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SEGA Mega Drive/Genesis - why was a filter used?

  • I'm not into digital audio that much. I understand how a low-pass filter works but I don't quite get why it was used on the SEGA Mega Drive.

    When using Genesis Plus GX, I feel like the audio is (and should be) much crisper and pleasant with the low-pass filter turned off.

    There's no clear answer to this when googling. Why did SEGA filtered their console's audio? What confuses me even more is that it seems (according to schematics) that even CDDA (audio from CDs) was going through the filter!

  • Global Moderator

    this seems a good explanation:

    TLDR: without the filter you get distortion. i am not sure if emulators with a 'low pass filter off' function accurately emulate these drawbacks. i'm curious to try it, now!

  • Interesting link and I've also been reading technical documents and diagrams since I posted this.

    It appears that cheap mass produced DACs at the time had severe distortion issues so a pass filter would "smudge" out those annoyingly pitched misinterpretations that occured. However, the YM2612 itself does already contain an integrated DAC according to wikipedia. Would that mean it's flawed by design?

    I also don't understand why quantisation noises occurred, while the same family of chips were used in keyboard instruments and synthesizers without filters. Could it be related to the particular samples that were used in the SEGA console?

    From what I've read previously, early Model 1 Mega Drive systems give out the best possible sound and that would roughly equal to filtered emulation set at 55%. I'm still not sure what happens with later models that indeed sound inferior. Maybe they give out unfiltered audio?

    As for Genesis Plus GX, with the filter off I don't get any annoying sounds. On the contrary, everything sounds much more "complex" and deep. I assume that it sounds more like it was intended to but the DAC within the chip couldn't deliver properly? Or could it be that I don't quite get the quantisation noises and I'm happy with a distorted sound?

    Edit: it gets more interesting in the following topic

    I'm suspecting that unfiltered YM2612 emulation gives out unpleasant pitches that my ears can't get because I'm in my 30s! I'm switching my Genesis Plus GX settings to MAME (asic YM3438) with the low-pass filter to false. It's a safer bet without the muffled sound I get with (any percentage of) filtering.

    Edit 2: there's even an "enhanced YM3438" option on GPGX! I see zero documentation on this :(

  • Bumping this, might help clarify matters for several other GPGX users!

    ekeeke replied to me about this, explaining the following regarding the new MAME sound engines:

    *ASIC version corresponds to the chip model used in later Model 1 consoles and most Model 2 consoles. It features the better (not distorded and less noisy) DAC output and full address decoding of status register (which causes slower music in Hellfire since it was developed for the discrete chip version) compared to discrete chip version.

    Enhanced version has same "enhanced" features as ASIC version + full resolution DAC (14-bit instead of 9-bit in real hardware versions) + zero busy flag time (busy flag in FM status is only emulated for real hardware chip versions). It corresponds to an "idealized" version of the chip, like it is emulated in other Genesis emulators.

    The low pass filter is unrelated to the chip DAC and exists in all console models (it is implemented by analog circuitry outside of the chip). It should be enabled for accurate emulation of analog sound output (and disabled if you prefer the clear sound of pure digital emulation). Filtering ratio is configurable because it varies for each console models (also between model 1 consoles or between model 2 ones) so it's up to you to find a value that matches your own memory of accurate sound. Default value should correspond to Model 1 VA4 console.*

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