This paper describes a set of procedures, named Toolbox, for spectral data reduction to be used in additive synthesis system based on sinusoidal model. The Toolbox has been developed in MATLAB with the specific aim to support the realization of a very large scale additive synthesis system (AddSynth) based on special purpose VLSI chips. Test signals and musical tones played by real acoustic instruments, in different conditions of execution: intensity, pitch etc, are considered. The Toolbox mainly performs spectral envelope data extraction, data reduction, spectral interpolation and produces proper data block set for both the AddSynth machine and the related software simulator.