[sdiy] Analysis of the TB-303 CPU timing

Julian Schmidt elfenjunge at gmx.net
Thu Mar 16 12:43:35 CET 2017

Am 15.03.2017 um 17:13 schrieb Bruno Afonso:
> 3) you mention jitter several times but there is no statistical 
> description of the phenomena (mean, std dev, etc) or even a graphic 
> depiction such as an histogram. Without this it's hard for the reader 
> to see how relevant it may or not be.
I calculated the standard deviation and mean value for the delay between 
the tempo clock and the gate signal and added a nice diagram to the PDF. 
I also added some diagrams showing the total on and off times for the 
gate signal at 2 different speeds.
> Personally, I feel it would be cool to simulate the clock interplay to 
> reproduce obseved experimental data and then explore the space in 
> order to get it more tight or more loose. Would it have a big impact 
> on sound and experience? Would people care or notice if they used a 
> much tighter 303 clock wise?
While I'm at it and have all the hardware here I could set up some A/B 
testing audio files. Do you think it would be worth the hassle?

I have a 303 with a socket for the CPU and not yet returned my loaned 
original Roland CPU.
Further I currently have a Quicksilver CPU here that has a much tighter 
timing (measured the tempo clock to gate on/off delay to be in the 0.1ms 
to 0.35ms range) and of course my own CPU with the emulated timing like 
in the original, but I could easily make it switchable that a button 
press would turn off the fixed 1ms/2.4ms interrupt gate latency and 
probably change the 1.8ms polling interrupt to a pin change triggered 
interrupt to follow the tempo clock tightly. But I guess the QS timing 
should be tight enough

So I could record a few patterns with different CPUs in the same synth 
hardware to exclude influences by differences in the analogue part of 
the machine or different knob settings.

Would be interesting to find out
A) If people can hear the difference between original sequencer, 
emulated sequencer and a tight sequencer
B) What listeners prefer if they are able to distinguish it.

maybe even add a 4th audio example to the test where the jitter and 
latencies are increased slightly?

> cheers
> b
> On Wed, Mar 15, 2017 at 11:14 AM Julian Schmidt <elfenjunge at gmx.net 
> <mailto:elfenjunge at gmx.net>> wrote:
>     I'm currently working on a TB-303 replacement CPU for the RE-303
>     project.
>     www.http://sonic-potions.com/re303
>     Since there is not too much information available, and a lot of
>     speculations are running wild on the net, I tried to clear things
>     up a bit.
>     Someone from the RE-303 forums loaned me an original CPU and I
>     hooked it
>     up to a logic analyzer and did some extensive testing and measurement
>     sessions.
>     I summed up my findings in a short paper available here:
>     http://sonic-potions.com/Documentation/Analysis_of_the_D650C-133_CPU_timing.pdf
>     I have to admit, a lot of it is definitely from the "hear the fleas
>     cough" category, as we say in Germany, but at least we now have data
>     coming from a real CPU.
>     Comments are welcome. I'm a bit rusty in writing proper papers ;)
>     Best,
>     Julian
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