Re: [hatari-devel] Improved YM2149 cycle accuracy

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

Le 27/07/2021 à 19:31, David Savinkoff a écrit :

> > As for YM2149_Next_Resample_Nearest(), I don't think it will give the
> > best result, because it will downsample a 250 kHz input to 44.1 kHz for
> > example, using only 1 input sample every 250/44.1=5.66 samples, thus not
> > taking into account a big part of the input samples.
> The IIR filter convolves many samples so that they are spread over each other.
> The output has high frequencies attenuated at 6dB per octave
> (not really enough for proper antialiasing).

[cc-ing hatari-devel to keep track of discussion]

Hi

even if IIR convolves many samples, it does so only every 5.66 input 
samples in the case of YM2149_Next_Resample_Nearest(), so you loose some 
information between those 5.66 samples that can be meaningful when 
period is <= 6.

> > This might be OK when emulating YM2149 for sound having period>=6, but
> > with very high freq sound, this gives bad results.
> >
> > IIRC this was hearable in the game Lethal Xcess in STF mode where they
> > play digi sound by setting period=0 on one voice and modulating volume
> > for this voice. In that case YM2149_Next_Resample_Weighted_Average_N()
> > gives much better result as it uses all samples from the 250kHz input
> > signal.
> >
> > Nicolas
>
>  From the snippet of code from sound.c below:
> - LowPassFilter(sample); is supposed to sound like a ST or STF
> - PWMaliasFilter (sample); is supposed to sound like a STE
>
>          /* Apply low pass filter ? */
>          if ( YM2149_LPF_Filter == YM2149_LPF_FILTER_LPF_STF )
>              sample = LowPassFilter ( sample );
>          else if ( YM2149_LPF_Filter == YM2149_LPF_FILTER_PWM )
>              sample = PWMaliasFilter ( sample );
>
>  From what you say... I suppose that PWMaliasFilter(sample); is ineffective
> and can be removed.

I'm not sure it should be removed. There's this part in audio.c

                if ( Config_IsMachineST() && nAudioFrequency >= 40000 )
                        YM2149_LPF_Filter = YM2149_LPF_FILTER_LPF_STF;
                else
                        YM2149_LPF_Filter = YM2149_LPF_FILTER_PWM;

so if PWMaliasFilter() is supposed to sound more like an STE, it should 
be kept.

> However, LowPassFilter(sample); is supposed to give the charactistic ST
> sound (less sharp). This is the same ST sound that everyone thought was OK
> a few years ago. I'll calculate some sharper sounding coefficients soon.

I don't disagree, it's just that the change is hearable between hatari 
2.3 and current devel version when I apply the patch you sent on 26/7/2021.
Maybe sound was too "sharp" before, I can't tell, some YM2149 musicians 
could give their opinion on this compared to a real STF.

Nicolas