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Introduced by Yamaha in 1994, XG (Extended General MIDI) is a proprietary extension to the General MIDI (GM) standard. It is similar in purpose to Roland's GS standard from 1991, but on a wider scope. Products compatible with the XG standard carry the XG logo.
As with the original General MIDI standard, the XG specification is a framework to ensure predictable playback of MIDI song data, and to facilitate the exchange of song data across compatible systems. This was seen as having natural benefits for musicians, but also for the wider computer market, although games had generally started to move away from MIDI technology at the time of its introduction.
Compared to the GM standard, which specified a fixed sound set of 128 normal instrument voices and a single drum kit, the XG standard included provisions for a larger palette of available instrument and drum sounds (480 normal voices and 11 drum kits minimum) and the means for calling these up using Bank Select MIDI messages. It also prescribed a widened set of controllers and parameters, also accessible via MIDI, that composers could employ to adjust the basic sounds and achieve greater subtlety and realism in their compositions. The XG standard stipulated an external input for instruments/microphones, which could be processed with the built-in effects and mixed with the synthesized sound.
The XG specification introduced a multi-purpose "Variation" effect processor in addition to the global Chorus and Reverb effects found in General MIDI devices. The Variation processor can be employed as a 'system' effect shared by all parts, or as an 'insertion' effect applying processing to a single part. The Variation block features effects like rotary speaker, compression, distortion, guitar amp simulation, wah-wah, etc. Yamaha's in-house songwriters often utilized these tools to demonstrate the power of the XG format, notably recreating guitar leads complete with feedback, flamenco guitar with distinct picked/hammered notes and finger slides, growling saxophones, and even a very convincing sitar.
Employing a scaled-down version of Yamaha's AWM2 (second generation Advanced Wave Memory) digital tone generator technology, the first generations of XG devices included an onboard 4 MB wave ROM chip containing sampled instrument sounds. Later products increased the size of the wave ROM as new instrument voices were added and sample quality was improved.
XG would eventually also include support for the PLG series plug-in expansion boards, that could be installed in compatible synthesizers. Available boards included models based on virtual analog, virtual acoustic and FM-synthesis technologies, a vocal harmony effect, and high-grade AWM2-based percussion and piano samples.
Many XG-compatible tone generators can be switched into a so-called TG300B mode, which provides access to extra instrument and drum sounds. This operating mode takes its name from an earlier tone generator module from Yamaha, the TG300 (1994), which predated XG, but in its B-mode offered unofficial compliance with the Roland GS standard. In 2001, Yamaha certified their MU500, MU1000 and MU2000 tone generator modules, as well as the S-YXG50 software synthesizer, for a licensed GS mode. On the hardware units, the GS mode replaced the TG300B mode via a free firmware update.
The XG standard evolved over time, and two superseding levels were added as products became more advanced:
| Specification | Tone Generator | Available instrument sounds (minimum) |
|---|---|---|
| XG Level 1 (1994) | 16 channels 32 note polyphony | 480 normal instruments 11 drum/SFX kits |
| XG Level 2 (1997) | 32 channels 64 note polyphony PLG support | 1074 normal instruments 36 drum/SFX kits |
| XG Level 3 (1998) | 64 channels 128 note polyphony PLG support | 1149 normal instruments 37 drum/SFX kits |
Additionally, a subset of XG known as XGlite was introduced in 2001. XGlite offers a set of 361 instruments (381 in some models), as well as a reduced number of available effect parameters and controllers.
Although the XG and XGlite specification do not actually specify polyphony, compatible tone generators generally provide a shared 32 notes of polyphony (when single-element voices are used) per 16 parts.
