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The Motif-Rack XS has Voice compatibility with the Motif XS. It shares the same 2,670 Waveforms in its resident Wave ROM. Therefore it is able to IMPORT any Motif XS VOICE libraries dependent on that exact Wave ROM. (As you are probably aware the keyboard Motif XS was RAM expandable via optional DIMMs - which allowed the instrument to extend its available Wave ROM - the Motif-Rack XS did not have this capability and therefore will NOT be able to IMPORT any new Wave data). This means you will NOT be compatible with any SAMPLE Libraries for the Motif XS; VOICE Libraries ONLY - those dependent on the resident Wave ROM.

In order to load the VOICES from a Motif XS Library you would use the "Yamaha Motif-Rack XS Editor Standalone/VST version 1.6.4" - Available (FREE) from http://download.yamaha.com
Go to FILE > IMPORT > Click on the OPEN FOLDER > Set the "FILES OF TYPE" option = "Motif XS Library (*.X0A)

You will see the types of Files your Motif-Rack XS can import. (Motif XS, Motif ES, MO6/MO8, Motif-Rack ES)

This will allow you to load the individual VOICES from a Motif XS Library.
This does not include PERFORMANCES, MASTER Programs, SONGS, or PATTERNS... only the Voices (obviously, because the Rack XS does not have any of these modes).

For example, at www.yamahamusicsoft.com you will find VOICE Libraries for the Motif XS. As long as they do not include/require RAM Sample data, you can IMPORT the Voice data to a Motif-Rack XS. So when reading the DETAILS on any Motif XS Library make sure it does not require RAM expansion - if it does you will NOT be able to successfully utilize that data. But there are many Libraries that are available and are compatible with the Rack XS.

Axxe
Vintage Keys
Pulse
B's Knees
Brass
etc.,

Read the descriptions carefully - if it mentions SAMPLES or RAM, you CANNOT use it. However, you can IMPORT Voices from a Motif XS ALL data file (.X0A), or ALL VOICE file (.X0V).

Hope that helps.
If you are not sure about a Library - please feel free to ask.