[sdiy] Henry, 256 Bw limited!

[Grant Richter]

Something is going on with the number of samples in a wavetable  
waveform.

Certain persons were nice enough to send me the binary images of the  
PPG 2.3 and Prophet VS wave ROMs.

The waveforms are all 128 bytes stored in 64 byte halves.

I wrote a simple QBasic program to fully expand the waves to 128 bytes.

Then I used the resample function in Sound Forge to turn them into  
256 byte waveforms.

This really mellowed them out, reduced their brightness considerably.

We ended up just putting two copies in series, this plays an octave  
higher, but keeps the original tone color.

The same thing happens if you take the 32 byte waveforms from the  
Digisound DVCO and resample them. They sound different at 256 bytes.

I believe the resample algorithm in Sound Forge does a DFT then and  
inverse DFT at the new sample rate.

This process somehow produces a transversal filtering effect (low pass).

The effect occured with and without the anti-aliasing option on or off.

It has always puzzled me, but the number of bytes in the waveform is  
involved in the tone color.

The quantization noise should be the same since all are 8 bit.

I assume it must be from harmonic aliasing, but I'm more of an s- 
plane kind of guy.

I admit z-plane math makes my head spin in a unit circle ;^)

On Dec 26, 2009, at 2:54 AM, karl dalen wrote:

> - Den fre 2009-12-25 skrev Andre Majorel <aym-htnys at teaser.fr>:
>> Från: Andre Majorel <aym-htnys at teaser.fr>
>
>> One thing that has annoyed me about wave table oscillators
>> is the
>> limited bandwidth. I kept wondering why the Monowave
>> sounded
>> muffled when its filter was, according to Paul, open. And
>> it has
>> 256 points per cycle. I wonder what it's like with 16 or
>> 10.
>
> Muffled? From where do you get the impression that
> 256 point single cycle table are limited in bandwidth?
> Are you sure Pauls filter was really open?
>
> Limited in frequency range yes,(i think he used 20bit acc) but not  
> in Bw!
>
> Reg
> KD
>
>
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