[Interest] Ultra precise timing in Qt

Nuno Santos nunosantos at imaginando.pt
Sat Apr 23 19:46:14 CEST 2016

Alright! Even that I feel completely dumb right now, I’m happy that I don’t need to reinvent the wheel!!! :)

In the mean time, with all of this refactoring I have changes that could probably lead to better timing mechanism.

With this in mind I will do further refactor. 

Thanks for your prompt reply.



> On 23 Apr 2016, at 18:20, Thiago Macieira <thiago.macieira at intel.com> wrote:
> On sábado, 23 de abril de 2016 18:06:38 PDT Nuno Santos wrote:
>> So far I was using QElapsedTimer to measure this. One particular behaviour I
>> observed was, once I started moving the virtual knobs and faders of the
>> synth, it would start to get out of sync with the machine. My conclusion is
>> that somehow, QElapsedTimer is tied to the event loop thus making it slower
>> when other interactions are happening. Does this make sense? I haven’t
>> looked to QElapsedTimer source code, i’m just guessing. Maybe I’m saying
>> silly things.
> Yeah, kinda... Your description above summarises to "the more code I add, the 
> more work needs to be done".
> The use of QElapsedTimer does not have any further impact on the event loop 
> besides executing more code and is not any different from any other further 
> addition of code that causes a system call. On some OSes, getting the system 
> time is a very fast operation (Linux); on others, it might be slow.
>> I search for options and started using mach_absolute_time. The problem seems
>> to be solved now. I have the soft synth running with the arpeggiator
>> enabled sending midi clock to the machine, and they keep their tempo
>> constant, even when the controls are being used or the device is under
>> stress.
>> Does Qt provide any kind of cpu absolute clock for every platform? I
>> couldn’t find it so far.
> Yes, it's called QElapsedTimer.
>> Does anyone know the equivalent to mach_absolute_time in Windows and Android
>> (probably the same as linux).
> On Windows, you use GetTickCount64 or QueryPerformanceCounter. On Android and 
> Linux, use clock_gettime for the CLOCK_MONOTONIC or CLOCK_MONOTONIC_COARSE.
> Do you know how I know this? I looked into qelapsedtimer_win.cpp and 
> qelapsedtimer_unix.cpp.
> -- 
> Thiago Macieira - thiago.macieira (AT) intel.com
>  Software Architect - Intel Open Source Technology Center
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