Re: [hatari-devel] YM2149 Anti-Alias Filter

[Nicolas Pomarède <npomarede@xxxxxxxxxxxx>]

On 10/06/2012 23:27, David Savinkoff wrote:
> Yes. By-the-way, the low-pass anti-alias cutoff frequency is not
> critical as long as it doesn't filter the music, thus, a convenient
> and existing filter is used. Note that this filter (and the previously
> disabled filter) are an alternative to BLEPs.
> https://ccrma.stanford.edu/~juhan/vas.html
>
> This anti-aliasing makes an improvement to the sound. Listen to
> P7ACTION (STE) and other tunes in 505's BLUBBER.PRG for
> clarity.
>
> The pull-up and pull-down resistance are of significance when making
> DSP filters; however, these resistances are usually the same. To
> model the YM2149, one must do circuit analysis and go beyond what
> is written in DSP textbooks. This exercise got me to 'at least' make
> some personal discoveries which others can now ponder over.

Hello

thanks for the explanations.

While applying the patch, I noticed in Audio_SetOutputAudioFreq in audio.c :

        if ((ConfigureParams.System.nMachineType == MACHINE_ST) &&
                (nAudioFrequency == 44100 || nAudioFrequency == 48000))
        {
                /* Apply YM2149 C10 filter. */
                UseLowPassFilter = true;
        }
        else
        {
                UseLowPassFilter = false;
        }

Should this be changed ? Should we use piecewise filter all the time ?

Nicolas