Frequencies TIA frequencies for unusual distortions

[Thomas Jentzsch]

While playing with Kylearan's new Tracker, I noticed that the frequencies of some (2, 3 and 8 ) distortions sound way off. I know the frequencies are based on Eckhard Stollberg's Frequency and Waveform Guide from 1997, so I wonder if no one ever has noticed this or if my ears are just bad.

 

If you look at the distortions 2+3, the base frequency is divided by 465. However to me it sounds, that for distortion 2 a divide by ~465/2 (= one octave higher) and for distortion a divide by ~30 (~8 octaves higher) matches what I hear. And for distortion 8, instead of 511 a divide by ~31 seems to match much better.

 

Can anyone confirm this? And maybe someone around has the knowledge how to calculate or measure the correct frequencies?

Edited March 14, 2016 by Thomas Jentzsch

[Kylearan]

I'd be very interested in that as well, as for TIATracker I've simply taken the frequencies from RandomTerrain's web page without really understanding or checking them (yet). And since I plan to add a feature to optimize matching desired notes to frequency (similar to WebTune2600), having correct frequencies/formulas as foundation would be nice.

[Thomas Jentzsch]

That website refers back to Eckhard's guide too.

[Crispy]

I ran a couple of simulations of the audio block in my FPGA based 2600 with AUDC = 2 and AUDC = 3, and here's what I saw.

 

AUDC = 2

When AUDC is 2 the bit sequence is indeed 465 bits long, but it's made up of two patterns that are essentially the same. The only differences are in the pulse widths, and the differences are quite small.

 

AUDC = 3

When AUDC is 3, the pattern is more complex, but does have some repetition in it as well. This might explain what you are hearing.

Edited March 14, 2016 by Crispy

[Thomas Jentzsch]

Many thanks for the analysis!

 

According to Eckhard's guide, the waveform are based on:

• 001010000111011 (15 bits, source pattern) and
• 0100000000000000000100000000000 (distortion 2) / 0010110011111000110111010100001 (distortion 3) (31 bits, clock modifier pattern).

I am not exactly sure, but at least for distortion 2 my findings seem to make sense. Would it be possible to extract the main frequency of distortion 3, e.g by using a Fourier-Transformation?

Edited March 15, 2016 by Thomas Jentzsch

[+Random Terrain]

If you guys discover something that needs to be changed, please let me know so I can update the page.