Bear in mind that most VGMs are compressed (VGZ): you'll need to decompress them first. The program I use to split VGM files into 32KB chunks. This includes the second part of the emulated PSG, which handles bytes written to the PSG and updates its internal registers as appropriate.
These levels are loaded into capture compare registers for TIM2, which is used in PWM mode to generate the analogue output signals. Two output levels are generated as I've implemented the Game Gear's stereo extension to the PSG (this simply lets you switch individual channels on or off for each ear). This is executed approximately 44,100 times a second (44.1kHz is the internal time step used in VGM files) and is used to update the internal PSG counters and shift register and generate the output level for that particular sample. The first is an interrupt handler written in STM8S assembly for speed. The emulated SN76489, or PSG, has been implemented in two parts. The other mode is white noise, which uses a feedback system to generate pseudo-random noise.
This has effect of producing a lower pitch with a distinctive "buzzy" timbre. When generating periodic noise a single bit shuttles around the shift register, generating a 1/15th or 1/16th duty cycle square wave. It uses a shift register (15- or 16-bit depending on the particular version of the chip) instead of a simple tone counter, and has two modes. The fourth channel proves rather more of a challenge.
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