goes in slot 5, midi channel 5, output 5/6, and is panned left.

You now have a multi-channel instrument with a bunch of instruments loaded. This stereo Audio Object will be used for the first two channels (the first and second violin in this example). All other instruments are handled with Aux objects.

Create a new Audio Object, call it Viola, and set it to the Viola midi channel, which is 3.
Turn this into an Aux Object.
The whole thing come together by declaring that the input for this object is the third channel of GPO.
Continue this, creating an Aux object for channels 4 and 5 of GPO.
These Aux objects are mono, so you have to pan them individually. What all this work has given you is that you can now set individual effects, or amounts of Bus send, on each channel.

We're mostly there. Now we can actually start making music in the Arrange window, but there are a couple of gotchas in that too.

The notes you are going to play will be a Midi sequence, so in the Arrange window you need tracks that have Midi input. These are the channels of the original Multi Instrument. (It's annoying that the channels are all called Grand Piano; it's easiest just to remember what they really are.)
At least you can give the track a decent name.
This way you make tracks for all channels, and put some music on them.
You have probably noticed that the channel strip you get for these instruments is not what you are used to seeing. The one you really want you can get by creating another track that contains the Aux object. Here you see a track for the Cello Aux object created, under the Cello midi sequences.
This track you can use for automating the effects and that sort of thing. Here I am modulating the frequency of a low shelving EQ.

And that's it. Now you'd better save this environment so that you can load it next time without needing half an hour for setup.