Re: [hatari-devel] STe DMA sound

[Christian Zietz <czietz@xxxxxxx>]

Vincent Rivière schrieb:

> First, see audacity1.png. We can see big stairs effect, which is typical of 
> upscaling with "nearest" algorithm.
> But if you look closely, there are big peaks at the beginning of each step. 
> This is particularly visible when zooming more, see audacity2.png.
> 
> That's a complete nonsense. I can't understand where such peaks come from. 

There's clearly something wrong with Hatari. The attached Audacity
screenshot shows what happens when I try to play a sine-wave with the
LMC1992 set to maximum treble boost (+12 dB).

I also have a theory: The big peaks followed by some ripple look like
the step response of a high-pass filter. Effectively the treble boost
*is* a high-pass filter. But why does it affect a sine wave? Easy:
Hatari first does the upsampling ("upscaling" as you called it),
introducing stair-like steps into the output signal. Only then it
applies the emulated LMC1992 filters, which will of course also act on
the steps, introducing these ugly artifacts.
Possible solutions: Either do the filtering on the original DMA sound
signal or upsample by smooth interpolation.

Regards
Christian
-- 
Christian Zietz  -  CHZ-Soft  -  czietz@xxxxxxx
WWW: http://www.chzsoft.de/
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Attachment: hatari-pcm-sound-wrong.png
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