[sdiy] Techniques for Multiplying MIDI Clock frequency?

[Benjamin Tremblay]

One more thing. I see a few examples online where the arduino is told to delayMicroseconds(n) to simulate the extra pulses. This seems inherently buggy because it makes the whole program only stable within a certain frequency range.

> On Dec 18, 2021, at 1:28 PM, Brian Willoughby <brianw at audiobanshee.com> wrote:
> 
> When measuring the incoming clock period, filtering those values should smooth out the jitter. i.e. Rather than use each newly measured clock period directly, run it through a filter that averages it out with recent clock periods. The DPLL shows a 3rd order filter.
> 
> For those commercial DPLLs, do they require 2 kHz as the multiplicand, or is that a master clock?
> 
> Brian
> 
> On Dec 18, 2021, at 03:26, Mikko Helin wrote:
>> There is at least one commercial MIDI clock multiplier:
>> https://tubbutec.de/product/audiowerkstatt-midi-clock-multiplier/
>> 
>> That device does 2x - 9x multiply. I don't think MIDI clock at least
>> if the source is a DAW is quite stable source so multiplying more than
>> 10x might not give expected results. Also 31.25 Kbaud is low speed
>> anyway, every byte takes at least 0.32 ms to transfer. Regarding
>> polyrhytmics which are the actual multipliers needed? Commercially
>> available DPLLs ("jitter attenuators" etc.) require quit high input
>> freq from 2kHz up or so. You might be implement slower one using FPGA,
>> maybe you could use something like PSoC 5LP ($10 board), see this PLL
>> demo: https://www.youtube.com/watch?v=ss75Jkio070
>> 
>> On Sat, Dec 18, 2021 at 12:11 PM Spiros Makris wrote:
>>> 
>>> There are all excellent suggestions!
>>> I happen to have a prototype with a teensy 3.2 and midi In/Out capabilities on the bench, so I think I'm going to experiment with that first.
>>> The counter method is an open-loop approach, pretty much, and what I'm going to try first. I suppose if I closed the loop somehow it would become a "proper" DPLL. Maybe if I build a software phase detector and use the error signal in combination with the counted value to get more steady results?
>>> I believe going for the maximum clock speed and resolution combination for the frequency chosen (clock frequency / counter size < max midi clock frequency ?) is what will give the most accurate results.
>>> 
>>> Spiros
>>> 
>>> On Fri, Dec 17, 2021 at 10:50 AM Roman Sowa <modular at go2.pl> wrote:
>>>> 
>>>> Measure incoming clock period with a timer. Then divide that measured
>>>> timer value by the number you want to multiply the frequency. If you say
>>>> anything between 10 and 100 is fine, then dividing by 32 sounds like
>>>> walk in a park. And that divided value (or simply shifted in case of 32)
>>>> is then loaded as period for another timer that outputs a pulse on overflow.
>>>> This way after each incoming clock pulse you get immediate response of
>>>> updated frequency multiplied by 64 (or whatever other value)
>>>> 
>>>> Roman
>>>> 
>>>> W dniu 2021-12-17 o 08:03, Spiros Makris via Synth-diy pisze:
>>>>> Hello List,
>>>>> I want to experiment with polymetric structures and create "non
>>>>> standard" subdivisions within the standard ones supported by MIDI. I
>>>>> have only worked with the 24ppq, using the timing messages to directly
>>>>> drive the sequencers. I would like a way to multiply the clock frequency
>>>>> in order to increase resolution, let's say by 10 or maybe 100 times, if
>>>>> that's feasible (I think it is?).
>>>>> I think this is achieved by a PLL. I understand the basic concepts of it
>>>>> and have used in the analog domain, but never did it in a digital only
>>>>> format. Furthermore, I suppose having an array of hardware peripherals,
>>>>> such as timers and interrupts, and a fairly fast processor (teensy 3.2)
>>>>> could open up other approaches that don't follow the standard PLL
>>>>> configuration.
>>>>> This has been on my mind for some time, but I don't really know where to
>>>>> start. Any advice, or resources are greatly appreciated
>>>>> 
>>>>> Regards,
>>>>> Spiros
> 
> 
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