Re: [hatari-devel] STe DMA sound

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Le 13/04/2018 à 09:31, Vincent Rivière a écrit :

I'm not really satisfied of Hatari DMA sound emulation.
Basically, Hatari lacks a low-pass filter, which is a key signature of STe DMA sound at low frequencies such as 6 or 12 KHz. This can particularly be heard in Lethal Xcess. Digidrums are very bass-boosted on real hardware. On the other hand, on Hatari (emulating sound at 44 KHz), we can hear some bogus-treeble sound. This is typical of samples upscaled with "nearest" algorithm, without filter.

I played Lethal Xcess with Effects only, and I recorded the sound with Hatari feature. See explosion sound as attachment.

I looked at it with Audacity.

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. They cause the apparition of bogus high frequencies, even more than standard upscaling. On the contrary, a normal low-pass filter would smooth such transitions. While currently, Hatari does the opposite, degrading the sound instead of improving it.

Definitely, Hatari STe sound can be improved.
I don't hear such problem in Steem.


Hatari has a low pass filter for STE DMA sound, see the functions DmaSnd_LowPassFilterLeft and DmaSnd_LowPassFilterRight in dmaSnd.c

But the filter is enabled only if the DMA sample freq is above the output freq of the audio card. As most PC will default to 44.1 kHz, this means that low pass only applies when STE sound is at 50 kHz.

This part was written by David Savinkoff, he might tell the reason for this choice. Maybe some fine tuning would be needed based on some real wav recording on an STE at different freq to get closer results to real HW.

In dmaSnd.c at line 530, can you force "DmaSnd_LowPass = true" to see if the sound is better according to you ?


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