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That is definitely opinion. I agree that it can sound smoother with its nice humming-type of instruments, but then again so can the NES. But as far as straight technical sound superiority, I'd have to say that the Genesis wins. Based on sound quality alone (not musical composition), listen to the following examples on the Genesis which the PCE could not do as well:
-Streets of Rage 1 & 2
-Mega Turrican
-Midnight Resistance
-Gauntlet 4
-Panorama Cotton
-Global Gladiators (?)
And although the Genesis is known for its scratchy voices, it could do voices better than the PCE as well from all the examples I've heard. I just think that more games for the Genesis used crappy samples than ones with good samples. I'm sure the same is true for the PCE, but even the best PCE voices I've heard sound pretty scratchy. I can post some good Genesis voice examples if you'd like (from real hardware, not an emulator).
If you have examples of PCE with technically superior sound quality to all of those titles above, please point me to it as I'd love to hear it.
It would be interesting if those games mentioned above did appear on the pc engine too as you'll then be able to make a direct comparison..
I know from hearing the originals of some megadrive/genesis games sounded awesome but this don't mean that the pce's sound chip is no slouch either.
You could however make some direct comparisions with the following:
PCE: Devil Crash/crush Genesis: Dragon's Fury
PCE: Tatsujin Genisis:Truxton
PCE: Street Fighter II Genesis: Super Street Fighter II
PCE: Kyuukyu Tiger Genesis:Kyuukyu Tiger
PCE: Aero Blaster Genesis:Aero Blaster
PCE: Populous Genesis:Populous
Yes I'll agree some Genesis version's sound better than the pc engine but then there are some pc engine games that sound better than the genesis ones which I don't think is the fault of the hardware but more of the musician programmers.