Genesis:Audio Chip Notes
At the heart of the Genesis/Mega Drive's sound hardware is the Yamaha YM2612 FM synthesizer chip, which is descended from the YM2203 chip which Sega had used in numerous arcade games (i.e. Hang-On, Space Harrier, others). The YM2612 has six 4-operator FM synthesis channels, with the sixth channel capable of 8-bit PCM sample playback. Unlike the YM2203 and the YM2151 which Sega had also used in arcade hardware, the YM2612 also has a built-in stereo DAC (digital to analog converter), saving both motherboard space and production costs. Accompanying the YM2612 are four PSG audio channels (three square wave and one noise) which originate from the VDP. The PSG core used in the VDP is based on the Texas Instruments SN76489, and was also used in the Master System and SG-1000. In many Genesis games, the PSG is typically used for sound effects, but was also used to great effect in some games for lead and background melodies.
Quantization Noise aka "Ladder Effect"
The YM2612 has a peculiar flaw with its 9-bit DAC, which causes low volume sounds to be quantized in an unusual way and is especially apparent during fades. Some composers were highly aware of this design flaw and used it in their compositions, with Yuzo Koshiro in particular being a fan of the technique.[1]
YM3438
The YM3438 was marketed as an improved version of the YM2612. Unlike the YM2612, it uses CMOS logic (as opposed to NMOS in the YM2612) which reduces power consumption and heat, and Yamaha also greatly reduced the "ladder effect" present in the YM2612. Sega began using the YM3438 in its discrete form in arcade hardware such as the System 32 and Genesis-based System C. The discrete YM3438 was also used in the TeraDrive, a combination of a Mega Drive and IBM-compatible PC which was only released in Japan.
Starting in 1992, Sega phased in the use of the Yamaha FC1004 ASIC in Genesis hardware, which combined the VDP, I/O, and a YM3438 into a single chip. The first console to use this ASIC was the original Wondermega, followed by the VA7 model 1 Genesis. All future variants of the Genesis would use this ASIC or later versions of it. The only two exceptions to this are the VA2 and VA2.3 model 2 consoles, which went back to using the YM2612 as the ASIC used in these models did not contain the YM3438 core. It is unknown why Sega chose to use the 2612 when they had already used the discrete 3438.
VA7 Model 1 and VA0-VA1.8 Model 2
In the VA7 model 1 and VA0 through VA1.8 model 2s, the YM3438's sound is largely spoiled by poor component choices used in the audio circuitry of these systems. These consoles tend to have very muffled sound with copious amounts of distortion, and are among the worst sounding Genesis systems as a result. Thankfully, this can be rectified by using audio modification boards such as the Mega Amp or the audio portion of the Triple Bypass, which use the raw audio straight from the ASIC and put it through a different amplifier circuit, resulting in much cleaner sound. The existing circuitry can also be altered with different components to improve the audio quality without resorting to more invasive mods.