[sdiy] TX81Z, YMxx chips, and aliasing

Quincas Moreira quincas at gmail.com
Fri Sep 21 02:01:24 CEST 2018


Sounds like a faulty unit, I had a DX7 mk1 for a few years and never
noticed any noise, sounded great

On Wed, Sep 12, 2018 at 5:51 AM Pomik <kobejn at seznam.cz> wrote:

> Hi,
> my first comment here...
> I have had big stress with my DX7 mk.I. digital sig noise,
> since I realised it on Headphones
> after some months from a spawnshop buy.
> I Googled a lot, found schematics,
> also looked over forums around,
> and if anybody ask for antialiasing,?
> Isnt this question about more or less filtering
> of output from DAC ?
> Cause of ol original DX7 noise for example
> is that thing of switching MOS 4 bit R2R add-on
> for 12 bit ADC, and S-H for 4(6 ?) chanell of voise,
>
> Thinking bout upgrading it for a cleaner sound of DX7 Mk II/ D FD
> found that these have S-H swithed to stereo out from one single PCM57 ADC.
>
> but signals and OPGenerator  out wiring are quite different ?(
>
>
>
> Antialiasing algorithms in FM ?
> Gues it as so DSP hungry and difficult,
> that original FM synth buiders didnt even think bout that.
>
> --
> s pozdravem
> Petr M
> (+420)608228257 ...<<<<<<< *<:]<> Pom.ixsss...
>
> ---------- Původní e-mail ----------
> Od: KD KD <pic24hj at gmail.com>
> Komu: synth-diy at synth-diy.org
> Datum: 8. 9. 2018 0:56:55
> Předmět: Re: [sdiy] TX81Z, YMxx chips, and aliasing
>
> Yes, i think all yamaha PM synths have keyboard scaling to change
> modulation index.
> e.g page 144 SY99 manual 4 breakpoints describes how the level of a
> modulator or operator over keyboard are freely scaled. All waveforms
> are mathematical transforms
> of sines.
>
> 2018-09-05 14:14 GMT+02:00, rburnett at richieburnett.co.uk
> <rburnett at richieburnett.co.uk>:
> > As Michael said, the FM spectrum is theoretically infinite even with a
> > sinusoidal carrier and modulator.
> >
> > However, I believe that synths like the DX-7 reduced audible aliasing by
> > employing mappings across the keyboard that decreased the modulation
> > index of patches progressively for higher pitched notes. Reducing the
> > modulation index in Frequency Modulation puts less energy into the
> > higher-order sidebands and more energy into the close-in sidebands from
> > the FM process, so it results in a signal that has less potential to
> > alias badly.
> >
> > -Richie,
> >
> > On 2018-09-05 12:56, Declare Update wrote:
> >> Ah! very good point. thanks.
> >>
> >> Chris
> >>
> >>> On Sep 5, 2018, at 4:14 PM, Michael Zacherl <sdiy-mz01 at blauwurf.info>
> >>> wrote:
> >>>> On 3 Sep 2018, at 09:43, Declare Update <declareupdate at gmail.com>
> >>>> wrote:
> >>>> Do the yamaha FM chips, such as the YM2414 in the TX81Z, do anything
> >>>> toward anti-aliasing? more specifically, does anyone know if they use
> >>>> band limited tables for their extra wave forms besides sine, and is
> >>>> there any attempt to band limit the signal after FM is applied?
> >>>
> >>> On the bottomline bandlimiting the source wave forms is moot, since
> >>> it’s the FM’s sidebands which cause the aliasing (and are responsible
> >>> for the rich spectrum).
> >>> So even when using just sine waves you may get side bands which get
> >>> above Nyquist and/or below zero.
> >>> m.
>
> >
>
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-- 
Quincas Moreira
Synth Diy Guy
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