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Genesis and Sega CD – Audio Filtering

This is a followup to the previous post on the Sega CD’s PCM sound chip. This post will start by going into more detail on why this chip’s audio output sounds pretty crummy by default, followed by one of two possible solutions that I know of to that problem.

At the end of the last post, I gave an example recording of the boss theme from Lunar 2: Eternal Blue, and I noted that it plays through the PCM chip with a sample rate of 0x0400 / 16276 Hz. One service manual has low-level circuit diagrams at the capacitor/resistor level, and I was able to identify where the two PCM low-pass filters are (there’s one for each of the two output channels), but I have exactly zero expertise in making sense of these. Here’s part of the Harley Quinn boss theme from The Adventures of Batman & Robin, which does some wild things with the YM2612 (also a good test of an emulator’s audio resampling code):

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