[sdiy] TX81Z, YMxx chips, and aliasing

[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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