Is OPLEMU inaccurate?

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Is OPLEMU inaccurate?

Postby superfury » 2018-8-07 @ 08:22

When I compare the Dosbox recordings I find on Youtube of Megarace soundtracks and compare them to my (UniPCemu's) cycle-accurate OPL2 emulation, some sound effects sound completely different, e.g. some blubbery high tones and echoes within songs like Maeva?

Is this a bug in UniPCemu, or is the oplemu Dosbox uses simply inaccurate on those?
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Re: Is OPLEMU inaccurate?

Postby gdjacobs » 2018-8-07 @ 15:41

You're better off comparing against Nuked OPL. It has far more fidelity to real OPL hardware.
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Re: Is OPLEMU inaccurate?

Postby NY00123 » 2018-8-07 @ 20:09

Also, if you really want to be sure, then compare to original OPL hardware *only*, and not to any OPL emulator's output.
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Re: Is OPLEMU inaccurate?

Postby superfury » 2018-8-08 @ 09:39

Oddly enough, it's probably either a Dosbox bug, or a OPL3 incompatibility with OPL2? My own emulator is itself based on some OPL2 reverse engineering reports and produces audio in a cycle-accurate way(sampling rate of exactly 14.31818MHz divided by 288, then downsampled&filtered to 44.1kHz).

The only thing that's floating point in my case(not based on the integer logic of the original chip) is the basic timing that drives the oscillators(floating point that drives the sinus lookup tables by converting timing to phase as input to the ROM quarter-sinus lookup tables). After that, it's all the original digital logic.
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Re: Is OPLEMU inaccurate?

Postby superfury » 2018-8-09 @ 05:35

The odd thing is:
1. I don't own a OPL2 adlib myself(nor can use it on my modern PC i7 mobo afaik).
2. All recordings I can find of the Megarace soundtracks are either from a YMF272-compatible clone or straight Dosbox recordings. So those don't count, do they? I need actual recordings from a OPL2(YM3812) chip.
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Re: Is OPLEMU inaccurate?

Postby nukeykt » 2018-8-09 @ 11:50

Differences between OPL2 and OPL3 in OPL2 mode are quite subtle and are not audible. Also DOSBox default OPL core does things closer to OPL2 rather than OPL3.
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