[sdiy] Trueno

rburnett at richieburnett.co.uk rburnett at richieburnett.co.uk
Thu Dec 14 10:54:22 CET 2017


Yes, the AK5720 contains a built-in anti-aliasing filter with a good 
steep response, so that should have taken care of band-limiting the 
audio before sampling.  Therefore it's likely that the aliasing seen in 
the example audio files is either:

1. Jitter in saw reset signals produced by the STM32 micro as Vladimir 
Pantelic wrote,

...or...

2. The result of some kind of poor quality sample-rate conversion 
process that was applied during the preparation of the online audio 
clips!

-Richie,



On 2017-12-13 14:58, Vladimir Pantelic wrote:
> On 13.12.2017 10:30, Roman Sowa wrote:
>> Here are more detailed pictures:
>> http://www.synthdiy.eu/files/truenobottom.jpg
>> http://www.synthdiy.eu/files/truenobottom2.jpg
>> http://www.synthdiy.eu/files/truenobottom3.jpg
>> http://www.synthdiy.eu/files/truenotop.jpg
>> http://www.synthdiy.eu/files/truenotop2.jpg
>> http://www.synthdiy.eu/files/truenotop3.jpg
>> 
>> You were right, Neil, that's pair of LM4250 in the filter.
>> Also on the bottom side:
>> 3rd VCO that was missing from yesterday's picture, with inevitable 
>> Kynar
>> job, STM32F042C6 for the brains, ADC - AKM5720, and silkscreen 
>> suggests
>> analog part works at +/-3.5V supply.
> 
> so now, where do the CVs for the VCOs come from? the STM32042 has no
> DACs and I see no external DAC either, so the CVs are all lowpass'd PWM
> from the STM?
> 
> or - and that could explain the aliasing seen in the signals - the 
> STM32
> itself provides the saw ramp reset timing signals and there is only a
> coarse CV for the current source to charge the timing caps?
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