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Class-compliant (no-driver) operation vs. driver install (midi devices)
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Q: My M-Audio Series II MIDI Device works without installing any driver in Windows XP, Windows Vista, and Windows 7 (this is also called "class-compliancy").  Why is there a driver available for download and when would I need to use it?

A: M-Audio Series II MIDI Devices can take advantage of standard USB drivers built into Windows.  If you connect your device to your Windows XP, Windows Vista, or Windows 7 system without first installing the M-Audio driver written specifically for that product family, Windows will automatically detect the device and load these built-in "class compliant" drivers.  Performance under the Microsoft class compliant driver is excellent, but users may wish to install and utilize the M-Audio driver if they plan to use their Series II MIDI Device in one of the following situations.  These situations are not supported by the Microsoft class compliant driver:

Multiple Clients

The M-Audio driver for USB MIDI controllers supports multiple applications opening both its MIDI In and Out ports simultaneously. This allows a user to have multiple applications running on the PC and to dynamically switch between them instead of having to shut down one application to free up the MIDI port so that another application can then use it. This is not possible when connecting the device without installing a driver.

Multiple devices

The M-Audio USB driver allows you to use Multiple devices of the same product family at the same time.  Thus more than one keyboard (e.g. Keystation Pro 88 and Trigger Finger) can be connected to the same PC simultaneously and be recognized and used by the system.  This allows a user to add drum trigger pads to his keyboard controller, or use multiple keyboards to control a Hammond B3 synthesizer supporting two keyboards, or for a user and one or more friends to jam using multiple keyboards connected to the same PC system.  These multiple instances can be a mix of different or the same keyboards from the M-Audio USB MIDI controller product family.  When multiple keyboards with the exact same product ID (PID; e.g. two Keystation 49e keyboards) are connected at the same time, the MIDI In and Out port names for these keyboards have a number appended so that the user can distinguish between them in the MIDI application software.  This is also not possible without installing the driver provided by M-Audio.

SysEx Messages

The M-Audio MIDI driver is also required to send or receive long SysEx (system exclusive) messages.  This is because the Windows output MIDI buffer is 1024 bytes.  Larger SysEx messages can exceed the size of the buffer and can become corrupted.  The M-Audio MIDI driver does not have this limitation.