Yamaha S-YXG50 was released in 1997. First versions were shipped with the simplified 2MB version of the wavetable, and the full 4MB version was published only in 2001. It seems that the cause of this is low amount of RAM in the computers of 90s. Both versions of the wavetable files were created in 1996 (4MB in February, simplified 2MB in July). Possibly, the full 4MB version is a dump of the WAVE ROM of the Yamaha MU80 / MU50 / QS300 / DB50XG / DB60XG / SW60XG / MU10, because the Yamaha S-YXG50 soft synthesizer is very similar to this series of hardware of 1994-1996.
The first software synthesizer from Yamaha was S-YG20, which was released in 1996. It allows only 22050 Hz sample rate, it supports only GM1 (General Midi level 1), and uses the 1.3MB wavetable file, which was created in July 1996, and it is obviously based on the 4MB wavetable of the S-YXG50. So, even though S-YG20 was released a year earlier than S-YXG50, the source of the wavetable is the same, and S-YXG50 offers the original 4MB version with much better quality. Variants of this set of samples had been used by Yamaha for almost 10 years in various hardware and software products. For example, a reduced version of this set of samples was used in the sound cards based on the Yamaha YMF7xx chipset series.
From 1996 to 2003, Yamaha also had released other software MIDI synthesizers based on the different versions of the S-YXG50 engine. In addition to the S-YG20 and S-YXG50, a little more advanced the S-YXG100 was released. It uses the same wavetable as the S-YXG50, but it offers a couple of additional sets of synthesized instruments (VL/PolyVL and SG extensions). Unfortunately, Yamaha had not ported it to the modern versions of Windows, so S-YXG100 is available only as a VxD driver for Windows 98, and it can not be used on the modern operating systems. However, the extensions of the S-YXG100 hadn't become popular, so no great loss there.
Also two special versions of this synthesizer were created. In 1998, S-YXG70 was released for the Final Fantasy 7 game. It is just a hybrid of the S-YG20 and S-YXG50, which allows on-the-fly switching between S-YXG50 and S-YG20 modes. S-YG20 requires much less system resources, so it is used in the most dynamic game scenes. In 1999, the DM extension was released for the Final Fantasy 8. It allows using of instruments from any external DLS file. Unfortunately, both of these special versions of the synthesizer are available as VxD drivers for Windows 98 only.
In 2003, Yamaha released a VSTi version of the S-YXG50 as a part of the Yamaha SOL2 package (S-YXG50.DLL). Also a limited version of the S-YXG50 VSTi (XGLite only, no GUI) was used as an internal component of the Yamaha MidRadio 6 (SGP.DLL). Soon, Yamaha discontinued all its software synthesizers. It seems that it motivated by decision to avoid unnecessary competition for Yamaha's hardware synthesizers. The VxD and WDM driver version of the S-YXG50 have a significant delay, which is suitable for the playing MIDI, but it is not acceptable for the writing music. VSTi-version doesn't have this issue just because VSTi-synthesizers are intended for the writing music. Fortunately, all VSTi versions of this synthesizer have a full version of the S-YXG50 engine. Thus, it is possible to remove trial restrictions in the S-YXG50.DLL and unlock Full XG support in the SGP.DLL.
However, it seems that Yamaha continued to develop its software synthesizer for internal using. Yamaha MidRadio 7 was released in 2006, and it uses SGP2.DLL for the MIDI playback. SGP2.DLL is a VSTi synthesizer (without a GUI), which uses heavily reworked code of the S-YXG50 VSTi. Its name is S-YXG2006LE. This synth sounds much different from the S-YXG50, because it uses the new wavetable, which uses the new high quality samples. But sometimes it sounds unbalanced, and the main drawback of the S-YXG2006LE is XGLite only support. At this moment it is unknown if it is possible to unlock Full XG support in this synth.