[sdiy] Frequency modulation vs Phase mod, was: Buchla 295 10-band comb filter topology

[Tom Wiltshire]

To add to what Richie said, there's supposed to be a 16KHz filter post-DAC too, so harmonics at 20KHz would definitely start being affected. According to this very good article:

https://ajxs.me/blog/Yamaha_DX7_Technical_Analysis.html


> On 28 Nov 2022, at 10:20, rburnett at richieburnett.co.uk wrote:
> 
> My understanding is that "FM" synths often decrease the modulation index with increasing pitch towards the top of the keyboard to curtail the extent of the modulation sidebands.  This is a crude but effective way to reduce the amount of high-frequency energy that would otherwise alias badly with a relatively low sample rate such as 48kHz.  So, you wouldn't necessarily expect the waveshapes in the top few octaves to match those lower down the pitch scale.
> 
> -Richie,
> 
> 
> 
> On 2022-11-28 08:32, Donald Tillman wrote:
>>> On Nov 27, 2022, at 5:44 AM, Ingo Debus via Synth-diy <synth-diy at synth-diy.org> wrote:
>>> We see that the waveforms of all keys except the highest key look similar, but not exactly the same. The highest key waveform looks very different. Maybe this is just a lowpass filter effect of the DX7 (certainly not of the scope, this is a 60 MHz scope). The frequency of the highest key is about 2 kHz.
>> That waveform has a heavy 10th harmonic going on.
>> And for a 2 kHz note, that would be 20 kHz.
>>  -- Don
>> --
>> Donald Tillman, Palo Alto, California
>> https://www.till.com
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