This thesis is focused on musical sound synthesis, in particular, the method of additive synthesis. The main goal is to implement a software musical instrument in the VST3 plug-in format, using the C++ programming language and the JUCE application framework. The final program offers spectral components editing capabilities and is able to morph between user-defined spectrum states in time. The introduction summarizes some common synthesis methods and their advantages or disadvantages. Next section deals with the technology used during the VST3 plug-in implementation and describes core parts that make up the final application. This analysis is focused mainly on the signal processing part, but there is also a brief description of the graphical...