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VOCALOID API refers to the core API of the VOCALOID engine which is used by all versions of the VOCALOID software.
The VOCALOID API is essentially the ensemble of the core processes that build VOCALOID vocals with no additional functions built upon it. Every VOCALOID vocal cross references this engine in some way or a version of it. It is rarely referenced unless it is just to the API itself. The VOCALOID API is designed to allow Yamaha to sell VOCALOID as a product to 3rd parties under license and use the engine for themselves, adapting it for the 3rd parties use. This allows VOCALOID to be used for video games and other software as a automated voice. The API has most of the functions of the normal VOCALOID3 or VOCALOID4 engine and can cross reference functions such as Cross-Synthesis.
Because not all versions call it by name as the "VOCALOID API", the history or overall inclusion of this variant in software of the API is vague outside of major VOCALOID software updates which mark updates of API. The older VOCALOID2 version is known often as "VOCALOID2 lite" by those who discovered its existence once the vocals of Masaoka Azuki and Kobayashi Matcha were inserted into VOCALOID3. The VOCALOID engine has always made use of being built of separate parts interacting with each other, as even VOCALOID had its engine built in Spain, but the interface built in Japan.
Due to the limit knowledge of the API, no much is known about it, such as the way developers who have made use of the API also has varied over time or the full extent of the usage, and more uses of the API may exist. It has rarely been noted as being in use and only Unity with VOCALOID and Piapro Studio have noted they have been using the API. A variant of this is seen with VOCALOID-flex, though there is little known about this version. Project 575 also used a version of this for their mobile game, this being a VOCALOID2 version. So the idea of the API has existed long before it was widely known. "Daigasso! Band Brothers P" also used the API for its game though fewer details are known.
The full extent of the usage of the API is unknown outside of VOCALOID itself, though has given rise to many of the alternative versions of VOCALOID2, VOCALOID3, VOCALOID4 within the franchise. The eVocaloid chip has also uses a built-in version of the API and all major version of VOCALOID use it.
Despite some myths over time about VOCALOID itself, all VOCALOIDs have the same potential as each other if they share the same API version. This means that only things such as samples, language set up and many other issues are often the flaw of a voicebank and not the API itself. However, any flaws with the VOCALOID API are shared by all VOCALOID voicebanks equally. Older versions are essentially less efficient at handling larger voicebanks than VOCALOID3 and VOCALOID4 versions of the API.
Another note is even within a engine version the API has seen an update. The VOCALOID4 API did not originally support VOCALOID3 XSY, though due to an update of this function, allowing XSY groups to form, VOCALOID3 and VOCALOID4 could finally XSY.
3rd Party usages
Unity with VOCALOID uses the API (version currently unknown) to produce live performances of synthesized sound. This allows the user to make music on the go and adapt it. It does not include a full editor and simply supplies the engine alone. Different layers of functions come with the API as a result.
Piapro Studio included the VOCALOID3 API with Hatsune Miku V3 and Hatsune Miku V3 English, MEIKO V3 and KAITO V3. Upon purchasing the VOCALOID4 softwares Megurine Luka V4X, Kagamine Rin & Len V4X, Kagamine Rin & Len V4 English, Hatsune Miku V4 English, Hatsune Miku V4X, Piapro Studio 2 was installed. Unlike Unity with VOCALOID, Piapro Studio has full access to most functions of VOCALOID4. It does not include the VOCALOID3 or VOCALOID4 interface and has its own instead, but otherwise functions as a full replacement for the VOCALOID software. Hatsune Miku V4 Chinese also has a separate "Standalone" version of the Piapro Studio which only it can use for the moment that also uses the API.
A variant of this, though unannounced, is used in the game "Daigasso! Band Brothers P" for use with the game. This version allowed manipulation of the player's own voice into a rudimentary VOCALOID. Though results are low in quality, this is the first time a user has been able to use a version of the VOCALOID™ software to make a "VOCALOID".
Project 575 used the core engine for use of a mobile app, with the vocals of Masaoka Azuki and Kobayashi Matcha. This is also an unannounced version and is referred to as a "lite" version of the VOCALOID2 editor. The app used a basic version of the VOCALOID2 engine to synthesize the two girls's vocals and allow them to sing, but reacted according to the players skill at the game.