The MSX-EQ PSG Spectrolyzer is a simple cartridge for the MSX platform that visualizes the spectrum of notes played by the programmable sound generator, such as the AY-3-8910 or YM2149 (PSG). The cartridge board is intended for the standard MSX cartridge slot. The board shows the effect of measuring the signal's level, in which each reproduced frequency (or frequency range) corresponds to one of 9 vertical LED indicators.
Design of the cartridge is from: RBSC
See also: https://github.com/RBSC/MSX-EQ